Designs for Ultra-Tiny, Special-Purpose Nanoelectronic Circuits

Abstract

Designs and simulation results are given for two small, special-purpose nanoelectronic circuits. The area of special-purpose nanoelectronics has not been given much consideration previously, though much effort has been devoted to the development of general-purpose nanoelectronic systems, i.e., nanocomputers. This paper demonstrates via simulation that the nanodevices and nanofabrication techniques developed recently for general-purpose nanocomputers also might be applied with substantial benefit to implement less complex nanocircuits targeted at specific applications. Nanocircuits considered here are a digital controller for the leg motion on an autonomous millimeter-scale robot and an analog nanocircuit for amplification of signals in a tiny optoelectronic sensor or receiver. Simulations of both nanocircuit designs show significant improvement over microelectronic designs in metrics such as footprint area and power consumption. These improvements are obtained from designs employing nanodevices and nanofabrication techniques that already have been demonstrated experimentally. Thus, the results presented here suggest that such improvements might be realized in the near term for important, special-purpose applications.

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2528 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 54, NO. 11, NOVEMBER 2007
Designs for Ultra-Tiny, Special-Purpose
Nanoelectronic Circuits
Shamik Das, Member, IEEE, Alexander J. Gates, Hassen A. Abdu, Student Member, IEEE,
Garrett S. Rose, Member, IEEE, Carl A. Picconatto, and James C. Ellenbogen, Member, IEEE
Abstract—Designs and simulation results are given for two
small, special-purpose nanoelectronic circuits. The area of
special-purpose nanoelectronics has not been given much con-
sideration previously, though much effort has been devoted to
the development of general-purpose nanoelectronic systems, i.e.,
nanocomputers. This paper demonstrates via simulation that
the nanodevices and nanofabrication techniques developed re-
cently for general-purpose nanocomputers also might be applied
with substantial benefit to implement less complex nanocircuits
targeted at specific applications. Nanocircuits considered here
are a digital controller for the leg motion on an autonomous
millimeter-scale robot and an analog nanocircuit for amplification
of signals in a tiny optoelectronic sensor or receiver. Simulations
of both nanocircuit designs show significant improvement over
microelectronic designs in metrics such as footprint area and
power consumption. These improvements are obtained from de-
signs employing nanodevices and nanofabrication techniques that
already have been demonstrated experimentally. Thus, the results
presented here suggest that such improvements might be realized
in the near term for important, special-purpose applications.
Index Terms—Design methodology, nanocircuit, nanocom-
puting, nano-electronics, nanotechnology, simulation.
I. INTRODUCTION
T
sibility for shrinking drastically the size and power consump-
tionoflarge-scale,general-purposeelectronicmemoriesandpro-
cessors.Theserecentadvancesinnanofabricationandnanoelec-
tronics could have conspicuous, pervasive impacts for general-
purpose computing in several years’ time. However, these ad-
vancesalso make it possibleto shrink theform factor and power
requirementsforawideclassofmuchsimplercircuits.Suchcir-
cuitsoftenarededicatedtospecializedapplicationsinthecontrol
andmonitoringofothersystems.Theyarelessconspicuousthan
the larger general-purpose processors and memories, but even
more pervasive. Moreover, the simplicity of these special-pur-
posenanocircuits issuchthattheymightbe realizedindustrially
in only a few years, and they might find great use immediately
thereafter in shrinking the larger systems in which they are em-
HE great progress toward building electronic circuits inte-
gratedonthenanometerscale[1]–[42]hasopenedthepos-
Manuscript received February 18, 2007; revised July 4, 2007. This work was
supported by the MITRE Technology Program. This paper was recommended
by Guest Editor C. Lau.
S. Das, A. J. Gates, H. A. Abdu, C. A. Picconatto, and J. C. Ellenbogen are
with the Nanosystems Group at The MITRE Corporation, McLean, VA 22102
USA (e-mail: sdas@mitre.org).
G.RoseiswiththeNanosystemsGroupatTheMITRECorporation,McLean,
VA 22102, USA, and also with Polytechnic University, Brooklyn, NY 11201
USA.
Digital Object Identifier 10.1109/TCSI.2007.907864
bedded. Thus, it is the design of these simple, special-purpose
nanoelectronic circuits that we begin to consider here.
Special-purpose nanoelectronic circuits might be employed
in a number of applications. For example, one class of these ap-
plications is control processing. Nanocircuits for digital control
mightbeusedtominiaturizeexistingapplicationsorenablenew
ones, such as autonomous microsensors (“smart dust”) [43],
medical microrobotics [44]–[46], or other micro/nano electro-
mechanical systems. Another class of applications that might
benefit from special-purpose nanoelectronics is communica-
tions. For example, novel, tiny optical or radio data transceivers
might be implemented using analog or mixed-signal nano-
electronic circuits. Also, field-programmable nanocircuits
could be utilized to implement or store codecs for a compact,
flexible software-defined radio system [47], [48]. Finally,
as an example bridging these two classes, radio-frequency
identification (RFID) systems [49] might be miniaturized or
augmented using nanoelectronic information storage, control,
or communications.
To begin to explore the utility of special-purpose nanoelec-
tronic circuits for applications such as these, the authors have
designed and simulated two such circuits for use in two ex-
ample applications. The first circuit we consider is for digital
control of a six-legged, millimeter-scale robot. This nanocircuit
is derived from architectures proposed for large-scale nanopro-
cessors [27], [51]. As presently designed, this special-purpose
nanocircuit will coordinate the motion of the legs of the robot.
However, the circuit design is adaptable or extensible to control
other functions on the robot.
The second nanocircuit design considered here is for an
analog nanoelectronic amplifier for use in an optoelectronic
communications system. This circuit could be used to imple-
ment very-wide-bandwidth optical communications. It also
could be embedded in the individual pixels of an optical sensor
array, enabling “smart pixel” capabilities [52].
The development of ultra-tiny, ultra-dense circuits such as
these will entail overcoming significant challenges. Many of
thesechallengesareinheritedfromthemorecomplexproblemof
developing extended, general-purpose nanoelectronic systems.
Theadvancedtechniquesthathavebeendevisedtoaddressthose
larger challenges likely will be even more effective in over-
coming the lower hurdles presented by simpler, special-purpose
nanoelectronic circuits. Section II discusses the techniques and
devices that have been selected for the designs presented here.
Following this discussion, Sections III and IV detail the de-
sign and the likely performance of the specific special-purpose
digitalandanalognanocircuitsmentionedabove.SectionVpro-
vides our conclusions based upon these design efforts.
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DAS et al.: DESIGNS FOR ULTRA-TINY, SPECIAL-PURPOSE NANOELECTRONIC CIRCUITS2529
II. NANODEVICES AND NANOFABRICATION FOR
SPECIAL-PURPOSE NANOELECTRONIC CIRCUITS
A number of novel nanoelectronic devices have been devel-
oped in the pursuit of extended nanocomputer systems. Such
nanodevices exhibit a wide variety of electronic behaviors.
These include classical behaviors such as Ohmic resistance
at low voltage [8] and rectification [53]–[55]. Less common
behaviors also have been demonstrated, such as negative differ-
ential resistance [13], Coulomb blockade [56], and hysteretic
switching [54].
Furthermore, devices such as nanotransistors [10], [11], [14],
[57] and molecular switches [5]–[8], [12], [58] have been incor-
poratedintoprototypesofsmallcircuits,suchasindividuallogic
gates, as wellas extended systems. In particular, effectiveuse of
such devices has been demonstrated in prototypes of extended
nanomemory systems integrated on the molecular scale [34],
[38], [40], [41]. As a result, methods now exist for fabricating
systems composed of hundreds of thousands of nanodevices.
Successful refinement of these methods should permit the fab-
rication of systems containing the many billions of devices that
will be required in a nanocomputer system. In the interim, the
fabrication of smaller, simpler circuitsconsisting of only tens or
hundreds of devices should be feasible.
In addition to the demonstrated prototype systems cited
above, a large number of proposals have been put forth for
system architectures that would integrate one or more of the
various molecular-scale devices demonstrated to date [24],
[27], [28], [30]–[33], [35]–[37], [39]. All of these proposals
and demonstrations are based upon a nanoelectronic system
architecture termed the crossbar array [25], [29], which calls
for the homogeneous distribution of nanodevices within tiled
arrays of crossed nanowires.
The primary reason for making this design decision is that
the fabrication of arbitrary, heterogeneous extended structures
at the nanoscale remains a significant unsolved problem [59].
At larger length scales with lower densities, this capability is
taken for granted, because at such scales, optical lithography is
capable ofprecise patterning.At themolecularscale, of thesev-
eral methods of integration that have been devised, the majority
produce homogeneous nanowire crossbar arrays [60]–[64].
Just using such simple crossbar structures, however, it should
befeasibletodevelopsmall,special-purposenanoelectroniccir-
cuits. Thus, the circuits presented in the following sections are
suitable for nanofabrication using established methods, such as
nanoimprinting, and using demonstrated nanodevices, such as
semiconducting nanowire transistors. The circuits also are suf-
ficiently simple to avoid many of the challenges [59] faced in
the development of extended nanoelectronic systems. The fol-
lowing sections of this paper will elucidate how nanocircuits
simple enough to ease fabrication also can be sufficiently com-
plex to carry out useful functions.
III. DESIGN AND ANALYSIS OF A SPECIAL-PURPOSE
DIGITAL NANOCIRCUIT: CONTROL NANOCIRCUIT FOR A
MILLIMETER-SCALE AUTONOMOUS ROBOT
A. Overview
Using the nanowire-based structures discussed in the pre-
vious section, we consider here the design of a portion of a dig-
Fig. 1. Design for a millirobot as originally conceived by Routenberg and El-
lenbogen [50]. The control circuit described in this article is designed to coor-
dinate the leg movements on tiny robots such as this insect-like walking robot.
ital control processor for an autonomous, walking, millimeter-
scale robot. Such a “millirobot” would be the size of a small
insect, and is an archetype for many other small systems that
would incorporate actuation, sensing, and control. In addition,
it could be employed as part of a swarm of millirobots for dis-
tributed computing and sensing.
An understanding of the overall system design options for
millirobots would provide useful guidance for the selection and
designof thenecessarynanocircuits.Presently, thereare several
different designs for millimeter-scale walking robots, including
a version that already has been constructed by researchers at
the University of California at Berkeley [65], [66]. All of these
designs have utilized off-robot power sources and control cir-
cuits, or they have dragged these necessary components behind
the main body, greatly decreasing the mobility and efficiency of
thetinyrobot.However,thedesignofamillirobotwithself-con-
tainedpowerandcontrolsystemshasbeenproposedbyRouten-
berg and Ellenbogen [50].
According to their design, silicon microelectromechanical
systems (MEMS) technology will be utilized to constitute the
millirobot. Thus, the body of this robot will be housed on a
silicon die. As depicted in Fig. 1, six legs will unfold from
the robot. Each will have two degrees of freedom. Prototype
components such as these legs have been developed, and efforts
are underway to prototype the complete mechanical subsystem
[67].
Walking will be accomplished using the tripod gait [68], [69]
employed by insects that are the same size as that projected for
themillirobot.Thetripodgaitrequiresthesixlegsoftherobotto
be split into two groups or tripods. Each tripod includes the two
end legs on one side and the center leg on the opposite side, as
represented in Fig. 2. To walk forward, the robot lifts one tripod
and moves it forward while pushing the other tripod backwards
untilitslegsreachtheirrearmostposition.Itisthismotionofthe
twin tripods that must be generated and modulated by a control
circuit.
Thedesignof suchacontrol circuitis governedbythreemain
constraints. First, the master control circuit and the circuits for
all six legs must occupy an insubstantial portion of the total sur-
face area of the robot. For this application domain, the avail-
able surface area is on the order of 10 mm . Second, the outputs

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2530IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 54, NO. 11, NOVEMBER 2007
Fig. 2. The operation of the millirobot segregates the legs into two “tripods.”
The forward and reverse motion of the legs is depicted as the millirobot takes a
half step from (a) to (b). Specifically, the triangles indicate the tripod that is to
be lifted and moved forward.
of the control circuit must be suitable to drive the MEMS ac-
tuators that couple to the individual legs of the robot. Finally,
because a self-contained millirobot can carry only very small
energy sources, the control subsystem must be designed to con-
sume as little power as possible.
Duetotheseconstraints,acircuitcomposedofnanoelectronic
devices and integrated on the nanometer scale would appear to
be especially suitable. Sections III-B and III-C describe the de-
sign and simulation of a simplified version of such a nanoelec-
tronic control circuit.
B. Design of the Nanoscale Control Circuit
In general, a nanoelectronic circuit that implements special-
purpose functions can be designed as follows. First, an appro-
priatearchitecturemustbeselected.Essentiallyallarchitectures
proposed thus far for nanoelectronics implement programmable
circuit styles such as field-programmable gate arrays [42], [70],
programmable logicarrays (PLAs)[27],or otherreconfigurable
fabrics [31]. These architectures specify how nanotransistors or
other gain-producing nanodevices can be interconnected using
post-fabricationmethodssuchastheprogrammingofmolecular
switches. As is illustrated below in Section III-C, the priorities
assigned to metrics such as power consumption and system size
can be used to guide the selection of a specific nanoelectronic
architecture.
Once an architecture is selected, a special-purpose nanoelec-
tronic circuit can be implemented by designing a logic network
for that circuit using the nanoelectronic components available
in that architecture. For example, in the CMOL architecture
[70], the desired circuit would be implemented with NOR gates,
whereas in the DeHon–Wilson architecture [27], the circuit
would be implemented in sum-of-products form. Both of these
architectures permit the implementation of arbitrary logic.
However, if greater control over the device-level implementa-
tion is required, other architectures, such as the complementary
symmetry array [31], could be used. This architecture provides
more flexibility in the transistor-level interconnection of the
circuit, as might be required for some digital logic styles, as
well as most analog circuits.
The final design step is the mapping of the desired logic cir-
cuit into the chosen architectural fabric. Given the relatively
small size of the special-purpose nanocircuits considered here,
this can be done by hand. However, optimization tools for this
task are under development by other researchers and would be
essential for mapping more extensive nanocircuitry into pro-
grammable hardware [51], [71], [72].
In thepresent example, a digital control nanocircuit for a mil-
lirobot,thechoice of circuitstyle is motivatedbythe constraints
describedinSectionIII-A,aswellasothersystemdesignissues.
For example, the system design of the millirobot under consid-
eration requires that the control circuits drive MEMS actuators.
For any of the legs of the millirobot to move, the electrostatic
comb drive motors that provide the mechanical power for each
of the legs must resonate at a specific frequency (typically in
thelowkilohertzrange).Inparticular,thecontrolcircuitoutputs
should be square waves at that frequency. Additionally, because
the millirobot designers plan for two electrostatic actuators in
a drive train arrangement for each tripod, the control signal to
each leg must be composed of two individual square waves that
are exactly 90 degrees out of phase. Each leg of a tripod set may
be controlled with the same signal, since the three legs move in
unison. However, because the motion of the opposing tripod is
exactly opposite, a second pair of square waves, the inverse of
the first pair, also must be generated.
These control signals could be generated by either an all-dig-
ital nanoelectronic circuit or a mixed-signal nanoelectronic cir-
cuit. Of these options, an all-digital design is desirable for two
reasons. First, the fabrication of a prototype based upon this de-
sign would be eased if the design were all digital rather than
mixed-signal. Mixed-signal design implies a degree of struc-
tural heterogeneity that is not required for a purely digital de-
sign. Second, a complete, practical design would need to pro-
vide many capabilities, such as high-level programmability and
the ability to respond intelligently to environmental data sensed
by the robot. These capabilities are implemented most easily
using digital logic. Although the design presented here is a sim-
plifiedversionintendedasaproofofprinciple,italsoisintended
to be scalable to a complete design. Thus, the design presented
here is a digital implementation.
Fig.3givesaschematicdiagramofasimpleall-digitalcircuit
thatproducesthedesiredcontrolsignals.Thisdesignissplitinto
three major components: two-bit counters, tripod switch, and
motor driver.
These components are driven by a clock signal generated
by an oscillator. This oscillator drives the circuit at four times
the desired resonant frequency. The oscillator could be imple-
mented as a conventional electronic circuit. Alternatively, new,
smaller nano-electromechnical system (NEMS) oscillators
presently under development may provide a smaller replace-
ment for these conventional oscillators [73]. As a third option,
ultra-tiny nanoelectronic oscillator circuits could be designed
to provide the required signals [74].
The output of the oscillator is fed into two divide-by-four cir-
cuits (i.e., two-bit counters) that are used to generate quadra-
ture outputs at the desired frequency. The motor driver is used
to multiplex the correct outputs onto the actuators. The “tripod
switch,” which controls the motor driver, obtains feedback from
the legs: when the forward-moving legs have reached their fur-
thest extent, a voltage pulse is sent to the tripod switch, which
indicates to the motor driver that its outputs should be inverted.
A proposed layout for a nanoelectronic circuit that imple-
mentsthesefunctionsis showninFig.4.Forthisnanoelectronic

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DAS et al.: DESIGNS FOR ULTRA-TINY, SPECIAL-PURPOSE NANOELECTRONIC CIRCUITS2531
Fig. 3. Schematic of a digital control circuit that produces a tripod gait for a
millimeter-scale walking robot.
layout, the nanocomputer architecture of DeHon and Wilson
[27]wasadapted.Thisarchitecturespecifieshownanowiretran-
sistors and diodes, such as those demonstrated by Lieber et al.
[10], [11], [54], [57], might be employed to construct a PLA .
DeHon and Wilson propose the use of extended arrays of tiled
nano-PLAs [25] to constitute a computer system.
Inasimilarmanner,itshouldbepossibletoutilizefewer,sim-
pler PLAs to develop small, special-purpose circuits such as the
nanoelectronic control circuit proposed here. Fig. 4 shows how
suitable nanowires might be arranged in a set of crossbar ar-
rays to construct a set of nano-PLAs. Field-effect transistors are
specified to be placed at the nanowire junctions indicated by
the boxes in the figure. The resulting PLA structure then can
be programmed by the method described by DeHon and Wilson
[27]inordertoproducethedesiredcontrol-circuitfunctionality.
The functionality required for this example is provided by pro-
gramming into the “on” state the diodes marked by black dots
in Fig. 4.
The use of nano-PLAs to synthesize a nanoelectronic con-
trol circuit provides several advantages. For example, the PLA-
basedarchitectureallowsfortheprogrammingofcontrol-circuit
functionality post-fabrication. Thus, the techniques already es-
tablishedforthenanofabricationofPLA-basedgeneral-purpose
nanocomputer systems also might be employed for the fabrica-
tion of the simpler, special-purpose circuits considered here. In
addition, because of the small size of these circuits, established
fabricationtechniques[26]mightbesufficienttoprovideimmu-
nity to the defect rates and process variations that are expected
to plague larger, general-purpose nanoelectronic systems [75].
The use of nano-PLAs also introduces performance trade-
offs that must be considered in the design of the control circuit.
Two logic styles have been proposed by DeHon and Wilson:
static and dynamic [27]. The dynamic logic style is likely to re-
sult in implementations that consume less power since the pro-
posed nanowire-based static logic draws significant static cur-
rent. Thus, the dynamic style would seem to be more desirable
forapplicationinthemillirobot.Unfortunately,dynamicvoltage
signals are not suitable to drive the MEMS actuators in the
proposed robot. These actuators require true square waves at a
fixed frequency. However, dynamic logic outputs would transi-
tion at both the desired output frequencyand the input oscillator
frequency. Thus, these outputs would have to be converted to
static voltage signals that switch only at the desired frequency.
Implementing such a dynamic-to-static logic converter would
be difficult in the array-based nanowire architectures [27], [51]
considered here. An additional drawback is that dynamic cir-
cuits are susceptible to current leakage because dynamic logic
is based upon charge storage. In order to overcome this leakage,
a dynamic implementation would need to be clocked at a much
higher rate than would be required for a static implementation.
This higher clock rate eliminates some power savings.
Thus, a static logic design was developed as shown in Fig. 4.
In the section that follows, the simulation and analysis of this
design is discussed.
C. Simulation and Analysis of the Nanoscale Control Circuit
In order to assess the performance of the nanoelectronic cir-
cuit given in Fig. 4, the circuit was laid out and simulated using
the Cadence DFII software package [76]. Examination of the
layout (provided in Fig. 4) demonstrates the millirobot control
circuit to be very compact. The nanowires in this circuit are as-
sumed to be 10-nm wide with 10-nm spacing. The microwires
are assumed to be those available in a 90-nm silicon process.
Based upon these assumptions, the proposed nanocircuit is only
3.6 m in size. In comparison, a gate-for-gate identical circuit
fabricated entirely using 90-nm standard cells [77] would mea-
sure approximately 92
m in size—roughly 25 times larger.
More importantly, a full microcontroller fabricated in a conven-
tional CMOS process would occupy an area of anywhere from
2.4to400mm ormore[78].Thisconventionalmicrocontroller
would be too large for the millirobot considered here. However,
a full nanocontroller 25 times smaller could fit easily within the
desiredformfactor. Thus,thenanocircuitdesignandsimulation
results presented here strongly support the possibility of minia-
turizing these conventional control circuits down to a size that
would permit their integration into tiny robots.
Simulations of special-purpose nanocircuitry were carried
out using a methodology devised originally for simulating
general-purpose nanomemories and nanoprocessor systems.
This methodology, plus the associated CAD environment and
device models, is described in detail in previous work by the
present authors [54], [59], [79]. In broad outline, four steps
were involved in that approach. First, empirical data were ob-
tained for the desired nanodevices and interconnect structures.
Second, these data were encapsulated into Verilog-A models
[54], [59]. Third, a system-level schematic representing Fig. 4
was assembled within the Cadence Virtuoso environment [76].
Finally, the performance of the circuit was simulated using the
Cadence Spectre simulator [76].
Inthissimulation,afrequencyof10kHzwasassumedforthe
inputoscillator.Thus,2.5-kHzsignalswereexpectedforeachof
thefouroutputs.Thesimulationwasrunfor2ms(i.e.,20cycles

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2532IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 54, NO. 11, NOVEMBER 2007
Fig. 4. Layout of an entire nanowire-based nanoelectronic controller to produce a millirobot’s tripod gait.
of the oscillator). During the simulation run, the tripod switch
input signal was pulsed twice to confirm correct switching of
the output signals.
TheresultsofthesesimulationsareshowninFig.5.Thesere-
sults reveal that the circuit produces the correct quadrature out-
puts. Specifically, output “out1B” lags output “out1A” by 90 ,
and outputs “out2A” and “out2B” are the inverses of outputs
“out1A” and “out1B,” respectively. Also, all the output signals
invert properly when the tripod switch input is pulsed.
The power consumption of the nanocircuit was measured
via simulation to be 1.9 W. This compares poorly with 156.9
nW for the implementation using 90-nm silicon standard
cells [77]. Thus, in contrast with expectations, this design
actually is predicted to consume 12 times as much power as
an equivalent conventional circuit. It was determined that the
primary reason for this unexpectedly high power consumption
is the use of the static version of the DeHon–Wilson archi-
tecture. The static circuit style employed in this architecture
is a “pseudocomplementary” style, in which some pulldown
chains are implemented using p-type transistors. Thus, one
conclusion that might be drawn from these simulation results
is that if static logic is required, nanoarchitectures that em-
ploy true complementary circuit layouts might be preferable.
This complementarity would reduce static power consumption
greatly. An example of a nanoarchitecture that offers this circuit
style is the complementary symmetry array proposed by the
Hewlett-Packard Corporation [51].
Nevertheless, based on the simulation results discussed here,
a nanoelectronic circuit fabricated to the design specifications
given in Fig. 4 is predicted to produce signals of the shape
needed for control of the millirobot. Such a circuit would be
much smaller than could be achieved using conventional silicon
fabrication processes. With further research, it is expected that
this circuit also could be made to consume very little power.
IV. DESIGN AND ANALYSIS OF A SPECIAL-PURPOSE ANALOG
NANOCIRCUIT: TRANSIMPEDANCE AMPLIFIER (TIA) FOR A
NANOSCALE OPTOELECTRONIC RECEIVER
A. Overview
In the previous section, we considered the design of a spe-
cial-purpose digital nanocircuit. In this section, we consider an
analog nanoelectronic circuit for use in optoelectronic applica-
tions.Akeyfunctioninsuchapplicationsistheabilitytoconvert