With appropriate device structures, bined lithographi and epitaxial scaling of HBTs,RTDs and S hottky diodes results in rapid in reases in device bandwidths.0.1 ¹m InGaAs RTDs have os illated at 650 GHz,Submi ron heterojunction bipolar sistors (HBTs)fabri ated with substrate transfer pro-esses have obtained 21 dB unilateral power gain at 100 GHz;if extrapolated at -20 dB/decade,this corresponds to a 1.1 THz power-gain uto ®frequency.HBT current-gain uto ®frequencies as high as 300 GHz have been obtained

Betser, Y.

Jaganathan, S.

Long, S.

Martin, S.C.

Mathew, T.

Rodwell, Mark

Scott, D.

Smith, R.P.

Sundararajan, PK

Urteaga, M.

Wei, Y.

English

AMS Acta - Alm@DL - Università di Bologna

1Submicron lateral scaling of HBTs and other
vertical-transport devices: towards THz bandwidths
Mark Rodwell, Y. Betser, S. Jaganathan, T. Mathew, PK Sundararajan, S.C. Martin
R.P. Smith, Y. Wei, M. Urteaga, D. Scott, S. Long
Abstract|With appropriate device structures, com-
bined lithographic and epitaxial scaling of HBTs, RTDs
and Schottky diodes results in rapid increases in de-
vice bandwidths. 0.1 ¹m InGaAs RTDs have oscillated
at 650 GHz, Submicron heterojunction bipolar tran-
sistors (HBTs) fabricated with substrate transfer pro-
cesses have obtained 21 dB unilateral power gain at 100
GHz; if extrapolated at -20 dB/decade, this corresponds
to a 1.1 THz power-gain cuto® frequency. HBT current-
gain cuto® frequencies as high as 300 GHz have been
obtained.
I. Introduction
DEVICE scaling {reduction of layer thicknessesand lithographic feature sizes { is central to high
frequency semiconductor device design. As CMOS
gate lengths have been reduced, clock rates have pro-
gressively increased. 0.13 ¹m Si MOSFETs exhibit
»70 GHz f¿ . Compared to silicon, III-V compound
semiconductors o®er higher electron mobilities, higher
electron saturation drift velocities, and stronger het-
erojunctions, hence III-V semiconductor devices can
obtain higher bandwidths than their silicon counter-
parts. Yet III-V devices must be aggressively scaled
to deep submicron dimensions if they are to remain
competitive with silicon.
III-V heterojunction bipolar transistors (HBTs), in
particular, have not been adequately scaled. GaAs-
and InP- based HBTs are typically fabricated at 1{2
¹m minimum feature sizes and operate at 1{2 £105
A=cm2. This is in marked contrast to both III-V
HEMTs, where 0.1 ¹m gate lengths are typical, and
Si/SiGe HBTs, where emitter widths are 0.1 ¹m [1]
and emitter current densities are as high at 106 A=cm2.
Because of this aggressive scaling, Si/SiGe HBTs are a
major competitive threat to III-V HBTs. Addressing
this, we have developed InAlAs/InGaAs HBTs with
submicron emitter and collector dimensions, and have
obtained 1 THz (extrapolated) power-gain cuto® fre-
quencies (fmax) and 300 GHz current-gain cuto® fre-
quencies (f¿ ). Challenges in HBT submicron scal-
ing include collector de¯nition, emitter parasitic resis-
tance, and support of high collector current densities.
Scaling laws and scaling examples
We will develop HBT scaling laws by ¯rst consider-
ing high frequency diodes. Dominant time constants
Mark Rodwell, Y. Betser, S. Jaganathan, T. Mathew, PK
Sundararajan, Y. Wei, M. Urteaga, D. Scott, and S. Long are
with the Department of Electrical and Computer Engineering,
University of California, Santa Barbara, CA 93106, USA. rod-
wellece.ucsb.edu. S.C. Martin is with Jet Propulsion Labs,
Caltech, Pasadena, CA. R. P. Smith was with Jet Propulsion
Labs, Caltech, Pasadena, CA, and is now with Cree Research,
Durham, NC, USA
D
W
L
Schottky-collector RTD
Transferred-Substrate HBT
base emitter
collector
drain
HEMT (Schottky gate FET)
source gate
Principle of Scaling
Deep Submicron Schottky
C ∝WL /D
R ∝1/ L
C ∝W /D
τ transit ∝D /velectron
→
s
Rs
R = kT /qI
C∝WL/ ID→
j
Rj ∝1 /JD
Rs
Rj
C
2:1 thinner epitaxial layers
4:1 smaller lithographic dimensions
4:1 increased current density
resistanceless (Schottky) top contacts
→2:1 increased device bandwidth
Fig. 1. Scaling laws for high-frequency devices
for Schottky diodes are shown in ¯g. 1. The device has
a junction impedance Rj = kT=qI, a depletion capac-
itance C and a parasitic series resistance Rs arising
from the bulk resistivity of the buried N+ layer and
its Ohmic contacts. Given junction depth D, there is
a transit time ¿transit / D=velectron. Given junction
width W , there is capacitance C /WL=D. Given de-
vice stripe length L, the buried layer & bottom contact
resistance Rs varies as 1=L.
We wish to double the device bandwidth. This re-
quires thinning the depletion layer 2:1 to proportion-
ally reduce ¿transit. In reducing D 2:1 with W left
unchanged, we have doubled the device capacitance,
doubling RsC. Our intent is instead to decrease RsC
by 2:1; to do this we must now decrease the contact
stripe width W by 4:1. With this combined scaling,
C has now been reduced to 1=2 its original value.
If the current I is left unchanged, the time constant
RjC = (kT=qI) ¢C is thus also reduced by the desired
2:1 ratio. Note that the device junction area WL has
been reduced 4:1; the device current density has thus
been increased fourfold.
These are the scaling laws for Schottky mixer diodes.
Each desired 2:1 increase in device bandwidth requires
a 2:1 reduction in epitaxial layer thicknesses, a 4:1 de-
crease in junction width (lithographic scaling), and a
4:1 increase in operating current density.
Exploiting such scaling, Siegel et al have reported
submicron Schottky mixers diodes with low noise at
2.5 THz [3], [4]. Figure 2 shows a 0.1 ¹m £0:5 ¹m T-
gate Schottky diode (with 150 ºA depletion thickness)
used for submillimeter-wave harmonic mixing [2].
Unlike Schottky diodes, PN diodes have both anode
2Fig. 2. Submicron (0:1¹m £0:5¹m) Schottky mixer diode with
150 ºA depletion depth. Calculated RC and transit-time
cuto® frequencies are » 15THz.
(top) and cathode Ohmic contacts. The top contact
has resistance Rtop = ½contact=LW , resulting in a time
constant RtopC / ½contact=D. Since a 2:1 desired im-
provement in device speed required a 2:1 reduction, in
D (for reduced transit time), the required 2:1 reduc-
tion on RtopC requires a 4:1 improvement in ½contact.
Devices with top Ohmic contacts can be very di±cult
to scale: each 2:1 improvement in device speed requires
a 4:1 improvement in contact resistivity.
Lack of minority carrier storage is an often-cited
speed advantage of Schottky diodes over their P-N
counterparts; elimination of the top Ohmic contact is
an advantage of equal signi¯cance. Zero gate contact
resistance is a crucial advantage of the (Schottky-gate)
MESFET / HEMT [5] over the JFET; in the JFET,
gate Ohmic contact resistance increases as the gate
length is reduced.
II. Schottky-collector RTDs
A resonant-tunnel diode (RTD) has an equivalent
circuit similar to that of the Schottky diode shown in
¯g. 1. The device has a negative resistance Rn re-
placing Rj of the Schottky diode, plus series resistance
Rtotal = Rs + Rtop and depletion capacitance C. Rn
varies as 1=Ipeak, the peak tunneling current density.
If the electron storage time in the quantum well ¿qw is
negligible, the device maximum oscillation frequency
is fmax = (1=2¼C)(RnRtotal)
¡1=2; if ¿qw is dominant,
fmax = (1=2¼)(RnRtotal)
¡1=4C¡1=2¿
¡1=2
qw .
RTD scaling follows the rules above. Current den-
sities must be high. Depletion layers must be thin
to avoid high transit times. Lithographic scaling re-
duces the RsC time constant. If the RTD has a top
Ohmic contact, then reduction of RtopC to values nec-
essary for submillimeter-wave oscillation requires very
low contact resistance, and InAs contacts [6] are then
employed. Yet, the RTD top Ohmic contact is unneces-
sary, and can be replaced by a direct Schottky contact
to the fully-depleted electron drift layer, thereby elimi-
nating Rtop. An RTD with a 0.1 ¹m T-gate Schottky
contact, 350 ºA depletion thickness, and 5 ¢ 105 A=cm2
Fig. 3. 0.1 ¹m InGaAs/AlAs Schottky-collector RTD.
Fig. 4. A 64-element, 650 GHz, RTD quasi-optical array oscil-
lator.
current density (¯g. 3) has a estimated (but not mea-
sured) 2 THz fmax. 64-element monolithic arrays of
Schottky-collector RTDs have oscillated at 650 GHz
(¯g 4) [2].
III. HBT scaling
An HBT is a pair of coupled diodes, and follows scal-
ing laws similar to the Schottky diode. A 2:1 desired
improvement in HBT bandwidth requires 2:1 reduction
in base and collector transit times, requiring a
p
2 :1
reduction in the base and a 2:1 reduction in the col-
lector epitaxial layer thicknesses. For a 2:1 improve-
ment of all HBT RC and delay terms, the emitter-
base and collector-base junction widths (lithographic
feature size) must also be reduced 4:1, the collector
current density increased 4:1, and the emitter Ohmic
contact improved 4:1.
A. Transferred-Substrate HBTs
An immediate di±culty is the required reduction
in the collector junction width. In mesa HBTs the
collector-base junction must lie under both the emitter
stripe and under the base Ohmic contacts, which must
3Fig. 5. Schematic cross-section of a transferred-substrate HBT
Fig. 6. E-beam HBT: test structure with 0.15 ¹m emitter-base
junction (a), and 0.4 ¹m Schottky collector stripe (b)
be a minimum width of one contact transfer length,
typically » 0:25 ¹m. A minimum collector-base junc-
tion width is thus de¯ned, and a lower limit is set for
HBT scaling
Using substrate transfer processes [7], HBTs can
be de¯ned with emitter-base and collector-base junc-
tions on opposing sides of the base epitaxial layer (¯g.
5). The minimum collector junction width is now de-
termined by the emitter width alone, and need not
include the widths of the base Ohmic contacts; col-
lector junctions can be scaled to submicron dimen-
sions. Submicron InAlAs/InGaAs devices fabricated
using electron-beam lithography (¯g 5) exhibit 21 dB
unilateral power gain at 100 GHz (¯g. 7). Extrapo-
lation at -20 dB/decade (to be treated with caution)
suggests an 1100 GHz fmax. In addressing limits to
collector scaling, one alternative to substrate transfer
is an undercut collector-base junction [8]. A second
alternative is very high (carbon) base doping within a
normal mesa HBT structure { at » 2 ¢ 1020=cm3 base
doping, the contact transfer length is » 0:1¹m, and
very narrow base mesas can then be employed.
The HBT of ¯g. 7 has narrow emitter and collec-
0
5
10
15
20
25
30
10 100 1000
G
a
in
s
, 
d
B
Frequency,GHz
f
max
=1080 GHz
f
τ
=204 GHz
H
21
Mason's gain, U 
MSG
I
c
= 6 mA, V
ce
= 1.2 V
Fig. 7. Gains of a 0.4 ¹m £ 6 ¹m emitter and 0.7 ¹m £ 10 ¹m
collector HBT fabricated using electron-beam lithography.
Theoretical -20 dB/decade (H21, U ) gain slopes are indi-
cated. The device exhibits an extrapolated 1.08 THz fmax
0
10
20
30
40
50
1 10 10
2
G
a
in
s
  
 (
d
B
)
Frequency   (GHz)
h
21
U
V
CE
 = 1 V,  J
C
 = 1.5 mA/um
2
f
MAX
 = 295 GHz
f 
τ=
 = 295 GHz
Fig. 8. Measured RF gains for an HBT with a 300 ºA base with
52 meV grading and a 2000 ºA collector.
tor junctions but thick 400 ºA base and 3000 ºA col-
lector layers, and hence has high fmax but only 200
GHz f¿ . While such a device will provide high gains
in tuned millimeter-wave ampli¯ers, more general cir-
cuits require simultaneously high values for f¿ and
fmax. Digital circuits, in turn, have gate delays in
which (¿b + ¿c), Cbc¢Vlogic=Ic, RbbCje, RexCje and
RF bb(¿b + ¿c)Ic=¢Vlogic are all signi¯cant terms.
To ensure a 2:1 improvement in all circuits employ-
ing the HBT, all HBT transit times and RC time con-
stants must be reduced 2:1. In addition to lithographic
scaling of emitter and collector junction widths{as
demonstrated in the high-fmax devices{epitaxial layer
thicknesses, emitter contact resistance, and emitter
current density then must also be scaled by the propor-
tions given earlier. Figure 8 shows measured RF gains
for a developed in our e®orts to realize logic gates at
70-100 GHz clock rates. Simultaneous 295 GHz f¿ and
fmax are obtained.
Circuit results in the substrate transfer process
include DC-80 GHz lumped and distributed ampli-
¯ers [10], 66 GHz true static frequency dividers [11],
4Fig. 9. 20 GHz-clock-rate ¢-§ ADC in the transferred-substrate
process.
W-band medium-power ampli¯ers, and 20-GHz-clock
delta-sigma analog-digital converters [12].
IV. Conclusions
With bipolar transistors, improved bandwidths are
obtained by vertical scaling (thinner base and collector
layers), combined with lateral scaling (narrower collec-
tor and emitter junctions), increased current density,
and progressive improvements in emitter Ohmic con-
tacts.
Substrate transfer processes facilitate lateral scaling;
remaining di±culties include the emitter contact resis-
tivity, loss of breakdown in thin collector layers, and
reliability and heatsinking under high-current-density
operation.
While III-V HBTs bene¯t from strong heterojunc-
tions, high mobilities, and high electron velocities,
Si/SiGe bipolar transistors have been much more ag-
gressively scaled. Aggressive submicron scaling of III-
V HBTs can result in THz device bandwidths.
Acknowledgments
HBT work was supported by the US O±ce of
Naval Research under the programs N00014-98-1-0068,
N00014-99-10041 , N00014-98-1-0830, by the ARO
Quasi-optical MURI, and by the AFOSR.
References
[1] K. Ohhata, T. Masuda, E. Ohue, K. Washio, \Design of a
32.7-GHz bandwdth AGC ampli¯er IC with wide dynamic
range implemented with SiGe HBT", IEEE J. Solid-State
Circuits, Vol. 34, No. 9, Sept 1999, pp. 1291{1297.
[2] M. Reddy, S.C. Martin, A.C. Molnar, R.E. Muller, R.P.
Smith, P.H. Siegel, M.J. Mondry, M.J.W. Rodwell, and S.J.
Allen, Jr., \Monolithic Schottky-Collector Resonant Tun-
nel Diode Oscillator Arrays to 650 GHz." IEEE Electron
Device Letters, Volume 18, No. 5, May 1997, pp. 218-221.
[3] Mehdi, I.; Martin, S.C.; Dengler, R.J.; Smith, R.P.; Siegel,
P.H. \Fabrication and performance of planar Schottky
diodes with T-gate-like anodes in 200-GHz subharmonically
pumped waveguide mixers " IEEE Microwave and Guided
Wave Letters, vol.6, (no.1), IEEE, Jan. 1996. p.49-51.
[4] Siegel, P.H.; Smith, R.P.; Graidis, M.C.; Martin, S.C.
\2.5-THz GaAs monolithic membrane-diode mixer". IEEE
Transactions on Microwave Theory and Techniques, vol.47,
(no.5), IEEE, May 1999. p.596-604
[5] C. A. Meade, \Schottky-Barrier Gate Field-E®ect Transis-
tor", Proc IEEE, 54, 307 (1966)
[6] E.R. Brown et. al., \Oscillations up to 712 GHz in
InAs/AlSb resonant-tunneling diodes", Appl. Phys. Lett,
vol. 58, no. 20, pp 2291-2293, May 1991.
[7] Q. Lee, S.C. Martin, D. Mensa, R.P. Smith, J. Guthrie, and
M.J.W. Rodwell, \Submicron transferred-substrate hetero-
junction bipolar transistors", IEEE Electron Device Let-
ters, Vol. 20, No. 8, August 1999, pp. 396-398.
[8] W. Liu, D. Hill, H. F. Chau, J. Sweder, T. Nagle and J.
Delany, \Laterally etched undercut (LEU) technique to re-
duce base-collector capacitance in heterojunction bipolar
transistors", IEEE GaAs IC Symp. Tech. Dig., pp. 167-
170, 1995.
[9] Y. Bester et. al, \High ft and fmax transferred-substrate
HBTs", 2000 Device Reseach Conference, Denver, June 19-
21.
[10] S. Krishnan, S. Jaganathan, T. Mathew, Y. Wei, M.J.W.
Rodwell \Broadband HBT ampli¯ers", 2000 IEEE/Cornell
Conference on Advanced Concepts in High Speed Semicon-
ductor Devices and Circuits, Cornell, NY, August.
[11] Q. Lee, D. Mensa, J. Guthrie, S. Jaganathan, T. Mathew,
S. Krishnan, S. Cerhan and M.J.W. Rodwell, \66 GHz
static frequency divider in transferred-substrate HBT tech-
nology" Presented at the 1999 IEEE RF/Microwave mono-
lithic circuits symposium, June, Anaheim, CA.
[12] S. Jaganathan, to be submitted.


Submicron lateral scaling of HBTs and other vertical-transport devices:towards THz bandwidths

Abstract|With appropriate device structures, com-bined lithographic and epitaxial scaling of HBTs, RTDs and Schottky diodes results in rapid increases in de-vice bandwidths. 0.1 m InGaAs RTDs have oscillated at 650 GHz, Submicron heterojunction bipolar tran-sistors (HBTs) fabricated with substrate transfer pro-cesses have obtained 21 dB unilateral power gain at 100 GHz; if extrapolated at-20 dB/decade, this corresponds to a 1.1 THz power-gain cuto ® frequency. HBT current-gain cuto ® frequencies as high as 300 GHz have been obtained. I

Mark Rodwell

Y. Betser

S. Jaganathan

T. Mathew

Pk Sundararajan

S. C. Martin

R. P. Smith

Y. Wei

M. Urteaga

D. Scott

S. Long

CiteSeerX

Submicron lateral scaling of HBTs and other vertical transport devices: towards Thz bandwidths

AMS Acta

1Submicron lateral scaling of HBTs and othervertical-transport devices: towards THz bandwidthsMark Rodwell, Y. Betser, S. Jaganathan, T. Mathew, PK Sundararajan, S.C. MartinR.P. Smith, Y. Wei, M. Urteaga, D. Scott, S. LongAbstract|With appropriate device structures, com-bined lithographic and epitaxial scaling of HBTs, RTDsand Schottky diodes results in rapid increases in de-vice bandwidths. 0.1 ¹m InGaAs RTDs have oscillatedat 650 GHz, Submicron heterojunction bipolar tran-sistors (HBTs) fabricated with substrate transfer pro-cesses have obtained 21 dB unilateral power gain at 100GHz; if extrapolated at -20 dB/decade, this correspondsto a 1.1 THz power-gain cuto® frequency. HBT current-gain cuto® frequencies as high as 300 GHz have beenobtained.I. IntroductionDEVICE scaling {reduction of layer thicknessesand lithographic feature sizes { is central to highfrequency semiconductor device design. As CMOSgate lengths have been reduced, clock rates have pro-gressively increased. 0.13 ¹m Si MOSFETs exhibit»70 GHz f¿ . Compared to silicon, III-V compoundsemiconductors o®er higher electron mobilities, higherelectron saturation drift velocities, and stronger het-erojunctions, hence III-V semiconductor devices canobtain higher bandwidths than their silicon counter-parts. Yet III-V devices must be aggressively scaledto deep submicron dimensions if they are to remaincompetitive with silicon.III-V heterojunction bipolar transistors (HBTs), inparticular, have not been adequately scaled. GaAs-and InP- based HBTs are typically fabricated at 1{2¹m minimum feature sizes and operate at 1{2 £105A=cm2. This is in marked contrast to both III-VHEMTs, where 0.1 ¹m gate lengths are typical, andSi/SiGe HBTs, where emitter widths are 0.1 ¹m [1]and emitter current densities are as high at 106 A=cm2.Because of this aggressive scaling, Si/SiGe HBTs are amajor competitive threat to III-V HBTs. Addressingthis, we have developed InAlAs/InGaAs HBTs withsubmicron emitter and collector dimensions, and haveobtained 1 THz (extrapolated) power-gain cuto® fre-quencies (fmax) and 300 GHz current-gain cuto® fre-quencies (f¿ ). Challenges in HBT submicron scal-ing include collector de¯nition, emitter parasitic resis-tance, and support of high collector current densities.Scaling laws and scaling examplesWe will develop HBT scaling laws by ¯rst consider-ing high frequency diodes. Dominant time constantsMark Rodwell, Y. Betser, S. Jaganathan, T. Mathew, PKSundararajan, Y. Wei, M. Urteaga, D. Scott, and S. Long arewith the Department of Electrical and Computer Engineering,University of California, Santa Barbara, CA 93106, USA. rod-wellece.ucsb.edu. S.C. Martin is with Jet Propulsion Labs,Caltech, Pasadena, CA. R. P. Smith was with Jet PropulsionLabs, Caltech, Pasadena, CA, and is now with Cree Research,Durham, NC, USADWLSchottky-collector RTDTransferred-Substrate HBTbase emittercollectordrainHEMT (Schottky gate FET)source gatePrinciple of ScalingDeep Submicron SchottkyC ∝WL /DR ∝1/ LC ∝W /Dτ transit ∝D /velectron→sRsR = kT /qIC∝WL/ ID→jRj ∝1 /JDRsRjC2:1 thinner epitaxial layers4:1 smaller lithographic dimensions4:1 increased current densityresistanceless (Schottky) top contacts→2:1 increased device bandwidthFig. 1. Scaling laws for high-frequency devicesfor Schottky diodes are shown in ¯g. 1. The device hasa junction impedance Rj = kT=qI, a depletion capac-itance C and a parasitic series resistance Rs arisingfrom the bulk resistivity of the buried N+ layer andits Ohmic contacts. Given junction depth D, there isa transit time ¿transit / D=velectron. Given junctionwidth W , there is capacitance C /WL=D. Given de-vice stripe length L, the buried layer & bottom contactresistance Rs varies as 1=L.We wish to double the device bandwidth. This re-quires thinning the depletion layer 2:1 to proportion-ally reduce ¿transit. In reducing D 2:1 with W leftunchanged, we have doubled the device capacitance,doubling RsC. Our intent is instead to decrease RsCby 2:1; to do this we must now decrease the contactstripe width W by 4:1. With this combined scaling,C has now been reduced to 1=2 its original value.If the current I is left unchanged, the time constantRjC = (kT=qI) ¢C is thus also reduced by the desired2:1 ratio. Note that the device junction area WL hasbeen reduced 4:1; the device current density has thusbeen increased fourfold.These are the scaling laws for Schottky mixer diodes.Each desired 2:1 increase in device bandwidth requiresa 2:1 reduction in epitaxial layer thicknesses, a 4:1 de-crease in junction width (lithographic scaling), and a4:1 increase in operating current density.Exploiting such scaling, Siegel et al have reportedsubmicron Schottky mixers diodes with low noise at2.5 THz [3], [4]. Figure 2 shows a 0.1 ¹m £0:5 ¹m T-gate Schottky diode (with 150 ºA depletion thickness)used for submillimeter-wave harmonic mixing [2].Unlike Schottky diodes, PN diodes have both anode2Fig. 2. Submicron (0:1¹m £0:5¹m) Schottky mixer diode with150 ºA depletion depth. Calculated RC and transit-timecuto® frequencies are » 15THz.(top) and cathode Ohmic contacts. The top contacthas resistance Rtop = ½contact=LW , resulting in a timeconstant RtopC / ½contact=D. Since a 2:1 desired im-provement in device speed required a 2:1 reduction, inD (for reduced transit time), the required 2:1 reduc-tion on RtopC requires a 4:1 improvement in ½contact.Devices with top Ohmic contacts can be very di±cultto scale: each 2:1 improvement in device speed requiresa 4:1 improvement in contact resistivity.Lack of minority carrier storage is an often-citedspeed advantage of Schottky diodes over their P-Ncounterparts; elimination of the top Ohmic contact isan advantage of equal signi¯cance. Zero gate contactresistance is a crucial advantage of the (Schottky-gate)MESFET / HEMT [5] over the JFET; in the JFET,gate Ohmic contact resistance increases as the gatelength is reduced.II. Schottky-collector RTDsA resonant-tunnel diode (RTD) has an equivalentcircuit similar to that of the Schottky diode shown in¯g. 1. The device has a negative resistance Rn re-placing Rj of the Schottky diode, plus series resistanceRtotal = Rs + Rtop and depletion capacitance C. Rnvaries as 1=Ipeak, the peak tunneling current density.If the electron storage time in the quantum well ¿qw isnegligible, the device maximum oscillation frequencyis fmax = (1=2¼C)(RnRtotal)¡1=2; if ¿qw is dominant,fmax = (1=2¼)(RnRtotal)¡1=4C¡1=2¿¡1=2qw .RTD scaling follows the rules above. Current den-sities must be high. Depletion layers must be thinto avoid high transit times. Lithographic scaling re-duces the RsC time constant. If the RTD has a topOhmic contact, then reduction of RtopC to values nec-essary for submillimeter-wave oscillation requires verylow contact resistance, and InAs contacts [6] are thenemployed. Yet, the RTD top Ohmic contact is unneces-sary, and can be replaced by a direct Schottky contactto the fully-depleted electron drift layer, thereby elimi-nating Rtop. An RTD with a 0.1 ¹m T-gate Schottkycontact, 350 ºA depletion thickness, and 5 ¢ 105 A=cm2Fig. 3. 0.1 ¹m InGaAs/AlAs Schottky-collector RTD.Fig. 4. A 64-element, 650 GHz, RTD quasi-optical array oscil-lator.current density (¯g. 3) has a estimated (but not mea-sured) 2 THz fmax. 64-element monolithic arrays ofSchottky-collector RTDs have oscillated at 650 GHz(¯g 4) [2].III. HBT scalingAn HBT is a pair of coupled diodes, and follows scal-ing laws similar to the Schottky diode. A 2:1 desiredimprovement in HBT bandwidth requires 2:1 reductionin base and collector transit times, requiring ap2 :1reduction in the base and a 2:1 reduction in the col-lector epitaxial layer thicknesses. For a 2:1 improve-ment of all HBT RC and delay terms, the emitter-base and collector-base junction widths (lithographicfeature size) must also be reduced 4:1, the collectorcurrent density increased 4:1, and the emitter Ohmiccontact improved 4:1.A. Transferred-Substrate HBTsAn immediate di±culty is the required reductionin the collector junction width. In mesa HBTs thecollector-base junction must lie under both the emitterstripe and under the base Ohmic contacts, which must3Fig. 5. Schematic cross-section of a transferred-substrate HBTFig. 6. E-beam HBT: test structure with 0.15 ¹m emitter-basejunction (a), and 0.4 ¹m Schottky collector stripe (b)be a minimum width of one contact transfer length,typically » 0:25 ¹m. A minimum collector-base junc-tion width is thus de¯ned, and a lower limit is set forHBT scalingUsing substrate transfer processes [7], HBTs canbe de¯ned with emitter-base and collector-base junc-tions on opposing sides of the base epitaxial layer (¯g.5). The minimum collector junction width is now de-termined by the emitter width alone, and need notinclude the widths of the base Ohmic contacts; col-lector junctions can be scaled to submicron dimen-sions. Submicron InAlAs/InGaAs devices fabricatedusing electron-beam lithography (¯g 5) exhibit 21 dBunilateral power gain at 100 GHz (¯g. 7). Extrapo-lation at -20 dB/decade (to be treated with caution)suggests an 1100 GHz fmax. In addressing limits tocollector scaling, one alternative to substrate transferis an undercut collector-base junction [8]. A secondalternative is very high (carbon) base doping within anormal mesa HBT structure { at » 2 ¢ 1020=cm3 basedoping, the contact transfer length is » 0:1¹m, andvery narrow base mesas can then be employed.The HBT of ¯g. 7 has narrow emitter and collec-05101520253010 100 1000Gains, dBFrequency,GHzfmax=1080 GHzfτ=204 GHzH21Mason's gain, U MSGIc= 6 mA, Vce= 1.2 VFig. 7. Gains of a 0.4 ¹m £ 6 ¹m emitter and 0.7 ¹m £ 10 ¹mcollector HBT fabricated using electron-beam lithography.Theoretical -20 dB/decade (H21, U ) gain slopes are indi-cated. The device exhibits an extrapolated 1.08 THz fmax010203040501 10 102Gains   (dB)Frequency   (GHz)h21UVCE = 1 V,  JC = 1.5 mA/um2fMAX = 295 GHzf τ= = 295 GHzFig. 8. Measured RF gains for an HBT with a 300 ºA base with52 meV grading and a 2000 ºA collector.tor junctions but thick 400 ºA base and 3000 ºA col-lector layers, and hence has high fmax but only 200GHz f¿ . While such a device will provide high gainsin tuned millimeter-wave ampli¯ers, more general cir-cuits require simultaneously high values for f¿ andfmax. Digital circuits, in turn, have gate delays inwhich (¿b + ¿c), Cbc¢Vlogic=Ic, RbbCje, RexCje andRF bb(¿b + ¿c)Ic=¢Vlogic are all signi¯cant terms.To ensure a 2:1 improvement in all circuits employ-ing the HBT, all HBT transit times and RC time con-stants must be reduced 2:1. In addition to lithographicscaling of emitter and collector junction widths{asdemonstrated in the high-fmax devices{epitaxial layerthicknesses, emitter contact resistance, and emittercurrent density then must also be scaled by the propor-tions given earlier. Figure 8 shows measured RF gainsfor a developed in our e®orts to realize logic gates at70-100 GHz clock rates. Simultaneous 295 GHz f¿ andfmax are obtained.Circuit results in the substrate transfer processinclude DC-80 GHz lumped and distributed ampli-¯ers [10], 66 GHz true static frequency dividers [11],4Fig. 9. 20 GHz-clock-rate ¢-§ ADC in the transferred-substrateprocess.W-band medium-power ampli¯ers, and 20-GHz-clockdelta-sigma analog-digital converters [12].IV. ConclusionsWith bipolar transistors, improved bandwidths areobtained by vertical scaling (thinner base and collectorlayers), combined with lateral scaling (narrower collec-tor and emitter junctions), increased current density,and progressive improvements in emitter Ohmic con-tacts.Substrate transfer processes facilitate lateral scaling;remaining di±culties include the emitter contact resis-tivity, loss of breakdown in thin collector layers, andreliability and heatsinking under high-current-densityoperation.While III-V HBTs bene¯t from strong heterojunc-tions, high mobilities, and high electron velocities,Si/SiGe bipolar transistors have been much more ag-gressively scaled. Aggressive submicron scaling of III-V HBTs can result in THz device bandwidths.AcknowledgmentsHBT work was supported by the US O±ce ofNaval Research under the programs N00014-98-1-0068,N00014-99-10041 , N00014-98-1-0830, by the AROQuasi-optical MURI, and by the AFOSR.References[1] K. Ohhata, T. Masuda, E. Ohue, K. Washio, \Design of a32.7-GHz bandwdth AGC ampli¯er IC with wide dynamicrange implemented with SiGe HBT", IEEE J. Solid-StateCircuits, Vol. 34, No. 9, Sept 1999, pp. 1291{1297.[2] M. Reddy, S.C. Martin, A.C. Molnar, R.E. Muller, R.P.Smith, P.H. Siegel, M.J. Mondry, M.J.W. Rodwell, and S.J.Allen, Jr., \Monolithic Schottky-Collector Resonant Tun-nel Diode Oscillator Arrays to 650 GHz." IEEE ElectronDevice Letters, Volume 18, No. 5, May 1997, pp. 218-221.[3] Mehdi, I.; Martin, S.C.; Dengler, R.J.; Smith, R.P.; Siegel,P.H. \Fabrication and performance of planar Schottkydiodes with T-gate-like anodes in 200-GHz subharmonicallypumped waveguide mixers " IEEE Microwave and GuidedWave Letters, vol.6, (no.1), IEEE, Jan. 1996. p.49-51.[4] Siegel, P.H.; Smith, R.P.; Graidis, M.C.; Martin, S.C.\2.5-THz GaAs monolithic membrane-diode mixer". IEEETransactions on Microwave Theory and Techniques, vol.47,(no.5), IEEE, May 1999. p.596-604[5] C. A. Meade, \Schottky-Barrier Gate Field-E®ect Transis-tor", Proc IEEE, 54, 307 (1966)[6] E.R. Brown et. al., \Oscillations up to 712 GHz inInAs/AlSb resonant-tunneling diodes", Appl. Phys. Lett,vol. 58, no. 20, pp 2291-2293, May 1991.[7] Q. Lee, S.C. Martin, D. Mensa, R.P. Smith, J. Guthrie, andM.J.W. Rodwell, \Submicron transferred-substrate hetero-junction bipolar transistors", IEEE Electron Device Let-ters, Vol. 20, No. 8, August 1999, pp. 396-398.[8] W. Liu, D. Hill, H. F. Chau, J. Sweder, T. Nagle and J.Delany, \Laterally etched undercut (LEU) technique to re-duce base-collector capacitance in heterojunction bipolartransistors", IEEE GaAs IC Symp. Tech. Dig., pp. 167-170, 1995.[9] Y. Bester et. al, \High ft and fmax transferred-substrateHBTs", 2000 Device Reseach Conference, Denver, June 19-21.[10] S. Krishnan, S. Jaganathan, T. Mathew, Y. Wei, M.J.W.Rodwell \Broadband HBT ampli¯ers", 2000 IEEE/CornellConference on Advanced Concepts in High Speed Semicon-ductor Devices and Circuits, Cornell, NY, August.[11] Q. Lee, D. Mensa, J. Guthrie, S. Jaganathan, T. Mathew,S. Krishnan, S. Cerhan and M.J.W. Rodwell, \66 GHzstatic frequency divider in transferred-substrate HBT tech-nology" Presented at the 1999 IEEE RF/Microwave mono-lithic circuits symposium, June, Anaheim, CA.[12] S. Jaganathan, to be submitted.

1
Submicron lateral scaling of HBTs and other
vertical-transport devices: towards THz bandwidths
Mark Rodwell, Y. Betser, S. Jaganathan, T. Mathew, PK Sundararajan, S.C. Martin
R.P. Smith, Y. Wei, M. Urteaga, D. Scott, S. Long
Abstract|With appropriate device structures, com-
bined lithographic and epitaxial scaling of HBTs, RTDs
and Schottky diodes results in rapid increases in de-
vice bandwidths. 0.1 ¹m InGaAs RTDs have oscillated
at 650 GHz, Submicron heterojunction bipolar tran-
sistors (HBTs) fabricated with substrate transfer pro-
cesses have obtained 21 dB unilateral power gain at 100
GHz; if extrapolated at -20 dB/decade, this corresponds
to a 1.1 THz power-gain cuto® frequency. HBT current-
gain cuto® frequencies as high as 300 GHz have been
obtained.
I. Introduction
DEVICE scaling {reduction of layer thicknessesand lithographic feature sizes { is central to high
frequency semiconductor device design. As CMOS
gate lengths have been reduced, clock rates have pro-
gressively increased. 0.13 ¹m Si MOSFETs exhibit
»70 GHz f¿ . Compared to silicon, III-V compound
semiconductors o®er higher electron mobilities, higher
electron saturation drift velocities, and stronger het-
erojunctions, hence III-V semiconductor devices can
obtain higher bandwidths than their silicon counter-
parts. Yet III-V devices must be aggressively scaled
to deep submicron dimensions if they are to remain
competitive with silicon.
III-V heterojunction bipolar transistors (HBTs), in
particular, have not been adequately scaled. GaAs-
and InP- based HBTs are typically fabricated at 1{2
¹m minimum feature sizes and operate at 1{2 £105
A=cm2. This is in marked contrast to both III-V
HEMTs, where 0.1 ¹m gate lengths are typical, and
Si/SiGe HBTs, where emitter widths are 0.1 ¹m [1]
and emitter current densities are as high at 106 A=cm2.
Because of this aggressive scaling, Si/SiGe HBTs are a
major competitive threat to III-V HBTs. Addressing
this, we have developed InAlAs/InGaAs HBTs with
submicron emitter and collector dimensions, and have
obtained 1 THz (extrapolated) power-gain cuto® fre-
quencies (fmax) and 300 GHz current-gain cuto® fre-
quencies (f¿ ). Challenges in HBT submicron scal-
ing include collector de¯nition, emitter parasitic resis-
tance, and support of high collector current densities.
Scaling laws and scaling examples
We will develop HBT scaling laws by ¯rst consider-
ing high frequency diodes. Dominant time constants
Mark Rodwell, Y. Betser, S. Jaganathan, T. Mathew, PK
Sundararajan, Y. Wei, M. Urteaga, D. Scott, and S. Long are
with the Department of Electrical and Computer Engineering,
University of California, Santa Barbara, CA 93106, USA. rod-
wellece.ucsb.edu. S.C. Martin is with Jet Propulsion Labs,
Caltech, Pasadena, CA. R. P. Smith was with Jet Propulsion
Labs, Caltech, Pasadena, CA, and is now with Cree Research,
Durham, NC, USA
D
W
L
Schottky-collector RTD
Transferred-Substrate HBT
base emitter
collector
drain
HEMT (Schottky gate FET)
source gate
Principle of Scaling
Deep Submicron Schottky
C ∝ WL /D
R ∝ 1/ L
C ∝ W /D
τ transit ∝ D /velectron
→
s
Rs
R = kT /qI
C ∝ WL/ ID→
j
Rj ∝ 1 /JD
Rs
Rj
C
2:1 thinner epitaxial layers
4:1 smaller lithographic dimensions
4:1 increased current density
resistanceless (Schottky) top contacts
→2:1 increased device bandwidth
Fig. 1. Scaling laws for high-frequency devices
for Schottky diodes are shown in ¯g. 1. The device has
a junction impedance Rj = kT=qI, a depletion capac-
itance C and a parasitic series resistance Rs arising
from the bulk resistivity of the buried N+ layer and
its Ohmic contacts. Given junction depth D, there is
a transit time ¿transit / D=velectron. Given junction
width W , there is capacitance C /WL=D. Given de-
vice stripe length L, the buried layer & bottom contact
resistance Rs varies as 1=L.
We wish to double the device bandwidth. This re-
quires thinning the depletion layer 2:1 to proportion-
ally reduce ¿transit. In reducing D 2:1 with W left
unchanged, we have doubled the device capacitance,
doubling RsC. Our intent is instead to decrease RsC
by 2:1; to do this we must now decrease the contact
stripe width W by 4:1. With this combined scaling,
C has now been reduced to 1=2 its original value.
If the current I is left unchanged, the time constant
RjC = (kT=qI) ¢C is thus also reduced by the desired
2:1 ratio. Note that the device junction area WL has
been reduced 4:1; the device current density has thus
been increased fourfold.
These are the scaling laws for Schottky mixer diodes.
Each desired 2:1 increase in device bandwidth requires
a 2:1 reduction in epitaxial layer thicknesses, a 4:1 de-
crease in junction width (lithographic scaling), and a
4:1 increase in operating current density.
Exploiting such scaling, Siegel et al have reported
submicron Schottky mixers diodes with low noise at
2.5 THz [3], [4]. Figure 2 shows a 0.1 ¹m £0:5 ¹m T-
gate Schottky diode (with 150 ºA depletion thickness)
used for submillimeter-wave harmonic mixing [2].
Unlike Schottky diodes, PN diodes have both anode
2
Fig. 2. Submicron (0:1¹m £0:5¹m) Schottky mixer diode with
150 ºA depletion depth. Calculated RC and transit-time
cuto® frequencies are » 15THz.
(top) and cathode Ohmic contacts. The top contact
has resistance Rtop = ½contact=LW , resulting in a time
constant RtopC / ½contact=D. Since a 2:1 desired im-
provement in device speed required a 2:1 reduction, in
D (for reduced transit time), the required 2:1 reduc-
tion on RtopC requires a 4:1 improvement in ½contact.
Devices with top Ohmic contacts can be very di±cult
to scale: each 2:1 improvement in device speed requires
a 4:1 improvement in contact resistivity.
Lack of minority carrier storage is an often-cited
speed advantage of Schottky diodes over their P-N
counterparts; elimination of the top Ohmic contact is
an advantage of equal signi¯cance. Zero gate contact
resistance is a crucial advantage of the (Schottky-gate)
MESFET / HEMT [5] over the JFET; in the JFET,
gate Ohmic contact resistance increases as the gate
length is reduced.
II. Schottky-collector RTDs
A resonant-tunnel diode (RTD) has an equivalent
circuit similar to that of the Schottky diode shown in
¯g. 1. The device has a negative resistance Rn re-
placing Rj of the Schottky diode, plus series resistance
Rtotal = Rs + Rtop and depletion capacitance C. Rn
varies as 1=Ipeak, the peak tunneling current density.
If the electron storage time in the quantum well ¿qw is
negligible, the device maximum oscillation frequency
is fmax = (1=2¼C)(RnRtotal)
¡1=2; if ¿qw is dominant,
fmax = (1=2¼)(RnRtotal)
¡1=4C¡1=2¿
¡1=2
qw .
RTD scaling follows the rules above. Current den-
sities must be high. Depletion layers must be thin
to avoid high transit times. Lithographic scaling re-
duces the RsC time constant. If the RTD has a top
Ohmic contact, then reduction of RtopC to values nec-
essary for submillimeter-wave oscillation requires very
low contact resistance, and InAs contacts [6] are then
employed. Yet, the RTD top Ohmic contact is unneces-
sary, and can be replaced by a direct Schottky contact
to the fully-depleted electron drift layer, thereby elimi-
nating Rtop. An RTD with a 0.1 ¹m T-gate Schottky
contact, 350 ºA depletion thickness, and 5 ¢ 105 A=cm2
Fig. 3. 0.1 ¹m InGaAs/AlAs Schottky-collector RTD.
Fig. 4. A 64-element, 650 GHz, RTD quasi-optical array oscil-
lator.
current density (¯g. 3) has a estimated (but not mea-
sured) 2 THz fmax. 64-element monolithic arrays of
Schottky-collector RTDs have oscillated at 650 GHz
(¯g 4) [2].
III. HBT scaling
An HBT is a pair of coupled diodes, and follows scal-
ing laws similar to the Schottky diode. A 2:1 desired
improvement in HBT bandwidth requires 2:1 reduction
in base and collector transit times, requiring a
p
2 :1
reduction in the base and a 2:1 reduction in the col-
lector epitaxial layer thicknesses. For a 2:1 improve-
ment of all HBT RC and delay terms, the emitter-
base and collector-base junction widths (lithographic
feature size) must also be reduced 4:1, the collector
current density increased 4:1, and the emitter Ohmic
contact improved 4:1.
A. Transferred-Substrate HBTs
An immediate di±culty is the required reduction
in the collector junction width. In mesa HBTs the
collector-base junction must lie under both the emitter
stripe and under the base Ohmic contacts, which must
3
Fig. 5. Schematic cross-section of a transferred-substrate HBT
Fig. 6. E-beam HBT: test structure with 0.15 ¹m emitter-base
junction (a), and 0.4 ¹m Schottky collector stripe (b)
be a minimum width of one contact transfer length,
typically » 0:25 ¹m. A minimum collector-base junc-
tion width is thus de¯ned, and a lower limit is set for
HBT scaling
Using substrate transfer processes [7], HBTs can
be de¯ned with emitter-base and collector-base junc-
tions on opposing sides of the base epitaxial layer (¯g.
5). The minimum collector junction width is now de-
termined by the emitter width alone, and need not
include the widths of the base Ohmic contacts; col-
lector junctions can be scaled to submicron dimen-
sions. Submicron InAlAs/InGaAs devices fabricated
using electron-beam lithography (¯g 5) exhibit 21 dB
unilateral power gain at 100 GHz (¯g. 7). Extrapo-
lation at -20 dB/decade (to be treated with caution)
suggests an 1100 GHz fmax. In addressing limits to
collector scaling, one alternative to substrate transfer
is an undercut collector-base junction [8]. A second
alternative is very high (carbon) base doping within a
normal mesa HBT structure { at » 2 ¢ 1020=cm3 base
doping, the contact transfer length is » 0:1¹m, and
very narrow base mesas can then be employed.
The HBT of ¯g. 7 has narrow emitter and collec-
0
5
10
15
20
25
30
10 100 1000
G
a
in
s
, 
d
B
Frequency,GHz
f
max
=1080 GHz
f
τ
=204 GHz
H
21
Mason's gain, U 
MSG
I
c
= 6 mA, V
ce
= 1.2 V
Fig. 7. Gains of a 0.4 ¹m £ 6 ¹m emitter and 0.7 ¹m £ 10 ¹m
collector HBT fabricated using electron-beam lithography.
Theoretical -20 dB/decade (H21, U ) gain slopes are indi-
cated. The device exhibits an extrapolated 1.08 THz fmax
0
10
20
30
40
50
1 10 10
2
G
a
in
s
  
 (
d
B
)
Frequency   (GHz)
h
21
U
V
CE
 = 1 V,  J
C
 = 1.5 mA/um
2
f
MAX
 = 295 GHz
f 
τ=
 = 295 GHz
Fig. 8. Measured RF gains for an HBT with a 300 ºA base with
52 meV grading and a 2000 ºA collector.
tor junctions but thick 400 ºA base and 3000 ºA col-
lector layers, and hence has high fmax but only 200
GHz f¿ . While such a device will provide high gains
in tuned millimeter-wave ampli¯ers, more general cir-
cuits require simultaneously high values for f¿ and
fmax. Digital circuits, in turn, have gate delays in
which (¿b + ¿c), Cbc¢Vlogic=Ic, RbbCje, RexCje and
RF bb(¿b + ¿c)Ic=¢Vlogic are all signi¯cant terms.
To ensure a 2:1 improvement in all circuits employ-
ing the HBT, all HBT transit times and RC time con-
stants must be reduced 2:1. In addition to lithographic
scaling of emitter and collector junction widths{as
demonstrated in the high-fmax devices{epitaxial layer
thicknesses, emitter contact resistance, and emitter
current density then must also be scaled by the propor-
tions given earlier. Figure 8 shows measured RF gains
for a developed in our e®orts to realize logic gates at
70-100 GHz clock rates. Simultaneous 295 GHz f¿ and
fmax are obtained.
Circuit results in the substrate transfer process
include DC-80 GHz lumped and distributed ampli-
¯ers [10], 66 GHz true static frequency dividers [11],
4
Fig. 9. 20 GHz-clock-rate ¢-§ ADC in the transferred-substrate
process.
W-band medium-power ampli¯ers, and 20-GHz-clock
delta-sigma analog-digital converters [12].
IV. Conclusions
With bipolar transistors, improved bandwidths are
obtained by vertical scaling (thinner base and collector
layers), combined with lateral scaling (narrower collec-
tor and emitter junctions), increased current density,
and progressive improvements in emitter Ohmic con-
tacts.
Substrate transfer processes facilitate lateral scaling;
remaining di±culties include the emitter contact resis-
tivity, loss of breakdown in thin collector layers, and
reliability and heatsinking under high-current-density
operation.
While III-V HBTs bene¯t from strong heterojunc-
tions, high mobilities, and high electron velocities,
Si/SiGe bipolar transistors have been much more ag-
gressively scaled. Aggressive submicron scaling of III-
V HBTs can result in THz device bandwidths.
Acknowledgments
HBT work was supported by the US O±ce of
Naval Research under the programs N00014-98-1-0068,
N00014-99-10041 , N00014-98-1-0830, by the ARO
Quasi-optical MURI, and by the AFOSR.
References
[1] K. Ohhata, T. Masuda, E. Ohue, K. Washio, \Design of a
32.7-GHz bandwdth AGC ampli¯er IC with wide dynamic
range implemented with SiGe HBT", IEEE J. Solid-State
Circuits, Vol. 34, No. 9, Sept 1999, pp. 1291{1297.
[2] M. Reddy, S.C. Martin, A.C. Molnar, R.E. Muller, R.P.
Smith, P.H. Siegel, M.J. Mondry, M.J.W. Rodwell, and S.J.
Allen, Jr., \Monolithic Schottky-Collector Resonant Tun-
nel Diode Oscillator Arrays to 650 GHz." IEEE Electron
Device Letters, Volume 18, No. 5, May 1997, pp. 218-221.
[3] Mehdi, I.; Martin, S.C.; Dengler, R.J.; Smith, R.P.; Siegel,
P.H. \Fabrication and performance of planar Schottky
diodes with T-gate-like anodes in 200-GHz subharmonically
pumped waveguide mixers " IEEE Microwave and Guided
Wave Letters, vol.6, (no.1), IEEE, Jan. 1996. p.49-51.
[4] Siegel, P.H.; Smith, R.P.; Graidis, M.C.; Martin, S.C.
\2.5-THz GaAs monolithic membrane-diode mixer". IEEE
Transactions on Microwave Theory and Techniques, vol.47,
(no.5), IEEE, May 1999. p.596-604
[5] C. A. Meade, \Schottky-Barrier Gate Field-E®ect Transis-
tor", Proc IEEE, 54, 307 (1966)
[6] E.R. Brown et. al., \Oscillations up to 712 GHz in
InAs/AlSb resonant-tunneling diodes", Appl. Phys. Lett,
vol. 58, no. 20, pp 2291-2293, May 1991.
[7] Q. Lee, S.C. Martin, D. Mensa, R.P. Smith, J. Guthrie, and
M.J.W. Rodwell, \Submicron transferred-substrate hetero-
junction bipolar transistors", IEEE Electron Device Let-
ters, Vol. 20, No. 8, August 1999, pp. 396-398.
[8] W. Liu, D. Hill, H. F. Chau, J. Sweder, T. Nagle and J.
Delany, \Laterally etched undercut (LEU) technique to re-
duce base-collector capacitance in heterojunction bipolar
transistors", IEEE GaAs IC Symp. Tech. Dig., pp. 167-
170, 1995.
[9] Y. Bester et. al, \High ft and fmax transferred-substrate
HBTs", 2000 Device Reseach Conference, Denver, June 19-
21.
[10] S. Krishnan, S. Jaganathan, T. Mathew, Y. Wei, M.J.W.
Rodwell \Broadband HBT ampli¯ers", 2000 IEEE/Cornell
Conference on Advanced Concepts in High Speed Semicon-
ductor Devices and Circuits, Cornell, NY, August.
[11] Q. Lee, D. Mensa, J. Guthrie, S. Jaganathan, T. Mathew,
S. Krishnan, S. Cerhan and M.J.W. Rodwell, \66 GHz
static frequency divider in transferred-substrate HBT tech-
nology" Presented at the 1999 IEEE RF/Microwave mono-
lithic circuits symposium, June, Anaheim, CA.
[12] S. Jaganathan, to be submitted.


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Submicron lateral scaling of HBTs and other vertical-transport devices:towards THz bandwidths

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