476 IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 6, JUNE 2007
Oxygen Ion Implantation Isolation Planar Process
for AlGaN/GaN HEMTs
Jin-Yu Shiu, Jui-Chien Huang, Vincent Desmaris, Chia-Ta Chang, Chung-Yu Lu, Kazuhide Kumakura,
Toshiki Makimoto, Herbert Zirath, Member, IEEE, Niklas Rorsman, and Edward Yi Chang, Senior Member, IEEE
Abstract—A multienergy oxygen ion implantation process was
demonstrated to be compatible with the processing of high-
power microwave AlGaN/GaN high electron mobility transistors
(HEMTs). HEMTs that are isolated by this process exhibited
gate-lag- and drain-lag-free operation. A maximum output power
density of 5.3 W/mm at Vgs= −4 V and Vds= 50 V and a
maximum power added efficiency of 51.5% at Vgs= −4 V and
Vds= 30 V at 3 GHz were demonstrated on HEMTs without
field plates on sapphire substrate. This isolation process results in
planar HEMTs, circumventing potential problems with enhanced
gate leakage due to the gate contacting the 2-D electron gas at the
(HEMTs), implantation, power density, pulsed I–V , transient.
Terms—GaN,high electronmobility transistors
etching to define the device active region . Implantation
isolation maintains the planarity of the device, which may in-
crease the yield and uniformity in GaN HEMT and monolithic
microwave integrated circuit (MMIC) processes.
Implantation isolation have been studied in pure GaN or
AlGaN material using H+, He+, N+, F+, Mg+, Ar+, and
Zn+ions –. The O+ion implant isolation was also
investigated on AlGaAs , InAlN , and GaN (n-type
doping)/GaN materials  to study the isolation quality, and
P/He, Ar+, and N+ions have been employed in AlGaN/GaN
In this letter, multiple energy O+ion implantation was
applied for isolation in the fabrication of AlGaN/GaN HEMTs.
EVICE isolation of GaN-based high electron mobility
transistors (HEMTs) is conventionally realized by dry
Manuscript received January 22, 2007; revised April 2, 2007. This work was
supported in part by the Ministry of Education, by the Ministry of Economic
Affairs, and by the National Science Council of China under Contract NSC 94-
2752-E-009-001-PAE and Contract 94-EC-17-A-05-S1-020. The review of this
letter was arranged by Editor J. del Alamo.
J.-Y. Shiu and J.-C. Huang are with the Department of Materials Science
and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan,
R.O.C., and also with Microwave Electronics Laboratory, Microtechnology
and Nanoscience (MC2-MEL), Chalmers University of Technology, 412 96
V. Desmaris, H. Zirath, and N. Rorsman are with the Microwave Elec-
tronics Laboratory, Microtechnology and Nanoscience (MC2-MEL), Chalmers
University of Technology, 412 96 Gothenburg, Sweden.
C.-T. Chang, C.-Y. Lu, and E. Y. Chang are with the Department of
Materials Science and Engineering, National Chiao Tung University, Hsinchu
300, Taiwan, R.O.C. (e-mail: firstname.lastname@example.org).
K. Kumakura and T. Makimoto are with the NTT Basic Research Laborato-
ries, NTT Corporation, Atsugi 243-0198, Japan.
Digital Object Identifier 10.1109/LED.2007.896904
The motivations for this are given as follows: First, O+ion
isolation implantation has better thermal stability compared to
light atomic mass ion (i.e., H+or He+). Second, the lower im-
plantation incident energy decreases the probability of surface
cident energy and higher implantation ion density were used to
ensure high-quality isolation in both AlGaN Schottky and GaN
buffer layers with good thermal stability. The latter motivations
are the biggest differences between the isolation tests and real
HEMT device processes.
Furthermore, in a mesa isolation process, the 2-D electron
gas (2DEG) channel is exposed on the sidewall. The gate
metallization contacts the 2DEG, causing an excessive gate
leakage current and a degradation of the breakdown voltage
of the device . This problem should be absent in an im-
plantation process for isolation. For InGaAs HEMTs, a method
of recessing the channel edge into the mesa sidewall with the
selective etching characteristics of Schottky and buffer layers
was developed . The AlGaN and GaN materials are chem-
ically stable at room temperature, and no convenient selective
wet etch process has been demonstrated.
In this letter, the gate leakages on mesa and implanted
HEMTs are compared. The multiple incident energies and dose
concentration are optimized to assure process compatibility
and minimize surface damage problem, as demonstrated by
pulsed current–voltage (I–V ) and load-pull measurements on
AlGaN/GaN heterostructures were grown on sapphire by
metal organic chemical vapor deposition (MOCVD) by
Hitachi Cable Corporation (material 1) and NTT Basic Re-
search Laboratories (material 2). They consist of a 2-µm-thick
unintentionally doped GaN buffer layer followed by 30 nm of
an undoped Al0.3Ga0.7N Schottky layer. From Hall measure-
ments, the sheet carrier concentration and electron mobility of
materials 1 and 2 were determined to be 1 × 1013cm−2and
900 cm−2/(V · s), and 1 × 1013cm−2and 1100 cm−2/(V · s),
The photoresist S1818 was used as an implantation mask
to define the active region of devices. The HEMT device
experiment consists of the following three pieces: one piece of
material 1 with 2 × 50 × 0.6 µm2gates defined in the middle
of the 4-µm source–drain spacing and two pieces of material 2
with 2 × 50 × 2 µm2gates defined in the middle of the 10-µm
source–drain spacing. The HEMTs were fabricated using an
0741-3106/$25.00 © 2007 IEEE
SHIU et al.: OXYGEN ION IMPLANTATION ISOLATION PLANAR PROCESS FOR AlGaN/GaN HEMTs477
ature. (b) Sheet resistivity versus temperature for BTS measurement at a bias
of 100 V.
(a) Sheet resistivity after 1-h annealing time versus annealing temper-
in-house process described in  with the exception of the
isolation process. The piece of material 1 and one piece of
material 2 were subjected to O+ion implantation with im-
plantation energies of 25, 50, and 75 keV, and the dose is
5 × 1014cm−2for each energy. The TRIM software was used
to simulate the implantation process . On these samples,
the ohmic contacts were formed before the isolation process.
Another sample of material 2 was subjected to a chlorine-based
inductively coupled plasma reactive ion etching mesa isolation
process. The isolation test structure used in this letter consists
of two 100-µm-wide ohmic contact with a separation of 5 µm.
III. RESULTS AND DISCUSSION
The thermal stability of the implantation isolation was inves-
tigated in two ways: First, the sheet resistivity versus annealing
temperature was measured on the isolation test structures after
1 h of annealing [Fig. 1(a)]. The sheet resistivity is higher than
1012Ω/? up to at least 450◦C annealing. The thermal stability
of the O+implantation is appropriate for subsequent HEMT
processes (i.e., CVD SiNxpassivation process at 300◦C) and
normal operation . After 100 h of annealing at 300◦C,
the implanted material sheet resistivity was 4.3 × 1012Ω/?,
time. The sheet resistance of these samples was measured as a
function of temperature, and in all cases, temperature-activated
behavior was obtained. From this, we obtained activation ener-
gies of 0.428, 0.520, 0.115, and 0.030 eV at 300◦C, 450◦C,
600◦C, and 850◦C, respectively (not shown). Second, bias
temperature stress (BTS) measurements were performed on the
isolation test structures at 100 V on the three pieces submitted
to full HEMT processing [Fig. 1(b)]. All samples were held at
each temperature 10 min before the measurement. The O+ion
implantation isolation technique showed higher sheet resistivity
at least up to 290◦C. (The sheet resistivity in a mesa structure is
the resistivity of the undoped GaN buffer.) These results show
the good thermal stability of this implantation technique on
samples submitted to full HEMT processing.
Fig. 2 shows the comparison of gate characteristics of the
HEMTs that are fabricated on material 2 using either implan-
tation or mesa isolation. The measurements were performed at
Vds= 0.1 V, and the gate bias was swept from −100 to +4 V.
oxygen ion implantation and (solid line) mesa isolation devices on material 2.
Comparison of gate characteristics at Vdsof 0.1 V for (dotted line)
from Vpinchto 0 V at Vds= 3 V). (b) Pulsed I–V characteristics from the
different quiescent states (dashed line) (Vgs, Vds) (0 V, 0 V) and (dotted line)
(Vpinch, 20 V) and (solid line) dc of O+-implanted HEMTs.
(a) Transient characteristics of O+-implanted HEMTs (Vgs pulsed
The planar implantation isolated devices have less gate-leakage
current, indicating that the gate may contact the 2DEG at the
sidewall of the mesa and increase the gate-leakage current .
The HEMTs with 2 × 50 × 0.6 µm2gates that are fabricated
on material 1 shows a gate breakdown of 90 V at 1 mA/mm
In order to investigate the trapping of electrons in the
epistructures and surface, the transient and pulsed I–V perfor-
mance of the O+-implanted HEMTs were evaluated using a
pulsed I–V system (Accent DiVA D225) , . Fig. 3(a)
shows the current recovery (IT/Idc) (i.e., stepping the gate
voltage from Vpinchto 0 V at a given constant drain bias and
monitoring the resulting transient drain current IT) measure-
ment at Vds= 3 V. No gate-lag problems can be noticed for
the O+-implanted HEMTs. The low Vds is chosen to avoid
complications associated with self-heating. In the literature, the
presence of traps at the surface and in the AlGaN Schottky
layer is used to explain the gate-lag problem seen in the gate
recovery measurement , . Comparing the dc and pulsed
and 0.1% duty cycle, is an effective way to clarify the gate-lag
and drain-lag effect at the same time , . Fig. 3(b) shows
478 IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 6, JUNE 2007
saturated output power density with a PAE of 51.5%.
Power sweep at Vds= 30 V and Vgs= −4 V, showing 4.5 W/mm
0 V and Vds= 0 V (pulsed 1); and pulsed from Vgs= Vpinch
and Vds= 20 V (mimicking a class B operation, pulsed 2) .
The pulsed 1 measurement shows higher current compared
to dc due to the absence of self-heating effect. The pulsed
2 measurement shows almost no current collapse. The knee
voltage in pulsed 1 and 2 measurements shows a difference
of 2 V. Based on the transient, dc, and pulsed I–V measure-
ments, we conclude that the O+-implanted isolation process
does not introduce trapping problems.
The extrinsic cut-off frequency fT and the maximum os-
cillation frequency fmaxcalculated from scattering parameters
measured up to 50 GHz vector network analyzer were 33 and
57 GHz, respectively. The large signal performance of the
devices was measured using continuous wave (CW) load-pull
measurements at 3 GHz without active cooling. The saturated
output power density is 4.5 W/mm with 51.5% power added
efficiency (PAE) at Vds= 30 V and Vgs= −4 V (Fig. 4). The
highest output power density was 5.3 W/mm at Vds= 50 V and
Vgs= −4 V.
This study has demonstrated a thermally stable electrical iso-
lation for AlGaN/GaN HEMT structure by multienergy implan-
tation of oxygen ions. The ion implantation isolation process is
demonstrated in the full processing of planar HEMTs, demon-
strating full compatibility with HEMT processing.
The planar nature of this isolation process circumvents po-
tential problems with enhanced gate leakage due to the gate
contacting the 2DEG at the mesa sidewall.
The effect of the ion implantation isolation process on
the dynamic performance of the HEMTs was investigated.
The HEMTs demonstrate dispersion-free operation in dynamic
measurements, which is further demonstrated by good CW
output power and efficiency at 3 GHz. A maximum output
power density of 5.3 W/mm and a maximum PAE of 51.5% on
HEMTs grown on sapphire substrate were demonstrated. Due
to its good power density and efficiency without gate-lag and
drain-lag problems, this planar process is a candidate for high-
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