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

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 . . .

Scaling for THz device bandwidths

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

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

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

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

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

50 mm transferred-substrate HBT Wafer: Cu substrate

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

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

20

25

30

35

1 10 100

Gai

ns, d

B

Frequency, GHz

fmax

=470 GHz

f=215 GHz

Mason'sGain, U

H21

(?)

0

5

10

15

20

25

100 1000

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

Emitter Profile: Stepper Device

0.15 m e/b junction

0.5 m emitter stripe

0

5

10

15

20

25

30

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

DC characteristics, stepper device

0

0.5

1

1.5

2

2.5

3

3.5

4

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

Given high fmax, vertical scaling exhanges reduced fmax for increased f

Transit times: HBT with 2kT base grading

0

0.2

0.4

0.6

0.8

1

1.2

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

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 ?

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

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

The wiring environment for100 GHz logic

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

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

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.

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

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

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

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

20

0 20 40 60 80 100

dB

Frequency, GHz

S21

S11

S22

0

2

4

6

8

10

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

Master-Slave Flip-Flops

CML: 47 GHz ECL: 48 GHz

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

Fiber OpticICs(not yet working !)

AGC / limiting amplifier CML decision circuit

PIN / transimpedance amplifier

Delta-Sigma ADC In Development (300 HBTs)

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