Antimonide-based compound semiconductors for electronic devices: A review

Abstract

Several research groups have been actively pursuing antimonide-based electronic devices in recent years. The advantage of narrow-bandgap Sb-based devices over conventional GaAs- or InP-based devices is the attainment of high-frequency operation with much lower power consumption. This paper will review the progress on three antimonide-based electronic devices: high electron mobility transistors (HEMTs), resonant tunneling diodes (RTDs), and heterojunction bipolar transistors (HBTs). Progress on the HEMT includes the demonstration of Ka- and W-band low-noise amplifier circuits that operate at less than one-third the power of similar InP-based circuits. The RTDs exhibit excellent figures of merit but, like their InP- and GaAs-based counterparts, are waiting for a viable commercial application. Several approaches are being investigated for HBTs, with circuits reported using InAs and InGaAs bases.

Full text

Review
Antimonide-based compound semiconductors
for electronic devices: A review
Brian R. Bennett
*
, Richard Magno, J. Brad Boos, Walter Kruppa, Mario G. Ancona
Electronics Science and Technology Division, Naval Research Laboratory, Washington, DC 20375-5347, USA
Received 14 April 2005; accepted 11 September 2005
Available online 4 November 2005
The review of this paper was arranged by Prof. C. Tu
Abstract
Several research groups have been actively pursuing antimonide-based electronic devices in recent years. The advantage of narrow-
bandgap Sb-based devices over conventional GaAs- or InP-based devices is the attainment of high-frequency operation with much lower
power consumption. This paper will review the progress on three antimonide-based electronic devices: high electron mobility transistors
(HEMTs), resonant tunneling diodes (RTDs), and heterojunction bipolar transistors (HBTs). Progress on the HEMT includes the dem-
onstration of Ka- and W-band low-noise amplifier circuits that operate at less than one-third the power of similar InP-based circuits. The
RTDs exhibit excellent figures of merit but, like their InP- and GaAs-based counterparts, are waiting for a viable commercial application.
Several approaches are being investigated for HBTs, with circuits reported using InAs and InGaAs bases.
Published by Elsevier Ltd.
PACS: 81.05.Ea; 81.15.Hi; 85.30.Pq; 85.30.Tv; 73.61.Ey; 73.63.Hs
Keywords: Antimonide; HEMT; RTD; HBT; MBE; InAs
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1876
2. InAs quantum wells and high-electron-mobility transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1876
2.1. Background . ......................................................................... 1876
2.2. InAs/AlSb single quantum wells ............................................................ 1877
2.3. Doping of HEMTs ..................................................................... 1877
2.4. HEMT design and performance ............................................................ 1878
2.5. Variations to standard HEMT design ........................................................ 1881
2.6. Microwave and low-frequency noise . ........................................................ 1881
2.7. Radiation effects ....................................................................... 1882
2.8. Circuits ............................................................................. 1882
2.9. InSb-channel HEMTs . . ................................................................. 1883
3. Resonant tunneling diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1883
3.1. Background . ......................................................................... 1883
3.2. Performance: peak current density and peak-to-valley ratio . ....................................... 1884
0038-1101/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.sse.2005.09.008
*
Corresponding author. Tel.: +1 202 767 3665; fax: +1 202 767 1165.
E-mail address: brian.bennett@nrl.navy.mil (B.R. Bennett).
www.elsevier.com/locate/sse
Solid-State Electronics 49 (2005) 1875–1895

3.3. Bias voltages and hysteresis ............................................................... 1886
3.4. Substrates, defects, and interfaces ........................................................... 1887
3.5. Radiation effects ....................................................................... 1887
3.6. Applications . ......................................................................... 1888
4. Heterojunction bipolar transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1889
4.1. Background and early work ............................................................... 1889
4.2. HBTs with InAs and InGaAs bases . . ....................................................... 1889
4.3. InGaSb/InAlAsSb npn HBTs.............................................................. 1890
4.4. pnp Structures and InP HBTs with GaAsSb bases . .............................................. 1891
5. Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1891
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1891
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892
1. Introduction
The binary compound semiconductors AlSb, GaSb,
InSb, and InAs along with their related alloys are candi-
dates for high-speed, low-power electronic devices. Appli-
cations could include high-speed analog and digital
systems used for data processing, communications, imag-
ing, and sensing, particularly in portable equipment such
as hand-held devices and satellites. The development of
Sb-based transistors for use in low-noise high-frequency
amplifiers, digital circuits, and mixed-signal circuits could
provide the enabling technology needed to address these
rapidly expanding needs. In Fig. 1, we show the trend
toward higher frequencies and lower power consumption
with increasing lattice constant. In recent years, consider-
able progress has been made in HEMTs, RTDs, and
HBTs in the antimonide–arsenide materials system (lattice
constants greater than 6.0 A
˚). In this paper, we will
review the progress in the design, growth, fabrication,
and performance of these electronic devices. We will dis-
cuss our work at NRL as well as major results from other
groups.
2. InAs quantum wells and high-electron-mobility
transistors
2.1. Background
The first high electron mobility transistors (HEMTs)
were fabricated with GaAs channels and AlGaAs barriers
[1]. These devices are also known as modulation-doped
field effect transistors (MODFETs). In order to achieve
higher electron mobility and velocity (translating to higher
frequency operation), In was added to the channel. Typical
structures have In
0.2
Ga
0.8
As channels that are pseudomor-
phically strained to the GaAs lattice constant (PHEMTs).
In order to improve performance further, additional In
was added to the channel and the barrier material was
changed to InAlAs; the larger lattice constants were
accommodated by using InP substrates. The logical pro-
gression of this trend is to use pure InAs as the channel
along with nearly lattice-matched AlSb, AlGaSb, or
InAlSb for the confining layer as the arsenides are not suit-
able barriers. Advantages of this material system include
the high electron mobility (30,000 cm
2
/V s at 300 K) and
Ge
Si
GaAs
GaP
InP
AlSb
GaSb
InAs
AlP
AlAs
0.45
0.50
0.60
0.80
1.00
1.30
2.00
5.00
0.8
0.4
1.2
1.6
2.0
2.4
2.8
0
5.4 5.6 5.8 6.0 6.2 6.4
Energy Gap (eV)
Wavelength (µm)
InGaAs
Lattice Constant (Å)
InSb
GaAs-based HEMTs and HBTs
First generation:
InP-based HEMTs and HBTs
Second generation:
Sb-based HEMTs and HBTs
Next generation:
Fig. 1. Energy gap versus lattice constant, showing the evolution of transistors to larger lattice constants and smaller bandgaps for high-frequency and
low-power operation.
1876 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

velocity (4 ·10
7
cm/s) of InAs [2], and a large conduction
band offset between InAs and AlSb (1.35 eV), as shown
in Fig. 2 [3–6].
2.2. InAs/AlSb single quantum wells
Most of the recent advances in antimonide-based
HEMTs have involved heterostructures grown by molecular
beam epitaxy (MBE). Antimonide growth by MBE was first
reported in the late 1970s [7–9]. The growth of antimonide
and mixed antimonide/arsenide structures by MBE has pre-
sented some challenges compared to growth of arsenides
[10]. For the most part, solutions have been found. Overall,
growth of antimonides is simpler than nitrides and does not
present the safety issues associated with phosphides. Scale-
up to production MBE systems should be feasible.
Crucial advances for transistor applications were
reported by Prof. KroemerÕs group at the University of
California at Santa Barbara (UCSB), beginning in the late
1980s. They grew InAs/AlSb single quantum wells and
showed that high mobilities could be achieved by control-
ling the interfaces [11,12]. The interface bonds can be either
InSb-like or AlAs-like as there is no common cation or
anion for a heterojunction between InAs and AlSb. Work
by Tuttle et al. showed that the bottom interface must be
InSb-like to achieve high mobilities. Bolognesi et al. varied
the thickness of the InAs quantum well and achieved high
mobilities for 125–200 A
˚[13]. The mobility of thinner wells
was suppressed by interface scattering. The decrease in
mobility for wells thicker than 200 A
˚presumably resulted
from the formation of misfit dislocations due to the 1.3%
lattice mismatch between the InAs and AlSb.
Although it is not the focus of this review, we note that
the growth of InAs quantum wells with very high mobilities
has lead to many interesting physics studies. For example,
see work on ballistic transport phenomena [14,15]. In addi-
tion, the Hall effect in these structures is of interest for
magnetic sensing [16–18] and magnetoelectronic logic
applications [19].
2.3. Doping of HEMTs
Key parameters for HEMT quantum wells are electron
mobility and sheet carrier density. Sheet carrier concentra-
tions for unintentionally doped InAs/AlSb single quantum
wells are known to be a function of the thickness of the
upper barrier, the cap material (usually InAs or GaSb),
the interface bond type, and the purity of the AlSb. In most
cases, the undoped heterostructures are n-type with densi-
ties less than or approximately equal to 1 ·10
12
/cm
2
[20].
Higher sheet charge densities are typically desirable for
HEMT applications. The arsenic-soak technique, first
reported by Tuttle et al., is one way to enhance the sheet
carrier concentration. After growing an InAs quantum well
and an AlSb spacer layer, the AlSb is subjected to an As
beam [12]. Apparently, As atoms replace Al atoms, provid-
ing extra electrons to the conduction band. Densities of
1–2 ·10
12
/cm
2
are typically achieved. High-speed HEMTs
have been fabricated using this technique [21].
Silicon is the most common n-type dopant in III–V
MBE systems. However, it is amphoteric in the III–VÕs,
producing n-type GaAs, InAs, AlAs, and InSb, but p-type
GaSb and AlSb on (1 0 0) surfaces [22]. Sasa et al. achieved
sheet carrier concentrations in the 2–4 ·10
12
/cm
2
range by
Si planar doping in an 18 A
˚InAs quantum well located
80 A
˚below a 150 A
˚InAs quantum well clad by AlGaSb
[23]. The energy levels in the very thin doped well are
higher than in the channel due to quantum confinement.
Fig. 2. Band alignments for selected binary and alloy semiconductors at 0 K.
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1877

Hence, electrons will transfer into the channel. This tech-
nique has been successfully applied to InAs HEMTs by
at least three groups [24–27].
Chalcogens such as S, Se, and Te are in column VI of the
periodic table and hence logical candidates for n-type
dopants in III–V semiconductors. As discussed by Furuk-
awa and Mizuta [28], elemental sources such as Te have a
very high vapor pressure and hence are not suitable for
III–V MBE. An alternative is to use chalcogenides such
as PbSe and GaTe. Wood demonstrated n-type doping of
GaAs using PbSe and PbS [29]. Furukawa and Mizuta suc-
cessfully used GaTe to make n-type AlGaSb layers in a
transistor [28]. Subbanna et al. used PbTe to achieve con-
trollable carrier concentrations from 10
16
to 10
18
/cm
3
in a
detailed doping study of AlSb and GaSb [30]. Several
groups have successfully used GaTe as a dopant source
for Sb-based transistors and other structures [31–33].
The relationship between mobility and carrier density is
plotted in Fig. 3 for several undoped, Si-doped, and Te-
doped HEMT structures grown at NRL. High mobility
and low sheet resistance are desired for HEMT applications.
Unfortunately, the highest mobilities are achieved for
undoped structures with carrier densities that are too low
to achieve sheet resistances less than 200 X/h.Fig. 3 demon-
strates that sheet resistances near 100 X/hhave been
reached for both Si- and Te-doping. We find that densities
near 3.0 ·10
12
/cm
2
are optimal for achieving both high
mobility and low sheet resistance. Note that these high
mobilities and low sheet resistances cannot be achieved by
GaAs- or InP-based HEMTs. Other groups have achieved
results similar to those shown in Fig. 3. To our knowledge,
no one has achieved the highest mobilities and high densities
simultaneously (e.g. 25,000–30,000 cm
2
/V s and 3·10
12
/
cm
2
). Based upon Shubnikov-de Haas measurements, elec-
trons begin to populate a second sub-band in InAs quantum
wells at densities of 1.4–2.4 ·10
12
/cm
2
[23,26,34]. Hence, for
higher densities we expect intersubband scattering and a
reduction in mobility, as shown in Fig. 3.
Both Si and Te doping appear to be viable for InAs
HEMTs. Advantages of Si include the fact that it is more
often available in MBE systems and has a much lower
vapor pressure, meaning that source depletion is generally
not a problem. A disadvantage of Si is that a decrease in
growth temperature may be required between the InAs
channel and the doped layer [26]. For Te doping, a con-
stant growth temperature can be used. In addition, Te
should act as a donor in any AsSb alloy.
2.4. HEMT design and performance
The layer structure and calculated band structure for a
HEMT with Te delta-doping are illustrated in Fig. 4.
Semi-insulating GaAs substrates are typically used for
InAs HEMTs as there are no suitable zincblende insulating
substrates with lattice constants near 6.1 A
˚. The 1.7 lm
Fig. 3. Electron mobility versus sheet density for InAs single quantum
wells. The doping technique is indicated next to each point. Ranges for
typical GaAs- and InP-based HEMTs are also indicated.
AlSb 5 nm
InAs 2 nm
In0.4Al0.6As 4 nm
SI GaAs substrate
AlSb 1.7 µm
InAs 12 nm
AlSb 50 nm
Al0.7Ga0.3Sb 300 nm
Te delta doping
AlSb 1.2 nm
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 20 40 60 80
Energy (eV)
Position (nm)
AlSb
AlSb
InAs
In0.4Al0.6As
Al0.7Ga0.3Sb
Te ∆-doping
(a) (b)
Fig. 4. Heterostructure cross-section (a) and calculated band structure (b) for InAs HEMT with Te delta-doping.
1878 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

buffer layer of AlSb is almost fully relaxed and accommo-
dates the 8% lattice mismatch [35]. The 0.3 lm layer of
Al
0.70
Ga
0.30
Sb allows for a mesa isolation etch that stops
in the AlGaSb and hence does not expose AlSb outside
the active region to air or moisture that will cause it to
deteriorate [36–38]. The InAlAs layer enhances the insulat-
ing properties of the barrier by providing a barrier to holes.
In addition, it serves as an etch-stop, enabling a gate recess
etch to be employed [21]. Despite the large lattice mismatch
and three-dimensional growth, the InAlAs layer does not
degrade the electron mobility, presumably because it is at
least 50 A
˚above the channel. The AlGaSb and InAlAs lay-
ers are now used by at least three groups investigating InAs
HEMTs [33,37,39].
The uniformity of transistors across a wafer will be
important for circuit applications. Non-uniformities can
be introduced in the growth or subsequent processing.
Contactless resistivity mapping of an as-grown wafer can
easily be done with a Lehighton system as a test of unifor-
mity. We have measured non-uniformities as low as 0.7%
across a 3-in. wafer (typical values were 1–3%) for Te-
delta-doped structures similar to the one in Fig. 4, grown
on a 3-in. Riber Compact 21T MBE system [35].
The low-power potential of these materials was demon-
strated by the fabrication of InAs HEMTs with a 60 nm
gate length [40]. The as-grown heterostructure had a sheet
density of 1.6 ·10
12
/cm
2
and a mobility of 21,300 cm
2
/V s.
The I–Vcharacteristics shown in Fig. 5 indicate that the
depletion-mode device has a transconductance of 1.1 S/
mm at V
DS
= 0.35 V and V
GS
=0.50 V. The increase in
output conductance at a drain voltage above 0.3 V is due
to the effects of impact ionization in the channel. At this
bias condition, the device exhibits an extrinsic f
T
of
160 GHz. The potential for low-voltage operation is dem-
onstrated by the plot of measured f
T
as a function of drain
voltage in Fig. 6.Anf
T
of 90 GHz is obtained at a drain
voltage of only 0.10 V. InP- and GaAs-based HEMTs can-
not reach these high frequencies at such low voltages.
In the last three years, funding from the DARPA anti-
monide-based compound semiconductor (ABCS) program
has resulted in substantial advances in InAs HEMT tech-
nology. A collaboration between the Naval Research
Laboratory and Northrop Grumman Space Technology
(NGST) focused on making the devices manufacturable.
Significant steps in this direction were the elimination of
an air bridge used for the device discussed above and the
successful fabrication of T-gates. Results from 0.1 lm
HEMTs fabricated from a heterostructure with
n
s
= 1.3 ·10
12
/cm
2
and l= 29,000 cm
2
/V s are shown in
Figs. 7 and 8 [37,41,42]. The devices displayed high trans-
conductance (G
m
) at low drain–source voltage (V
DS
), and
low on-state resistance (R
ON
). The average G
m
peak was
1.05 S/mm and 1.6 S/mm measured at a V
DS
of 0.2 V and
0.3 V, respectively. The average off-state reverse gate-drain
breakdown voltage (BV
GD
) was 1.42 V (measured at a
gate current of 1 mA/mm). DC characteristics in Fig. 7
were measured from an 80-lm-gate-width device with typ-
ical extrinsic R
ON
of 0.67 Xmm. (The high peak for DC
G
m
@V
DS
= 0.4 V is anomalous and related, in part, to
impact ionization effects.) These characteristics illustrate
the combination of low-drain-voltage operation, low knee
voltage, and high transconductance, which are critical
parameters for low-power, high-frequency operation.
VGS = 0 V
Vstep = 0.1 V
0.0 0.1 0.2 0.3 0.4 0.5
Drain Voltage (V)
0
200
400
600
800
Drain Current (mA/mm)
Fig. 5. HEMT drain characteristics for an InAs HEMT with a 60 nm gate
length, L
DS
= 1.0 lm, and W
G
=50lm[40].1999, AVS The Science
and Technology Society.
VGS = -0.4 V
0.0 0.1 0.2 0.3 0.4 0.5
Drain Voltage (V)
0
40
80
120
160
fT (GHz)
Fig. 6. Extrinsic cut-off frequency, f
T
, versus drain voltage for a 60 nm
InAs-channel HEMT. Note that an f
T
of 90 GHz is achieved at a bias of
only 0.10 V [40].1999, AVS The Science and Technology Society.
0
50
100
150
200
250
300
350
400
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Gate Voltage (V)
fT
(GHz)
0
500
1000
1500
2000
2500
DC Transconductance
(mS/mm)
TOP CURVE: VDS = 0.4 V
STEPS OF -0.1 V
Fig. 7. Extrinsic cut-off frequency, f
T
, and DC transconductance versus
gate voltage for 0.1 lm T-gate InAs-channel HEMT [37].IEEE.
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1879

Small signal RF tests were also performed on-wafer, and
the maximum available gain was 10 dB at 100 GHz. The
normal slope (6 dB/octave) was used to extrapolate beyond
100 GHz and gave an average peak f
T
of 153 GHz and
212 GHz at V
DS
of 0.2 and 0.4 V, and drain current densi-
ties of 115 and 340 mA/mm, respectively. These corre-
spond to DC power dissipations of 22 and 180 mW/mm.
Compared to f
T
-DC power performance of state-of-the
art 0.1-lm gate length InAlAs/InGaAs/InP HEMTs, our
AlSb/InAs HEMTs provide equivalent f
T
at 5–10 times
lower power dissipation, as shown in Fig. 8. The use of a
T-gate resulted in values of f
max
greater than f
T
, with
f
max
= 270 GHz at V
DS
= 0.4 V.
HRL Laboratories recently reported InAs-channel
HEMTs with InAlAs barrier layers. The composition and
thickness of the In
x
Al
1x
As is not given, but presumably
the layers are In-rich and pseudomorphic. For a 70 nm gate
length, an intrinsic f
T
of 308 GHz was achieved at
V
DS
= 0.70 V. The peak intrinsic f
max
was 110 GHz. The
device did not have a T-gate [39].
A Rockwell-UCSB team also achieved excellent low-
power performance. They report 0.25-lm T-gate HEMTs
fabricated from a heterostructure with n
s
= 3.7 ·10
12
/cm
2
and l= 19,000 cm
2
/V s [33]. The extrinsic values of both
f
T
and f
max
are 162 GHz at V
DS
= 0.35 V, V
GS
=1.1 V,
I
d
= 6.1 mA, and I
g
=20.9 lA. In a subsequent report,
0.1-lm T-gates were written on a heterostructure with
n
s
= 2.1 ·10
12
/cm
2
and l= 18,000 cm
2
/V s. These devices
reached an f
T
of 235 GHz at V
DS
= 0.45 V, and an f
max
of 235 GHz at V
DS
= 0.30 V, as shown in the contour plots
of Fig. 9 [43]. Note that both f
T
and f
max
exceed 100 GHz
at a drain bias of only 0.10 V, again highlighting the low-
power potential of these transistors.
The threshold voltage of the n
s
= 3.7 ·10
12
/cm
2
Rock-
well device discussed above was 1.2 V. We have also
observed large negative threshold voltages (V
th
<1.0 V)
for some devices with high sheet charge (3·10
12
/cm
2
).
These high threshold voltages are generally not desirable
for low-power circuits. One way to make V
th
less negative
is to decrease the sheet density. The disadvantage to this
approach is evident in Fig. 3.At3·10
12
/cm
2
, mobilities
near 20,000 cm
2
/V s are possible, yielding sheet resistances
near 100 X/h. If the density is reduced to 1 ·10
12
/cm
2
, the
mobility may increase to 30,000 cm
2
/V s, but the sheet
resistances will increase to 200 X/h. This will mean
higher access resistance for the HEMTs. An alternative is
to reduce the gate-to-channel spacing. To quantify this
approach, we performed Silvaco modeling of V
th
as a func-
tion of sheet density and vertical spacing. We use the dis-
tance between the gate (which rests on the InAlAs) and
the center of the channel as a parameter. Surface state
and quantum effects are not included. The results are
shown in Fig. 10. For low densities, the threshold voltage
is small (approximately 0.5 V) regardless of the gate–
channel separation. For higher densities, there is only a
0
50
100
150
200
250
1 10 100 1000
DC Power Dissipation (mW/mm)
Cut-off Frequency, fT (GHz)
Increasing VDS
VDS = 0.1V
0.2V
0.5V
VDS = 0.2V
0.5V
1.0V
Fig. 8. Extrinsic cut-off frequency, f
T
, versus power dissipation for InAs/
AlSb HEMT (solid lines) at V
DS
= 0.1, 0.2, and 0.5 V and InAlAs/InGaAs
HEMT (dashed lines) at V
DS
= 0.2, 0.5, and 1.0 V [37].IEEE.
00.1 0.2 0.3 0.4 0.5
0
100
200
300
400
500
600
700
800
Drain–Source Voltage (V)
Drain Current (mA/mm)
50
50
50
50
50
100
100
100
100 100
150
150
150
150
150
200
200
200
200
235
fτ(GHz)
0 0.1 0.2 0.3 0.4 0.5
0
100
200
300
400
500
600
700
800
50
50
50
100
100
100
100
100
100
150
150
150
150
150
200
200
200
200
235
Drain–Source Voltage (V)
Drain Current (mA/mm)
f
max
(GHz)
(a)
(b)
Fig. 9. Drain current versus drain–source voltage for Rockwell 0.1-lm-
gate-length, InAs/AlSb HEMT with contours of f
T
and f
max
illustrating
high-frequency performance at low power [43].IEEE.
1880 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

small difference between the 162 and 214 A
˚lines, but a sub-
stantial increase in V
th
for 300 A
˚. We also include experi-
mental points on the plot which generally confirm the
predictions. (We have several measured values of
0.4 < V
th
<0.7 for 1–2 ·10
12
/cm
2
but do not include
all of them on the plot for clarity.) It was these results, in
part, that led us to select the Te-delta-doped structure
shown in Fig. 4. Using structures similar to Fig. 4, we have
achieved densities of 1–3 ·10
12
/cm
2
with gate–channel sep-
arations of 130–214 A
˚. The reduced spacing is also advan-
tageous because it allows good aspect ratios as gate lengths
are reduced.
2.5. Variations to standard HEMT design
A number of variations to the InAs HEMTs described
above have been investigated over the last decade. Several
involve changes to the channel. Yang et al. fabricated FETs
from a heterostructure with an 80 A
˚InAs/50 A
˚GaSb com-
posite channel [44]. By varying the gate voltage, the carriers
in the channel can be changed from electrons to holes,
resulting in a novel V-shaped transfer characteristic. Boos
et al. modified the conventional InAs HEMTs by adding
a42A
˚InAs subchannel, separated from the 100 A
˚InAs
channel by 30 A
˚AlSb [45]. The subchannel has a larger
bandgap than the channel due to quantization. Hot elec-
trons can tunnel from the channel into the subchannel
before gaining enough kinetic energy for impact ionization.
HEMTs using this design exhibited an intrinsic f
T
of
250 GHz for a 0.1 lm gate length. A similar approach
was taken by Lin et al. who used a composite channel of
InAs and InAsP (InAlAs) with the InAsP (InAlAs) having
a larger bandgap [46,47]. The resulting HEMTs exhibited
increased breakdown voltage compared to InAs-channel
devices. A final variation to the channel is to incorporate
alloys of InAsSb. Using digital alloys of InAsSb, we
showed that the band alignment between the InAs
1x
Sb
x
and AlSb changes to type-I for x> 0.15 [48]. The type-I
alignment should enable better confinement of holes gener-
ated thermally or as a result of impact ionization in the
channel. We fabricated 0.1 lm HEMTs with InAs
0.8
Sb
0.2
channels and achieved intrinsic values of f
T
= 180 GHz
and f
max
= 120 GHz, and a voltage gain of 9, despite the
fact that the mobility was only 13,400 cm
2
/V s [49]. More
recently, we have improved the mobility to 22,000 cm
2
/
V s using random alloys of InAs
0.7
Sb
0.3
and barriers of
In
0.2
Al
0.8
Sb. Transconductances of 1.35 S/mm were mea-
sured at V
DS
= 0.3 V for a 0.2 lm gate length [50,51].
Another variation we investigated included a 100–200 A
˚
layer of p-type GaSb (doped in the mid 10
17
/cm
3
by Si or
Be) 500 A
˚below the InAs channel [21]. The intention of
this layer is to drain a portion of the impact-ionization-gen-
erated holes back to the source contact rather than having
them remain in the AlSb buffer layer where they are likely
to cause deleterious trapping effects or be collected at the
gate contact and thereby increase the gate leakage current.
A related technique is to use an epitaxial back-gate placed
below the quantum well to remove holes [52].
Another method to minimize impact ionization effects is
the use of dual gates. The use of a dual gate for InAs
HEMTs was proposed in 1990 [53] and demonstrated by
two groups in 1996 [54,55]. This design enables the reduc-
tion of leakage current by using a second gate to modify
the E-field under the first gate. A disadvantage is the
increased process complexity required to achieve low access
resistance with the dual-gate design.
As mentioned earlier, even undoped InAs quantum
wells usually have 10
12
/cm
2
electrons in the channel.
Hence, InAs HEMTs normally operate as depletion-mode
devices. Zhao et al. incorporated Be delta-doping in the
upper barrier to achieve n-channel enhancement-mode
transistors [56]. Recently, other groups have also been
working toward this goal [39,57,58].
The typical InAs HEMT structures include a thin
(2 nm) fully-depleted InAs cap which is removed to allow
a recessed gate on the InAlAs barrier. An alternative, com-
monly used in InP-based HEMTs, is to grow a thicker n
+
cap layer. This can result in low access resistance and low
threshold voltage. In a recent report, HEMTs were fabri-
cated on a heterostructure with a 20 nm n
+
-InAs cap above
an InAlAs/AlSb composite barrier [59]. At NRL, we are
investigating structures with a 20 nm n
+
-InAs cap above
an InAlSb barrier [60].
2.6. Microwave and low-frequency noise
As discussed earlier, several groups have now demon-
strated low-power InAs HEMTs with values of f
T
and f
max
exceeding 100 GHz. If this technology is to become viable,
however, the microwave noise must be low. As shown in
Fig. 11,F
min
> 1 dB was measured for frequencies of 2–
20 GHz in the first noise measurements reported in 1997
[61]. These relatively large noise figures were caused by
the high gate leakage currents. Simulations indicated that
a reduction in gate leakage current should result in much
lower noise figures (e.g. 0.3 dB at 4 GHz). Devices recently
fabricated by Rockwell [43,62] and NGST/NRL [37,63]
0
-0.5
-1
-1.5
-2
0 0.5 1 1.5 2 2.5 3 3.5 4
Sheet Char
g
e (1012cm-2)
Threshold Voltage (V)
300Å
214Å
162Å
265Å
214Å
315Å
164Å
250Å
264Å
240Å
300Å
Fig. 10. Calculated threshold voltage as a function of sheet charge at three
different values of gate-to-center-of-channel separation. Experimental data
points are also shown.
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1881

exhibited lower gate leakage current and a substantial
reduction in F
min
, as shown in Fig. 11. These results are
very encouraging: they demonstrate that InAs HEMTs
have attained microwave noise performance comparable
to GaAs- and InP-based HEMTs.
The low-frequency noise of InAs- and InAsSb-channel
HEMTs is also of considerable interest for two primary
reasons. First, this noise is a very sensitive indicator of
material quality, particularly at the interfaces, and there-
fore can be used to fine-tune material composition and
growth parameters. Secondly, since this noise is upcon-
verted in non-linear HEMT circuits, it has a direct bearing
on the phase noise of microwave oscillators and phase jitter
of high-speed digital circuits. At this time, measurements of
the low-frequency noise characteristics of AlSb/InAs/AlSb
[64,65] and AlSb/InAsSb/AlSb [66] HEMT structures have
been performed. Typical devices whose noise spectra were
studied consisted of small HEMT structures, 30 lm wide,
with a source–drain length of 3 lm, and gate length of
0.1 lm. The various layer structures examined included
InAs channels, digital alloy superlattice channels of InAs/
InSb, and random alloy channels of InAsSb. Both undoped
and doped devices were included in the study. The mea-
surements were usually done open-channel to avoid gate
leakage current whose associated shot noise could affect
the results.
Although considerable variations among the noise char-
acteristics were observed, several points can be made to
summarize the results of the measurements. The low-fre-
quency noise in the HEMTs consists of the superposition
of 1/fnoise and a local-level component which contributes
noise in the temperature range between 150 K and 300 K.
An Arrhenius plot of the level yields an activation energy
of about 0.38 eV. The value of the Hooge parameter, a
H
,
was found to range between 10
3
and 10
2
. No correlation
between the channel mobility and the Hooge parameter
was found in the samples measured. A correlation between
the noise and the channel strain, however, was suggested by
the results. Specifically, the devices with InAs
0.8
Sb
0.2
chan-
nels, which are unstrained, had the lowest noise, while
those with InAs-channels (in tension) and those with
InAs
0.7
Sb
0.3
(in compression) were noisier.
It should be noted that no effort has been made at this
point to optimize the layer structure, the growth parame-
ters, or the processing procedure in order to minimize the
low-frequency noise. Although the Hooge parameter in
these devices is about one order of magnitude higher than
in the best InP- or GaAs-based HEMTs, the values of a
H
appear reasonable for a relatively immature technology
with a dislocation density of 10
8
–10
9
cm
2
due to lattice
mismatch with respect to the substrate [67,68]. It should
also be noted that in all the devices measured at NRL,
the noise increases greatly with illumination at low temper-
atures, and displays a strong Lorentzian spectrum. This
noise appears to involve a deep level in the AlSb which
can host the photo-excited electrons and thereby also con-
tribute to negative photoconductivity.
2.7. Radiation effects
The low power consumption of InAs HEMTs makes
them candidates for applications in space where toler-
ance to radiation effects is clearly an important issue. Pre-
liminary measurements of laser-induced single-event effect
(SEE) characteristics have been performed [69]. The results
indicate SEE performance will be similar to that observed
previously for InP HEMTs and GaAs FETs under similar
excitation conditions. A recent study used proton bom-
bardment to compare the radiation-induced change in
drain current for several HEMT material systems [70].
InAs HEMTs were not included in the study, but the con-
duction band offset between the channel and barrier mate-
rials was found to be a key parameter, with larger offsets
resulting in smaller changes in drain current. Hence, the
1.35 eV conduction band offset in InAs/AlSb HEMTs
should result in excellent tolerance to radiation. Recent
experimental results confirm this, with the radiation-
induced decrease in HEMT drain currents a factor of 150
lower than for typical GaAs/AlGaAs HEMTs [71].
2.8. Circuits
The first InAs HEMT circuits were reported in 2003–
2005. HRL reported an 18-transistor inverter-buffer [39].
The circuit was not designed for depletion-mode devices
but the DC transfer curves exhibit the correct functionality.
Using a conservative design, NGST/NRL demonstrated an
X-band low-noise amplifier (LNA) with greater than 7 dB/
stage peak gain between 12 and 14 GHz and 6 mW/stage
DC power dissipation [37]. Rockwell fabricated low-power
Ka-band LNAs with a noise figure of 1.46 dB at 35 GHz
(see Fig. 11) and an associated gain of 22 dB.[72] Most
Fig. 11. Noise figure, F
min
, as a function of frequency for InAs-channel
HEMTs [37,61–63,72].
1882 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

recently, a W-band LNA was reported by NGST/NRL.
The 3-stage amplifier has a noise figure of 5.4 dB with an
associated gain of 11.1 dB at a total chip dissipation of
only 1.8 mW at 94 GHz [63]. The dc power consumption
for both the Ka and W-band LNAs was less than one-tenth
(one-third) the power for comparable GaAs(InP)-based
circuits.
2.9. InSb-channel HEMTs
InSb has a room-temperature electron mobility of
78,000 cm
2
/V s, the highest of any semiconductor. In addi-
tion, its electron saturation velocity is reported to be
greater than 5 ·10
7
cm/s, compared to 4 ·10
7
and
1·10
7
cm/s for InAs and GaAs, respectively [73]. In prin-
ciple, transistors with InSb channels could reach higher fre-
quencies than the InAs-channel HEMTs. Quantum wells
can be formed with InSb wells and InAlSb barriers.
Room-temperature mobilities as high as 40,000 cm
2
/V s
have been achieved [74]. The mobility decreases with
increasing carrier density, resulting in sheet resistances
greater than 200 X/h. The high mobility of InSb should
enable low-voltage operation. The high intrinsic carrier
concentration (n
i
10
16
/cm
3
) and thermal generation rate
result in high gate leakage currents. This problem has been
mitigated by the use of a carrier extraction technique
invented at QinetiQ Corporation [73]. The basic device
structure is shown in Fig. 12. The contact to the p
+
-InAlSb
layer allows the minority carrier density to be reduced by
many orders of magnitude. For a gate length of 0.2 lm,
an f
T
of 150 GHz was achieved at V
DS
= 0.5 V [75].
3. Resonant tunneling diodes
3.1. Background
Resonant tunneling diodes (RTDs) were first fabricated
in the GaAs- and InP-based material systems and later in
the antimonide system, similar to the situation for HEMTs
[76]. In 1988, Luo et al. fabricated RTDs with InAs contact
layers, AlSb barriers, and an InAs well [77]. The band dia-
gram is shown in Fig. 13a. Electrons tunnel via conduction
band states in the well in this type-I structure. The follow-
ing year, So
¨derstro
¨m et al. reported on the first resonant
interband tunneling diode (RITD) with the InAs well in
the RTD structure replaced by GaSb [78]. As shown in
Fig. 13b, the band alignment is type II, with the valence
band edge of the GaSb above the conduction band of the
InAs. Interband tunneling through the structure occurs
when electrons from the InAs conduction band tunnel
through the AlSb barrier, into the GaSb valence band,
through the second barrier, and into the InAs conduction
band. To minimize power dissipation in circuits using res-
onant tunneling devices it is desirable to have the current
peak occur at a low voltage. SchulmanÕs calculations
shown in Fig. 14 [79] illustrate the difference between the
peak positions of Sb-based RITDs and RTDs. Note that
the current peaks at a bias near 100 mV (defined as V
p
)
for the RITD compared to 200 mV for the RTD. The liter-
ature for Sb-based RITDs shows a wide range of V
p
values
with some as low as 100 mV. The Sb-based RITD peak
voltages are also much smaller than the bias voltages of
1–1.5 V that are generally required to reach I
p
in GaAs-
and InP-based RTDs with the smallest being near 0.4 V
[80]. While the RTD and RITD structures in Fig. 13 have
semi-insulating GaAs substrate
InSb well
InAlSb buffer
Source Gate Drain
Extraction (optional)
p+ InAlSb buffer
InAlSb barrier
InAlSb barrier Modulation doping (optional)
Doped channel (optional)
Fig. 12. Cross-section of InSb-channel HEMT with additional contact for carrier extraction [75].
InAs InAs
AlSb AlSb
ER
tt
InAs
(a)
InAs InAs
AlSb AlSb
GaSb
ER
tt
(b)
Fig. 13. Conceptual band structure for antimonide-based (a) resonant
tunneling diode (RTD) and (b) resonant interband tunneling diode
(RITD).
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1883

been the most often studied structures, the unusual align-
ment of the valence and conduction bands and the large
differences in the bandgaps allow a wide variety of struc-
tures to be made. Many of them have been reported in
the literature [81–91].
3.2. Performance: peak current density and peak-to-valley
ratio
We show typical I–Vdata from one of our RITDs in
Fig. 15. The structure was grown on an InAs substrate
and was symmetric, consisting of a 1.0 lm InAs (n3·
10
18
/cm
3
) buffer layer, 300 A
˚InAs (n1·10
17
/cm
3
),
120 A
˚undoped InAs, 15 A
˚AlSb, 80 A
˚GaSb, 15 A
˚AlSb,
120 A
˚undoped InAs, 300 A
˚InAs (n1·10
17
/cm
3
), and
0.2 lm InAs (n3·10
18
/cm
3
). The data shown here are
for a 3-lm-diameter circular mesa. For positive biases,
the figures of merit are: peak current density, I
p
= 1.2 ·
10
4
A/cm
2
and peak-to-valley ratio (PVR) = 16. For nega-
tive biases, I
p
=1.3 ·10
4
A/cm
2
and PVR = 15. Large
values of I
p
are required for high-speed operation in order
to rapidly charge and discharge internal and circuit capa-
citances [92,93]. The low valley currents are also desirable
for low power dissipation. The peak and valley currents
in Fig. 15 are found at the small biases of 120 mV and
300 mV, respectively. This is an important feature of this
material system as these low voltages offer the possibility
of building low-power-dissipation electronic devices. We
have measured devices with peak currents at voltages as
low as 65 mV.
Calculating the tunneling current in the InAs/AlSb/
GaSb system is very difficult as it requires using at least 8
bands to calculate the wave functions in the various layers
for many different values of the in-plane and out-of-plane
momentum for each bias. The solution is further compli-
cated by the need to satisfy both the Schro
¨dinger and Pois-
son equations. In addition to ideal elastic tunneling, the
current is determined by a number of phenomena such as
phonon interactions [94], interface roughness scattering
[95], and impurity scattering [96]. Most of these interac-
tions have been studied in detail in the GaAs/AlGaAs sys-
tem but not the interband system discussed here.
A starting point for an investigation of resonant inter-
band tunneling is determining the dependence of I
p
and
the PVR on the barrier thickness. An exponential depen-
dence of I
p
on the AlSb barrier thickness is anticipated
because the transmission probability for an electron to
cross a barrier increases exponentially with decreasing bar-
rier thickness. In Fig. 16a, we plot I
p
versus barrier thick-
ness for a set of six samples we grew under nominally
identical conditions. Data from other groups is also shown
and is in good agreement with our results. The exception is
the data point from the first reported RITD; presumably
growth procedures were not yet optimized. Fig. 16a indi-
cates that the current increases almost exponentially with
decreasing AlSb barrier thickness for thicknesses greater
than 10 A
˚, and somewhat slower for smaller thicknesses.
Kitabayashi et al. have reported on the dependence of
the tunneling current on the width of the GaSb well and
AlSb barrier for barrier widths down to zero [97]. They
analyze the difference between the peak current and the val-
ley current and explain their observations in terms of the
location of the resonance level in the GaSb well relative
to the conduction band edge in the InAs, and the width
of the resonance. Their qualitative discussion is based on
several assumptions including one that the light-hole band
is the dominant band in the GaSb well. They assume that
the resonance level moves towards the GaSb valence band
edge as the well widens, and that it depends on the barrier
thickness. For thin barriers there is a large overlap between
the InAs conduction band and GaSb valence band states
leading to a smaller effective mass for thin barriers com-
pared to thick barriers. The effective mass contributes to
determining the location of the resonance level in the well.
The dependence of the PVR on the barrier thickness has
also been examined. In Fig. 16b we show the PVR values
corresponding to the data in Fig. 16a and observe a maxi-
mum of 20 at an AlSb barrier thickness of 21 A
˚. There are
many possible contributing factors in determining the
Fig. 14. Calculated I–Vcharacteristics for antimonide-based RTDs and
RITDs. Note the small bias voltages [79].
Fig. 15. Current–voltage data for an NRL InAs/AlSb/GaSb/AlSb/InAs
RITD with 15 A
˚barriers.
1884 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

valley current, and the difference in the thickness dependen-
cies of I
p
and PVR suggests that different effects dominate
at different thickness ranges. Shen measured the tempera-
ture dependence of the valley current at several different
voltages and proposed that field-assisted thermionic hole
emission dominates at room temperature and above, and
that Fowler-Nordheim hole tunneling is dominant at low
temperature and high fields [98]. More recently, Xu et al.
examined the temperature dependence for several devices
with barriers from 6 to 20 A
˚thick [99]. The valley current
increased exponentially in all cases but the increase was fas-
ter for the thicker barriers. This led to the conclusion that
the predominance of thermal emission over hole tunneling
increases with increasing barrier thickness. Tehrani et al.
reported some success in reducing valley currents by add-
ing a monolayer of AlAs to the barriers to increase the bar-
rier height [100]. This has limited usefulness as it also
reduces the peak current and there is a limit to how thin
the barrier can be.
Defects have also been shown to add to the valley cur-
rent. Shiralagi et al. noted this in a study of the thickness
of the InAs buffer needed when using a GaAs substrate
to obtain the same PVR as found for an InAs substrate
[101]. Magno et al. reported observing features in AFM
measurements on RITDs with small PVR that were not
present in the topography of diodes with larger PVR
[102]. These features are believed to be associated with
defects that propagate from the substrate through the
RITD layers. RITDs have been exposed to 2 MeV protons
in order to study the incremental changes in the peak and
valley currents due to defect-assisted processes. Diodes
with a thick, 13 ML barrier were more sensitive than those
with 5 ML barriers. In both cases the valley current
increased faster than the peak current [103].
Ternary alloys have been used in variations of the stan-
dard RITD structure shown in Fig. 13 to increase the peak
current and PVR. Schulman et al. modified the RITD
structure by replacing the GaSb well with GaAs
x
Sb
1x
[104]. The strain should move the heavy-hole band to an
energy below the light-hole band. This is expected to
reduce the high-bias valley current through the heavy-hole
resonance resulting in an improvement in the PVR. Results
for several samples with values of xfrom 0 to 0.3 revealed a
maximum PVR = 11 at x= 0.1 compared to PVR = 8 at
x= 0 and PVR = 3 for larger x. Unfortunately, the peak
current dropped significantly from a high at x=0. At
NRL, we replaced AlSb barriers with AlGaSb and InAlSb
in RITD structures. The smaller bandgaps resulted in
increases in I
p
by as much as a factor of three [105]. PVR
values generally decreased for the alloy barriers but
remained above 10. An additional advantage of these
alloys is that their oxidation rates are substantially lower
than pure AlSb which should improve device reliability
[38]. The peak current density and PVR for InAs/AlSb
RTDs (see Fig. 13a) from several groups are shown in
Fig. 17. Compared to the RITDs, higher peak currents
have been achieved for antimonide RTDs, with several
reports of I
p
greater than 1 ·10
5
A/cm
2
[82,106,107]. The
PVRs for the RTDs are smaller than those for the RITDs,
and the PVR for the RTD data of So
¨derstro
¨m-1990
increases monotonically with thickness unlike the RITD
that has a maximum [108]. When an RITD is biased
beyond resonance, the electrons must tunnel through the
bandgap of the GaSb well in addition to the AlSb barriers.
Hence, tunneling contributions to valley currents are min-
imized, resulting in higher PVRs for RITDs [78].
The values of I
p
and PVR for Sb-based RTDs and
RITDs are clearly adequate for many potential applica-
tions. There have been reports of RTDs using InGaAs
alloys for the electrodes and wells along with strained AlAs
barriers or unstrained InAlAsSb barriers on InP substrates.
PVRs near 50:1 were obtained but with relatively low peak
currents of 5.8 ·10
3
A/cm
2
[109,110]. The peak voltages
reported for both these cases were over 750 mV. An
Fig. 16. Peak current density (a) and peak-to-valley ratio (b) as a function
of AlSb barrier thickness for symmetric antimonide-based RITDs
reported by several groups [78,97,101,102,104,112].
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1885

I
p
= 4.1 ·10
5
A/cm
2
has been reported for InP-based RTD
with a strained In
0.8
Ga
0.2
As well and strained AlAs barri-
ers. The PVR = 3.8 was reported along with a peak voltage
of 680 mV [80].
3.3. Bias voltages and hysteresis
Some care needs to be used in comparing the peak volt-
ages reported in the literature, as all devices will contain
some series resistance due to the geometry and quality of
the ohmic contacts. The peaks may also be shifted by a
voltage drop across depletion regions in the electrodes
adjacent to the barriers. The size of the variations in V
p
that can result from series resistance effects are illustrated
in Fig. 18 [111] by the I–Vcharacteristics for RITDs on
the same wafer with mesa diameters of 3 lmand95lm.
Here the small device has V
p
90 mV while it is shifted
to 400 mV for the large device. The peak shift can be
understood by recognizing that the applied bias voltage is
given by the sum of the voltage drop across the RITD
and that across the series resistance. The series resistance
is not expected to scale with diode area as there are many
contributing factors in addition to mesa diameter. For
the 3 lm device, the intrinsic RITD resistance dominates,
but for the 95 lm device the series resistance is significant,
causing an increase in V
p
. This data also illustrates the
large hysteresis in the negative resistance region that can
result from parasitic series resistance. We have consistently
obtained values of V
p
near 100 mV for RITDs by using
small-diameter mesas with short paths through the InAs
to the ohmic contacts as well as short wires to the electron-
ics. Other groups have also reported values near 100 mV
[78,97,112]. For Sb-RTDs, values of V
p
as low as
200 mV have been reported [113] while higher values
are also found in the literature, presumably due to series
resistance effects.
The hysteresis exhibited by the RITD in Fig. 18bis
believed to be largely an extrinsic effect since it disappears
for the small devices on the same wafer. Hysteresis and
peak shifts may be due in part to the storage of charge in
the well which is an intrinsic phenomena. Charge stor-
age may also lead to bistabilities that would result in
Fig. 18. I–Vcharacteristics of RITD devices: (a) 3-lm-diameter mesa and
(b) 95-lm-diameter mesa. The solid and dashed lines in (b) represent
voltage sweeps of different directions, as indicated by arrows [111].
IEEE.
Fig. 17. Peak current density (a) and peak-to-valley ratio (b) as a function
of AlSb barrier thickness for symmetric antimonide-based RTDs reported
by several groups [77,106–108,113].
1886 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

high-frequency oscillations. Chow and Schulman [114]
found that RTDs with AlGaSb barriers and InAs wells
and contact layers exhibit an intrinsic bistability. Jimenez
et al. also demonstrated intrinsic bistability for RTDs with
AlSb barriers and GaSb wells and contact layers [115].
Diode bistability could be useful in the design of high-fre-
quency oscillators and bistable logic circuits.
3.4. Substrates, defects, and interfaces
As discussed in Section 2, there are no suitable semi-
insulating substrates with a lattice constant near 6.1 A
˚.
Discrete RTDs and RITDs can be grown on conducting
InAs or GaSb substrates, but circuit applications will gen-
erally require SI-GaAs or -InP substrates or wafer transfer
techniques. Brown et al. compared InAs/AlSb RTDs
grown on GaAs and InAs substrates and found little differ-
ence in the PVR values, despite the high density of disloca-
tions in the GaAs sample [116]. For RITDs, we have
generally found that some of the diodes on GaAs sub-
strates have PVR values equal to devices on InAs sub-
strates, but the uniformity is not as good. AFM imaging
reveals that devices which contain a defect have substan-
tially lower PVR values [102]. Shiralagi et al. looked at
RITD performance on GaAs substrates as a function of
the InAs buffer layer thickness. PVR values increased as
the InAs thickness increased, reaching values of 14 for
1lm buffers, compared to 16 for InAs substrates [101].
If antimonide R(I)TDs are to be used in circuit applica-
tions, the uniformity and reproducibility of I
p
will be
important. State-of-the-art production MBE systems can
achieve thickness uniformities of 1% across large-diame-
ter substrates. Hence, we do not expect significant variation
in I
p
as a result of thickness variation across wafers. The
presence of defects, especially in the case of lattice-mis-
matched growth, could be an issue. We have found that
defects primarily influence the PVR rather than I
p
. In most
applications, PVR uniformity will be less important than I
p
uniformity. In one case we found that the PVRs across an
RITD wafer varied from 18 (no defect) to 9 (defect present)
[102]. If, for example, the minimum PVR required for a cir-
cuit was 5, then this defect-related non-uniformity would
be acceptable. A more serious issue may be run-to-run
reproducibility. As mentioned above, I
p
is a strong func-
tion of AlSb barrier thickness. To obtain high values of
I
p
(>1 ·10
4
A/cm
2
), AlSb barriers less than 15 A
˚thick
are required for RITDs. Hence, variations of 1 A
˚will cause
significant changes in I
p
(see Fig. 16a), and the quality of
the interfaces is expected to play an important role.
We grew sets of RITDs in which the growth temperature
was varied from 350 to 500 C with other parameters fixed.
Remarkably, NDR behavior was observed across this
entire range, with values of I
p
and I
v
varying by less than
20% (for example, the data in Fig. 15 are from a device
grown at 350 C). This suggests that precise control of
growth temperature will not be necessary for good repro-
ducibility. The variations that we did observe can be
explained by smoother interfaces at higher growth temper-
atures, resulting in barriers that are effectively thicker,
yielding lower I
p
values [102].
As discussed in Section 2, either InSb- or AlAs-like
bonds can form the interface between InAs and AlSb.
Unlike the situation with the HEMTs, however, RITDs
can be produced using either bond type [100,101]. In fact,
PVR values as high as 30:1 for negative biases were
achieved using AlAs interfaces [100]. (These structures were
intentionally asymmetric, with much larger PVR but lower
I
p
for negative biases than for positive bias. Hence, we do
not include them in Fig. 16.) Using STM, we found that a
quarter monolayer of roughness is introduced at the InAs/
AlSb interface due to a change in surface reconstructions
when growing InSb interface bonds. This roughness can
be eliminated by intentionally depositing 1.25 ML In
(rather than 1.0 ML) at the interface [117]. We grew RITDs
in which only the InSb-like interface was varied (1.0 and
1.25 ML In). The room-temperature I–Vcharacteristics
were very similar. These samples were also characterized
by Shubnikov-de Haas oscillations at 1.7 K. The sample
with 1.25 ML In showed better-defined oscillations, con-
firming that the interface was indeed smoother [118].
Some potential applications for antimonide R(I)TDs
require very high densities of devices and hence small-diam-
eter diodes. Nomoto et al. fabricated Sb-RTDs with diame-
ters ranging from 100 lmto20nm[119,120]. They observed
a reduction in PVR for devices smaller than 1 lm and attrib-
uted it to a contribution of the surface current to the valley
current. Even at 20 nm, NDR behavior was observed. In
contrast, the contact layers are pinched off by surface deple-
tion in GaAs RTDs for diameters less than 100 nm [121].In
related work, Shiralagi et al. used epitaxial regrowth on
patterned substrates to define sub-micron Sb-RITDs with-
out using any fine-line lithography [122].
3.5. Radiation effects
As mentioned in Section 2, radiation tolerance may be
important for Sb-based devices. We reported preliminary
measurements for InAs/AlSb/GaSb RITDs [103]. The
diodes were able to sustain significant proton irradiation
before the I–Vcharacteristics deteriorated. As mentioned
earlier, diodes with 5 ML barriers were less sensitive than
ones with 13 ML barriers. This is fortunate because the
thinner barriers will be required to achieve the large current
densities needed for high-frequency operation. Two differ-
ences have been noted in the response of an InP-based
RTD and an RITD to radiation. The peak current for
the InP-based diode decreased with fluence while it
increased for the RITD, and an InP-based RTD was more
sensitive than the 5-ML-barrier RITD. Additional work is
necessary to fully understand these differences. We have
also investigated laser-induced single-event effect character-
istics [111]. The measurements reveal complex behavior
depending on whether the device is biased at a voltage
below or above the voltage of the current peak. The
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1887

response also occurs with two decay times: a nanosecond
and 100 picoseconds for an RITD with a peak current den-
sity of 10
3
A/cm
2
. A qualitative description based on the
different time scales for the decay of electrons and holes
stored in the well has been proposed.
3.6. Applications
Several applications for Sb-R(I)TDs have been pro-
posed and demonstrated. Chow and Williamson et al. com-
bined RITDs and Schottky diodes to produce a complete
set of logic gates with a maximum operating frequency
greater than 12 GHz [112,123]. Shen et al. used RITDs to
fabricate static random-access memories and demonstrate
bistability and switching [124]. They combined RITDs with
InGaAs-channel FETs to demonstrate an exclusive-NOR
logic gate [125]. Fay et al. also combined Sb-RITDs with
an InGaAs-channel FET and demonstrated an integrated
circuit based upon the monostable/bistable logic element
(MOBILE) [126]. At NRL, we integrated Sb-RITDs with
Sb-HEMTs, as shown in Fig. 19. Both the transistor and
diode exhibited good performance (HEMT f
T
= 220 GHz;
RITD I
p
= 1.4 ·10
4
A/cm
2
and PVR = 11:1) [127].Cir-
cuits have not been fabricated but modeling suggests that
MOBILE circuits with very low power dissipation should
be feasible [128]. As discussed in Section 2, Sb-HEMTs
exhibit lower power dissipation than InP-based HEMTs,
due to operation at lower bias. We have also combined
RITDs with giant magnetoresistance elements and demon-
strated monostable–bistable transition logic elements [129].
The fast response times were discussed in Section 3.5. Since
RITDs with current densities near 10
5
A/cm
2
have been
reported, and the frequency scales with current density,
RITDs can be expected to operate at frequencies above
100 GHz. In a landmark study, Brown et al. reported an
InAs/AlSb RTD operating at 712 GHz [130].
The final application we will discuss in this section is the
mm-wave backward diode. The basic structure consists of a
single AlSb barrier, separating n-InAs and p-GaSb, as
shown in Fig. 20a. Luo et al. [131] and Chen et al. [132]
demonstrated negative resistance from this structure.
Schulman and co-workers applied these diodes to mm-
wave detection, translating low-level rf power into dc
1000 Å n+InAs
300 Å n-InAs
120 Å InAs
15 Å AlSb barrier
80 Å GaSb well
15 Å AlSb barrier
120 Å InAs
300 Å n-InAs
1000 Å n+InAs
Etch Stop
20-50 Å InAs
40 Å In0.4Al0.6As
12 Å AlSb
12 Å n+InAs(Si) doping
125 Å AlSb
100 Å InAs channel
30 Å AlSb
42 Å InAs sub-channel
500 Å AlSb
100 Å p+GaSb(Si)
2 m AlSb buffer
SI GaAs (001) substrate
Source
Gate
Drain
Fig. 19. Cross-section for antimonide-based RITD and HEMT grown in a single heterostructure at NRL [127].2000, AVS The Science and Technology
Society.
Fig. 20. Band diagram and I–Vcharacteristics from HRL backward-
diode for mm-wave detection [133,134].IEEE.
1888 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

voltage or current with extreme linearity and low noise
[133–135].InFig. 20b, we show their I–Vcharacteristic
for a 1.5 ·1.5 lm
2
detector. The large curvature at zero
bias translates into good responsivity at a frequency of
95 GHz. Unlike other Sb-based devices discussed in this
review, the mm-wave detector is already used in a commer-
cial product.
4. Heterojunction bipolar transistors
4.1. Background and early work
The design and growth of an HBT is generally more
challenging than a HEMT or R(I)TD because of the need
to obtain the appropriate conduction- and valence-band
offsets at the emitter–base and base–collector heterojunc-
tions. High current gains require low carrier recombination
rates in the base and at the emitter–base junction, thus low
defect concentrations are desirable. High mobilities are
desirable to minimize the transit time across the base and
collector depletion regions, enabling high-frequency opera-
tion. High mobilities and low resistance ohmic contacts
also help to minimize parasitic resistances that slow devices
through large RC time constants. High-frequency opera-
tion requires large collector currents. Hence, power dissipa-
tion becomes a problem that can be handled by using
high-thermal-conductivity materials and devices that oper-
ate at low voltage. To attain high performance requires
making wise choices particularly in the choice of materials
given that optimum device geometry may be obtained in
any material system. These facts, combined with the rela-
tive immaturity of the antimonide-based materials systems,
resulted in relatively little work on antimonide-based/nar-
row-bandgap HBTs until recently. This system has advan-
tages in that materials such as InAs and InGaSb have
narrow bandgaps and high electron and hole mobilities
that make them attractive for use as the base. The small
bandgaps should lead to low power dissipation through
use of small emitter–base and emitter–collector voltages,
and the high mobilities should lead to high-frequency oper-
ation. The choice of semiconductors for the emitter and
collector with suitable bandgaps and band offsets becomes
a major consideration. They should be closely lattice
matched to the base, and have a large valence band offset
relative to the base. While there is data in the literature
on the bandgaps and band offsets of some of the relevant
materials there are many uncertainties in the details. There
are a number of publications listing interpolation routines
and bowing parameter data that are useful as a starting
point. The lack of a commercial semi-insulating substrate
with a lattice constant near that of InAs and GaSb is
another difficulty.
Vergurlekar et al. published their results on the develop-
ment of a hot-electron AlSb/InAs bipolar transistor in
1990. They showed that the large conduction band discon-
tinuity between InAs and AlSb resulted in impact ioniza-
tion when electrons were injected into a p-InAs base
across the AlSb/InAs heterojunction [136]. Two years later,
Pekarik et al. demonstrated the first AlSb–InAs–AlSb pnp
HBT. The devices exhibited current gain but were limited
by extremely large base currents, apparently due to exten-
sive interface recombination [137]. Dodd et al. fabricated
npn InAs bipolar transistors on InP in an attempt to
achieve pseudo-HBT performance due to bandgap narrow-
ing. Current gains of 30 were achieved at room temperature
but junction leakage currents were very high. The dc per-
formance was improved when the structure was grown
on an InAs substrate [138]. Transistors grown on a conduc-
tive substrate, however, cannot be used at microwave fre-
quencies. For this reason, Moran et al. used wafer
bonding to transfer an antimonide-based npn HBT from
a GaSb substrate to an insulating sapphire substrate
[139]. The transistors exhibited a dc current gain of 5.
4.2. HBTs with InAs and InGaAs bases
One approach to narrow-gap HBTs is to use InAs as the
base with alloys as the emitter and/or collector. Averett
and co-workers fabricated npn HBTs with an InAs base
and collector and InAlAs or InAsP emitters [140–142].
Thomas et al. at HRL used InAsP as both the emitter
and collector [143].InFig. 21, we plot the collector current
density, J
C
, as a function of the emitter–base voltage, V
BE
,
for several materials systems [144,145]. As expected, as the
bandgap of the base decreases, the emitter–base voltage for
a given collector current also decreases. The HRL devices
demonstrated low-power operation, with gain for V
BE
between 0.1 and 0.3 V. When transferred to a sapphire sub-
strate, the HBTs exhibited excellent microwave perfor-
mance with an f
T
of 181 GHz. Divide-by-16 circuits
consisting of 62 transistors were demonstrated and exhib-
ited a maximum operating frequency of 10 GHz [143].
Conventional HBTs lattice-matched to SI-InP sub-
strates use In
0.53
Ga
0.47
As as the base with either InP or
In
0.52
Al
0.48
As for the emitter and InP or In
0.53
Ga
0.47
As as
the collector. A narrow-gap alternative is to use InAlAs
0.2 0.4 0.6 0.8
VBE (V)
10-1
100
101
102
103
104
105
InAsP/InAs/
InAsP (HRL)
InAlAsP/InGaAs/
InAlAs (NGST)
SiGe
InP
InAlAsSb/InGaSb/
InAlAsSb, 6.2 Å
simulation (NRL)
InAlAsSb/InGaSb/
InAlAsSb, 6.3 Å
simulation (NRL)
Current Density (A/cm2)
Fig. 21. Collector current density versus base-emitter voltage for npn
HBTs [143–146,149].
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1889

and InGaAs with indium concentrations greater than 80%
in all three layers. Monier et al. at NGST demonstrated
npn HBTs using an In
0.86
Al
0.14
As/In
0.86
Ga
0.14
As/In
0.86
A-
l
0.14
As structure with a 6.0 A
˚lattice constant [146]. The
devices were grown on InP substrates using graded InAlAs
buffer layers [147]. Gain was achieved with V
BE
between 0.2
and 0.4 V, as shown in Fig. 21. Microwave measurements
yielded a peak cut-off frequency of 170 GHz. Initial test cir-
cuits include 28 GHz dividers that dissipate half the power
of InP-based circuits [148].
4.3. InGaSb/InAlAsSb npn HBTs
At NRL, we surveyed materials and band offsets and
selected a material system with a lattice constant near
6.2 A
˚[149]. The willingness to try working with a large lat-
tice mismatch was motivated in part by the success in the
InP community in developing metamorphic growth tech-
niques allowing the use of GaAs substrates in the growth
of HBTs and HEMTs lattice matched to InP. Past success
at NRL in growing InAs/AlSb HEMTs on GaAs also was
considered. For developmental purposes commercial GaSb
substrates were chosen to minimize the lattice mismatch
rather than the preferred semi-insulating GaAs or InP.
In
x
Ga
1x
Sb was chosen for the base because of its small
bandgap and to exploit the good hole transport characteris-
tics of these alloys. In addition, low-resistance ohmic con-
tacts to p-InGaSb have been achieved [150–152]. These
properties are important factors in minimizing the base
resistance. Equally important is the fact that a narrow
bandgap InGaSb base can be used with a collector and an
emitter made from a variety of InAlAsSb alloys. While little
is known about the details of the band offsets and bandgaps
of these materials, extrapolations from known binary and
ternary alloys indicate that it should be possible to have a
large valence band offset of 300 mV or more over a wide
range of InAlAsSb alloys, particularly with alloys having
a 6.2 A
˚lattice constant [3]. This is important in minimizing
the parasitic hole current flow from the InGaSb base to the
emitter. The InAlAsSb alloys are predicted to be direct
bandgap semiconductors over a range of bandgaps from
0.2 to 1.4 eV. This leads to a wide range of possible conduc-
tion band offsets with the InGaSb base because the valence
band offset is almost insensitive to the InAlAsSb composi-
tion. The above listed properties allow a great deal of lati-
tude in using band-structure engineering to optimize HBT
performance. One possible emitter–base–collector structure
with estimated band offsets is illustrated in Fig. 22. The neg-
ative side to using InAlAsSb alloys with the desired compo-
sition is that they are expected to be hard to grow as
controlling the As/Sb composition is difficult, and there
may be miscibility gap problems. Methods for obtaining
n-type doping also are required.
The structure in Fig. 22 was grown on an undoped GaSb
substrate with a 1 lm AlSb buffer layer [153]. The collector
was 1 lm thick with a nominal n=3·10
16
/cm
3
doping; the
base was 0.1 lm thick with a nominal p=5·10
18
/cm
3
doping; the emitter was 0.2 lm thick with a nominal
n=3·10
17
/cm
3
doping. The heterostructure also included
a heavily doped 0.1 lm layer grown between the collector
and the AlSb buffer. This layer has the same composition
as the collector and it was included to improve the electrical
contact to the collector. Heavily doped contact layers have
been used on the top of the emitter to aid in contacting it.
Rather conservative layer designs were used in this struc-
ture. A relatively low base doping was chosen to avoid pos-
sible problems associated with Be segregating into the
emitter. A thick base was used to make it easier to etch
down to it using an iterative etch test procedure. A thick
collector was used to accommodate the lattice mismatch
between the AlSb with a 6.136 A
˚lattice constant and the
6.2 A
˚HBT structure. A variety of emitter shapes were fab-
ricated with relatively large areas of 30–60 lm
2
.
The common emitter curves for the layer structure in
Fig. 22 are shown in Fig. 23. It has a DC current gain of
25 and has been measured to V
CE
= 5 V without damage
to the device. The maximum collector current, I
C
,in
Fig. 23 corresponds to a density of 1.8 ·10
4
A/cm
2
. The
low collector–emitter offset voltage of 220 mV supports
the possibility of low power dissipation. Current–voltage
measurements on the emitter–base and collector-base p-n
junctions indicate that series resistances are spreading the
voltage over which I
C
increases from zero in Fig. 23. Much
higher collector currents densities are expected at lower
base-emitter voltages for the InGaSb devices compared to
Fig. 22. Predicted band diagram for InAlAsSb/InGaSb/InAlAsSb npn
HBT with a
o
= 6.20 A
˚[149].
Fig. 23. I–Vcharacteristics from InAlAsSb/InGaSb/InAlAsSb npn HBT
[153].IEE.
1890 B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895

conventional InP or SiGe HBTs. This is important because
large collector currents are required for high-frequency
operation, and small values of V
BE
indicate that low power
dissipation is possible.
4.4. pnp Structures and InP HBTs with GaAsSb bases
With the exception of the 1992 paper by Pekarik et al.,
all the above work involves npn structures. Recently,
Rockwell demonstrated a pnp structure with an InAs base
and AlGaSb emitter and collector [154]. The bandstructure
is shown in Fig. 24. The device exhibits gain for base-emit-
ter voltages as low as 0.2 V.
Our discussion here has focused on HBTs with very nar-
row bandgaps and large lattice constants (6.0–6.2 A
˚),
regardless of whether the heterostructures included Sb.
There has been an important advance involving the use
of Sb in InP-based HBTs. Specifically, the In
0.53
Ga
0.47
As
base can be replaced with GaAs
0.51
Sb
0.49
to give a type-II
band alignment to reduce collector-current blocking.
Excellent HBT performance has been achieved with these
structures [155–157].
5. Summary and outlook
In this paper, we have reviewed the recent progress on
three antimonide-based electronic devices: HEMTs,
R(I)TDs, and HBTs. For all three devices, the advantage
of Sb-based structures over Si-, GaAs-, or InP-based struc-
tures is the attainment of high-frequency operation with
much lower power consumption. For the HEMT, recent
advances include the demonstration of Ka- and W-band
LNA circuits that operate at less than one-third the power
of similar InP-based circuits. Applications may also include
low-power logic; advantages and obstacles are discussed in
a recent article [158]. Active areas of HEMT research
include reduction of the gate-to-channel spacing for sub-
100-nm gate length and reduction of gate leakage currents.
The R(I)TDs have been heavily studied and considered for
application in MOBILE circuits, but, like their InP- and
GaAs-based counterparts, are waiting for a viable commer-
cial application. Spintronic concepts using RITDs both
with and without ferromagnetic semiconductor materials
have also been reported [159–163]. Several groups have
been investigating HBTs based upon narrow-bandgap
materials in the last three years. Most approaches involve
lattice constants of 6.0–6.1 A
˚with InAs or In
0.86
Ga
0.14
As
as the base and InAs or related alloys such as InAsP or
InAlAs as the emitter and collector. Divider circuits have
been reported. An alternative approach is to work at a
6.2–6.3 A
˚lattice constant using InGaSb as the base and
InAlAsSb alloys as the emitter and collector.
Future systems could also include unconventional
devices that take advantage of the properties of antimo-
nides. A good example of this is the backward diode being
developed for use in THz imaging [133]. THz source con-
cepts using these materials have also been suggested and
need to be examined [164]. The large conduction band off-
sets between AlSb and InAs and the large electron mobility
in InAs make this system useful in the fabrication of one-
dimensional quantum wires. Advances in the lateral pat-
terning by C.H. Yang and colleagues have allowed them
to demonstrate low-temperature electron elastic mean free
paths greater than 1.4 lm and coherence lengths greater
than 3 lm[165]. Such structures could form the basis of
future electronic devices.
Antimonide-based electronics may also benefit from
growth and processing advances resulting from work on
electro-optic devices. The wide variety of bandgaps and off-
sets along with the high mobilities are being exploited to
develop long-wavelength (8–25 lm) superlattice detectors
that are predicted to have superior performance to the
HgCdTe detectors [166]. Lasers emitting in the 2–5 lm
range are also being developed [167].
As we have discussed, there has been considerable pro-
gress in Sb-based electronic devices in the last few years.
The advances are especially significant considering the rel-
atively low level of funding and effort, compared, for exam-
ple, to InP-based electronics in the 1980s. In our view, it is
unrealistic to think that Sb-based electronics will replace a
large fraction of GaAs- or InP-based devices. However,
they are promising for specific applications where both
high-frequency operation and low power consumption
are needed. System applications that have already transi-
tioned from GaAs- to InP-electronics to achieve lower
power consumption may be candidates for transition to
antimonides.
Acknowledgements
The Office of Naval Research supported the develop-
ment of this review as well as much of the NRL research
discussed. The Defense Advanced Research Projects
Agency (ABCS Program) also supported portions of the
research. The authors thank colleagues at NRL including:
Fig. 24. Band diagram for pnp HBT demonstrated by Rockwell [154].
IEEE.
B.R. Bennett et al. / Solid-State Electronics 49 (2005) 1875–1895 1891

R. Bass, A.S. Bracker, P.M. Campbell, J.C. Culbertson,
E.R. Glaser, M. Goldenberg, K.D. Hobart, D. McMor-
row, N. Papanicolaou, D. Park, B.V. Shanabrook, B.P.
Tinkham, R.J. Wagner, B.D. Weaver, and M.J. Yang;
and collaborators at Northrop Grumman Corporation:
W.R. Deal, A. Gutierrez, M.D. Lange, and R. Tsai.
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