Transferred-Substrate Heterojunction Bipolar Transistor Integrated Circuit Technology
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Transferred-Substrate Heterojunction Bipolar Transistor Integrated Circuit Technology
M Rodwell , Q Lee, D Mensa, J Guthrie, Y Betser, S Jaganathan, T Mathew, P Krishnan, S LongUniversity of California, Santa Barbara
SC Martin, RP Smith, NASA Jet Propulsion Labs
Supported by ONR (M Yoder, J Zolper, D Van Vechten), AFOSR ( H Schlossberg )
1999 IEEE Symposium on Indium Phosphide & Related Materials
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Why are HEMTs smaller & faster than HBTs ?FETs have deep submicron dimensions.
0.1 µm HEMTs with 400 GHz bandwidths (satellites).5 million 1/4-µm MOSFETs on a 200 MHz, $500 CPU.FET lateral scaling decreases transit times.FET bandwidths then increase.
HBTs have ~1 µm junctions. vertical scaling decreases electron transit times.vertical scaling increases RC charging times.lateral scaling should decrease RC charging times.HBT & RTD bandwidths should then increase.
But, HBTs must first be modified . . .
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Scaling for THz device bandwidths
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Current-gain cutoff frequency in HBTs
collex
Ebc
Ejecollectorbase RR
qIkTC
qIkTC
f
21
nbbase DT 22 satccollector vT 2
Collector velocities can be high: velocity overshoot in InGaAsBase bandgap grading reduces transit time substantiallyRC terms quite important for > 200 GHz ft devices
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Excess Collector-Base Capacitance in Mesa HBTs
• base contacts: must be > 1 transfer length (0.3 m) sets minimum collector width sets minimum collector capacitance Ccb
• base resistancespreading resistance scales with emitter scalingcontact resistance independent of emitter scaling sets minimum base resistance sets minimum RbbCcb time constant
fmax does not improve with submicron scaling
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0
200
400
600
800
1000
0 0.5 1 1.5
f max
, G
Hz
emitter width, microns
Transferred-Substrate HBTs: A Scalable HBT Technology
• Collector capacitance reduces with scaling:• Bandwidth increases rapidly with scaling:
ecb WC
eWf 1max
Ohmics base m .01
Ohmics base m 5.0
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Thinning base, collector epitaxial layers improves ft, degrades fmaxLateral scaling provides moderate improvements in fmaxRegrowth (similar to Si BJT !) should help considerablyTransferred-substrate helps dramatically
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Integrated Circuit Technology
• very high HBT bandwidths, low interconnect capacitance, lowground-return inductance, low thermal resistance
metal 1
bypass capacitortransistor resistor capacitor microstrip
BCB
GaAs carrier wafer
In/Pb/Ag solder
polyimide metal 2 SiN NiCr contact
C
gold ground plane
goldthermal via ground
via
BE
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50 mm transferred-substrate HBT Wafer: Cu substrate
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AlInAs/GaInAs graded base HBT
Band diagram under normal operating voltagesVce = 0.9 V, Vbe= 0.7 V
• 400 Å 5E19 graded base ( Eg = 2kT), 3000 Å collector
-2
-1.5
-1
-0.5
0
0.5
0 1000 2000 3000 4000 5000 6000Distance, Å
Gradedbase
Collector depletion regionEmitter
Schottkycollector
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Transferred-Substrate Heterojunction Bipolar Transistor
0.25 µm devices should obtain >1000 GHz fmax
Device with 0.6 µm emitter & 1.8 µm collectorextrapolated fmax at instrument limits, >400 GHz
0
5
10
15
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35
1 10 100
Gai
ns, d
B
Frequency, GHz
fmax
=470 GHz
f=215 GHz
Mason'sGain, U
H21
(?)
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Gai
ns, d
B
Frequency. GHz
Mason's gain, U
MSG, common emitter
MSG,common base
H21
, common emitterfmax
= 820 GHz
Submicron Transferred-Substrate HBT
0.4 m x 6 m emitter, 0.4 m x 10 m collector
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Emitter Profile: Stepper Device
0.15 m e/b junction
0.5 m emitter stripe
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0
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100 1000
Gai
ns, d
B
Frequency, GHz
Mason's gain, U
H21
fmax
= 805 GHz f = 147 GHz
Transferred-Substrate HBT: Stepper Lithography
0.4 m emitter, ~0.7 m collector
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DC characteristics, stepper device
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0 0.2 0.4 0.6 0.8 1 1.2
Vce, Volts
Ib step,0.01 mA
We=0.2 X 6 m2
Wc=1.5 X 9 m2
=50
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Given high fmax, vertical scaling exhanges reduced fmax for increased f
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Transit times: HBT with 2kT base grading
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0 0.1 0.2 0.3 0.4 0.5 0.6
1/2
f (ps)
1/Ic (1/mA)
0.51 ps
2000 Å InGaAs collector400 Å InGaAs base, 2kT bandgap grading
ps 045.0
ps 065.0 ps 114.0
ps 41.0
mcb
mje
cbex
cb
gC
gCCR
ps 634.0 total GHz 252f
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Digital microwave / RF transmitters (DC-20 GHz)direct digital synthesis at microwave bandwidthsmicrowave digital-analog converters
Digital microwave / RF receiversdelta-sigma ADCs with 10-30 GHz sample rates 16 effective bits at 100 MHz signal bandwidth ?
Basic Science: 0.1 µm Ebeam device: 1000 GHz transistor (?)transistor electronics in the far-infrared
Fast fiber optics, fast digital communications:200 GHz f, 500 GHz fmax device: ~ 75-90 Gb/s160 Gb/s needs ~350 GHz f, 500 GHz fmax
Why would you want a 1 THz transistor ?
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Transferred-Substrate HBT ICs: Key Features
100 GHz clock-rate ICs will need: very fast transistors short wires –> high IC density –> high thermal conductivity low capacitance wiring low ground inductance –> microstrip wiring environment
Transferred Substrate HBT ICs offer: 800 GHz fmax now , > 1000 GHz with further scaling 250 GHz ft now, >300 GHz with improved emitter Ohmics copper substrates / thermal vias for heatsinking low capacitance (= 2.5) wiring
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THz-Bandwidth HBTs ???
1) regrown P+++ InGaAs extrinsic base --> ultra-low-resistance 2) 0.05 µm wide emitter --> ultra low base spreading resistance3) 0.05 µm wide collector --> ultra low collector capacitance4) 100 Å, carbon-doped graded base --> 0.05 ps transit time5) 1kÅ thick InP collector --> 0.1 ps transit time.
Projected Performance: Transistor with 500 GHz ft, 1500 GHz fmax
1
2
3
45
deep submicron transferred-substrateregrown-base HBT
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The wiring environment for100 GHz logic
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Why is Improved Wiring Essential?
ground return loops createinductance
Wire bond createsground bounce betweenIC & package
30 GHz M/S D-FF in UCSB - mesa HBT technology
Ground loops & wire bonds:degrade circuit & packaged IC performance
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ADC digitalsections
inputbuffer
ground returncurrents
Lground
Vingroundbouncenoise
Ground Bound Noise in ADCs
Ground bounce noise must be ~100 dB below full-scale inputDifferential input will partly suppress ground noise coupling
~ 30 to 40 dB common-mode rejection feasibleCMRR insufficient to obtain 100 dB SNR
Eliminate ground bounce noise by good IC grounding
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Microstrip IC wiring to Eliminate Ground Bounce Noise
a
Brass carrier andassembly ground
interconnectsubstrate
IC with backsideground plane & vias
near-zeroground-groundinductance
IC viaseliminateon-wafergroundloops
Transferred-substrate HBT process provides vias & ground plane.
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Power Density in 100 GHz logic
Transistors tightly packed to minimize delays 105 W/cm2 HBT junction power density. ~103 W/cm2 power density on-chip 75 C temperature rise in 500 m substrate.
Solutions: Thin substrate to < 100 m Replace semiconductor with metal copper substrate
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Transferred-Substrate HBT Integrated Circuits47 GHz master-slave flip-flop
7 dB, 5-80 GHz distributed amplifier
11 dB, 50+ GHz AGC / limiting amplifier
10 dB, 50+ GHz feedback amplifier
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Transferred-Substrate HBT Integrated CircuitsW-band VCO
Clock recovery PLL
multiplexer
2:1 demultiplexer (120 HBTs)
16 dB, DC-60 GHz amplifier
6.7 dB, DC-85 GHz amplifier
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Darlington Amplifier - 360 GHz GBW
• 15.6 dB DC gain• Interpolated 3dB bandwidth of 60 GHz• 360 GHz gain-bandwidth product
-15
-10
-5
0
5
10
15
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0 20 40 60 80 100
dB
Frequency, GHz
S21
S11
S22
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0
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0 20 40 60 80 100
Forw
ard
Gai
n, S
21, d
B
Frequency, GHz
6.7 dB, 85 GHz Mirror Darlington Amplifier
• 6.7 dB DC gain• 3 dB bandwidth of 85 GHz• f-doubler (mirror Darlington) configuration
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Master-Slave Flip-Flops
CML: 47 GHz ECL: 48 GHz
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66 GHz Static Frequency Divider in Transferred-substrate HBT Technology
Q. Lee, D. Mensa, J. Guthrie, S. Jaganathan, T. Mathew, Y. Betser, S. Krishnan, S. Ceran, M.J.W. RodwellUniversity of California, Santa Barbara
IEEE RFIC’99, Anaheim, California
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Fiber OpticICs(not yet working !)
AGC / limiting amplifier CML decision circuit
PIN / transimpedance amplifier
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Delta-Sigma ADC In Development (300 HBTs)
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Transferred Substrate HBTsAn ultrafast bipolar integrated circuit technology
Ultrahigh fmax HBTs
Low capacitance interconnects
Superior heat sinking, low parasitic packaging
Demonstrated: HBTs with fmax > 800 GHz
fast flip-flops, 85 GHz amplifiers, ...
Future: 0.1 m HBTs with fmax > 1000 GHz
100 GHz digital logic ICs --> DACs, DDS, ADCs, fiber