Ultra High Speed InP Heterojunction Bipolar Transistors

Mattias Dahlström

Trouble is my business, (Raymond Chandler)

Ultra High Speed InP Heterojunction Bipolar Transistors

• Introduction to HBT’s

• How to make a fast HBT…– Delay terms– The graded base– The base-collector grade

• Recent results– Record fmax mesa DHBT*

– Record f DHBT

*details regarding this to follow

The transistor

0

5

10

15

20

25

30

0

1

2

3

4

5

6

7

0 0.5 1 1.5 2 2.5

J (A/um

2 )I C (

mA

)V

CE (V)

Ib in 200 A steps

Schematic of an HBT Typical common-emittercharacteristics

Small change in base current large change in collector current

InP lattice structure

Nearest neighbor: 2.5 ALattice constant:5.86 A

InP and InGaAs have -L separations of ~0.65 eV, vs ~0.4 eV for GaAs→ larger collector velocityInGaAs has a low electron effective mass → lower base transit time

InGaAs

InP

Objectives and approachObjectives:fast HBTs → mm-wave power, 160 Gb fiber opticsdesired: 440 GHz ft & fmax, 10 mA/m2, Ccb/Ic<0.5 ps/Vbetter manufacturability than transferred-substrate HBTsimproved performance over transferred-substrate HBTs

Approach:

narrow base mesa → moderately low Ccb

very low base contact resistance required, and good alignment → carbon base doping, good base contact process

high ft through high current density, thin layers

bandgap engineering: small device transit time with wide bandgap emitter and collector

Potential uses of InP HBT

Communication systems:• wireless communication, fiber optics transceivers, • digital processing in radar (ADCs, DACs)

Types of circuits:• broadband amplifiers, power amplifiers, laser/modulator drivers• comparators, latches, fast logic

Circuit characteristics • 1-10 000 HBTs per IC• Very high demands for speed (40-200 GHz)• Fast logic with moderate power consumption (~20 mW/gate)• Moderate Output Power

mmwave power amps, optical modulator drivers ~6 V at Jc=4 mA/μm2 , ~2 V at Jc=8 mA/μm2

DHBT band diagram: under bias

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

0 50 100 150 200 250 300 350 400

E (

eV)

Distance (Å)

Ec

Ev

base

emitter

collector

High speed HBT: some standard figures of merit

• Small signal current gain cut-off frequency (from H21)

• Maximum power gain ( from U)

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

bcibbCR

ff

8max

VI

C

c

cb

•Collector capacitance charging time when switching :

Scaling laws for fast HBTsfor x 2 improvement of all parasitics: ft, fmax, logic speed…base 2: 1 thinnercollector 2:1 thinneremitter, collector junctions 4:1 narrowercurrent density 4:1 higheremitter Ohmic 4:1 less resistive

Challenges with Scaling:Collector mesa HBT: collector under base Ohmics. Base Ohmics must be one transfer lengthsets minimum size for collector Emitter Ohmic: hard to improve…how ?Current Density: dissipation, reliabilityLoss of breakdownavalanche Vbr never less than collector Eg (1.12 V for Si, 1.4 V for InP) ….sufficient for logic, insufficient for power

emitterbase contact

collectorcontact

SI substrate

InGaAs subcollector

InP collector

InGaAscollector

InP subcollector

InGaAs base

undercutcollector junction

narrow collector mesa

transferred-substrate

Contact resistance: tunneling through barrier

• High doping: 1-9 1019 cm-3

• Small bandgap: InAs<InGaAs<InP<GaN• Surface preparation: no interstitial oxide• Metal reactions

Theory: idealized contact

barrierd

bi

barrier

d

c

Em

qNV

Emx

*221exp

1

*2exp

1

d

bisd qN

Vx

2

Pd-based contacts

• Pd/Pt reacts with III-V semiconductor:

InGaAs+Pd As + (In,Ga)Pd+(In,Ga)(Pd,As)

• Pd reaction depth ~4 x thickness• 25 Å Pd for 300 Å base

•Contact resistance: 100-500 -m2 1-20 -m2

from TLM and RF-extraction

Ohmic contact to p-type material 10-100 times worse than n-type.Work function line-up, electron/hole effective mass

Yu, J.S.; Kim, S.H.; Kim, T.I.  ‘’PtTiPtAu and PdTiPtAu ohmic contacts to p-InGaAs’’, Proceedings of the IEEE Twenty-Fourth International Symposium on Compound Semiconductors, San Diego, CA, USA, 8-11 Sept. 1997

Emitter resistance

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

ee

ecex WL

R ,

Emitter resistance: grades removed

At degenerate doping levels grades are not necessary

-1.2

-0.8

-0.4

0

0.4

1012

1013

1014

1015

1016

1017

1018

1019

0 50 100 150 200 250 300 350 400

E (

eV)

n (cm-3

)

Distance (Å)

Ec

Ev

electrons

-1.6

-1.2

-0.8

-0.4

0

1015

1016

1017

1018

1019

0 50 100 150 200 250 300 350 400

E (

eV)

n (cm-3

)

Distance (Å)

Ec

Ev

electrons Contact resistance:50 m2 25 m2 15 m2

High doping 3 1019 cm-3

No InGaAs-InP grade necessary at very high doping

Thin undepleted n- emitter

Small emitter area increases Rex

InGaAscap layer InP

emitter

light doping

heavy doping

Base resistance

bcibbCR

ff

8max

Rbb is a critical parameter for fmax, and in npn HBT the base contact resistance dominates.Rbb is minimized through high base doping and improved base contact metallization, small undercut Wgap, and long emitter Le

eesspread

egapsgap

ecscontb

spreadgapcontbbb

LWR

LWR

LR

RRRR

12

2

2,

,

TLM measurement

Problems with very thin bases

• Etching and depletion effects reduce the effective base thickness Tb, and increases the base resistance.

• At 500 nm scaling generation, best base thickness is 30-40 nmbetter fmax , lower Rbb-related delay terms in gate delay ,minimal improvement in f between 25 & 30 nm

High resistance

Increase of sheet resistance with thin base layers

A 51~

intrinsic,extrinsic, bb TTT-1

-0.5

0

0.5

1

1012

1013

1014

1015

1016

1017

1018

1019

1020

0 50 100 150 200 250 300

E (

eV) p (cm

-3 )

Distance (Å)

Ec

Ev

InGaAs base doped 6 1019 cm-3, surface pinned at 0.18 eV.

Surface depletion decreases base thickness 40 Å.

d

bisd qN

Vx

2 extrinsic,

intrinsic,intrinsic,extrinsic,

b

bbb R

RTT

Rb,extrinsic=800-1000 Ω/sqRb,intrinsic=600-750 Ω/sq

A 17~

intrinsic,extrinsic, bb TTT

Base protected by E/B grade(contacts diffused through 160 Å grade)

Surface depletionWet etching

Base surface exposed :

T~

Collector resistance

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

Rc: access resistance between collector contact and the mesa. Minimized by large collector contacts, and low resistance subcollector

Subcollector design

Some still use all InGaAs subcollector…Subcollector resistivity500 A InGaAs + 2000 A InP ~ 11 /sq125 A InGaAs + 3000 A InP ~ 9 /sq

Etching selectivity of InGaAs vs. InP main limit 50 A InGaAsContact resistance better to 125 A than 50 A after annealing

Goals: • minimize electrical resistance • minimize thermal resistance• limit thickness to improve

manufacturabilityThermal conductivity of InGaAs ~5 W/mKThermal conductivity of InP ~68 W/mK

Tsubc

Etch stop layer provides collector undercut – less Cbc

- 53 % of thermal resistance

Base-emitter capacitance

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

Cje is the junction capacitance between the emitter and baseCje corresponds to ~100 Å depletion thickness

Minimized by shrinking the emitter area at fixed or at increasing current Ic

Base-collector capacitance

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

Cbc is the junction capacitance between the base and subcollector.

c

bcbc T

AC

Base-collector capacitance

c

bcbc T

AC

Tc = 3000 A 2150 A 1500 A

Abc must be kept small:• narrow emitter• narrow base contacts• undercut of base contacts

• implant or regrowth

Breakdown limits thickness

Thickness (A) Breakdown (V)

2150 7.5

1500 4-5

Collector thickness reduced due to speed requirements:

Ccb increases !

yinstabilit thermal

heating,by ndestructio

J highat decreases

ingby tunnell limited

collectors Very thin

/

collectors Thin

:collectorsThick

,

br

br

InPgapbr

cbr

V

V

qEV

TV

Theory of the base

If gain is limited by Auger recombination in the base:

22

,2

ba

baseeRB

TN

TAk

basen

bb D

T

,

2

2

is 100-250 fs)(calcb

is 10-50 • Decreasing increases . • High Na and Tb for low s decreases• Grade gives 30-50 % improvement

b

babasehs TNq ,

1

The base sheet resistance:

The base transit time:

ps is 400-900 /sq

Base Transit Time

dzdxznzD

zN

xN

xndx

J

Q bbb T

x iz

a

T

a

i

T

c

bb

)()(

)(

)(

)(2

0

2

0

)N - (N D )N-N D (

e D eN e N )N e D e N e

a1i11a1i11

) WN WD - D - W(-N1

)(-D a1

)(-Di1i1

)D - W(-D1

)D - W(-Da1

)D - W(-D

int,

ba1b12bi1222b12b12b1

b

bT

bi

i

a

baexit Tn

xn

xN

TN

02

2

)(

)(

)(

)(

)N-)(N(Tnv

)e- (e )(TN

a1i1b2is

)N-(N)N-TN-NT(Nba

a2i2a2ba1i2bi1

exit

Fitting of relevant parameters of the form )()( BAzexf

With doping as

)( 21 aa NzNa eN

Intrinsic carrier concentration )(2 21 ii NzN

i en

Diffusivity

)( 21 DzDeD

Kroemer’s double integral:Drift-Diffusion equation for base current:

Exit term

Solution used for evaluation of the base transit time:

2bT

exitb vT Ballistic injection: bT

qdx

dqJ n

nn

0pJ

Base grading

Graded bandgap Graded doping

Doping 8 5 1019 cm-3Change in In:Ga ratioInAs: Eg=0.36 eVGaAs: Eg=1.43 eV

Base grading: induced electric field

Induced electric field accelerates electrons towardscollector – decreases base transit time and increases gain

Limits: strain Limits: Bandgap narrowing, needs degenerate doping

The effect of degenerate doping

Evidence: Observed Vbe increase Von ~ φbi , increases with Ev

Nb=4 1019cm30.75 VNb=8 1019cm30.83 Vfor graded base-emitter

Strong variation in Fermi-level with doping at high doping levels

Base bandgap narrowing

Model after V. Pavlanovski

Bandgap grade

Doping grade

BGN provides an electric field opposing the doping-induced field.

~1:5 in magnitude

0

0.5

1

1.5

200 300 400 500 600 700 800 900 1000

Base transit time

bandgap grade

constant

doping grade

Ba

se tr

ans

it tim

e (

ps)

Base thickness (A)

Base Transit time

Ballistic effects may arise when Tb<180-200 @5 1019 cm-3 (Tessier, Ito)

Results: Bandgap graded Doping gradedDC gain 25 18ft 250 GHz 282 GHz

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

Bandgap grade and doping grade give same b

Collector design

Transit time:

Close inspection show velocity near base most important

effective

cc v

T

2

Grade No Grade

dxxv

xT

Tc

T

cc

c

)(

1

0

-Use grade

-Use setback

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

-2

-1.5

-1

-0.5

0

0.5

1

0 50 100 150 200

E (

eV)

Distance (Å)

Ev

Ec

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 50 100 150 200

E (

eV)

Distance (Å)

Ev

Ec

Base-collector grade

Early grade designs:• Too coarse• No setback layer

Recent grade designs:• 15 A period• 200 A setback layer

Gain: 7f: 128 GHz (Tc=3000 A)Jkirk: 1.3 mA/μm2

Gain: 27f: 282 GHz (Tc=2150 A)Jkirk: 4 mA/μm2

InAlAs/InGaAs super lattice

• Why super lattice?– MBE is more suited for super lattice than

quaternaries.– InP/InGaAs gives poor quality material due to

phosphorous-arsenic intermixing

• MOCVD growth → InGaAsP grade

• GaAsSb base needs no grade

• Quantum well trapping• Electron/hole in the InGaAs well• 500 meV InAlAs potential barrier

A rough approximation: the infinite potential well.

....3,2,1....2 2

222

nma

nEn

If En> 500 meV (InGaAs/InAlAs potential) no electron confinement ~31 A is the maximum allowed InGaAs width by this model

Quantum mechanical trapping in grade

The delta-doping

grade

cr

Tq

ETN

2

H. Kroemer : a conduction band difference can be offset with a grade and a delta-doping

With this choice the conduction band will be smooth

No delta-doping Delta-dopingVbc=0.3 V Vbc=0.3 V

The setback layer

• An InGaAs layer beneath the base – Margin for Base dopant diffusion– Increases Electron speed at SL

SetbackVbc=0.3 V

No setbackVbc=0.3 V

Collector design: doping

2max,

min,

)(2

dopingcollector allowable maximum thespecifies This

ECL)in Volts 0.0 (e.g. specified minimum someat

)(low collector depletedfully a want illdesigner wCircuit

CqT

VN

V

C

cbd

cb

cb

Collector design: velocity and scattering

No -L scattering

-L scatteringpossible

Collector band profile designed for greatestpossible distance without -L scattering

Collector under current (simulation)

-2

-1.5

-1

-0.5

0

0.5

0 100 200 300 400

J=0mAJ=1mAJ=2mAJ=3mAJ=4mAJ=5mAJ=6mAJ=7mAJ=8mA

E (

eV

)

Position (A)

Nc reduced by Jc/q/vsat

Current blocking

Metal resistance

• Resistance of e-beam deposited metals higher than “book” values.

• Metal resistance increases when T<1000 A

2.2

2.4

2.6

2.8

3

3.2

3.4

0 500 1000 1500 2000 2500 3000 3500

Au

cm

Gold thickness (Å)

Problem for base contact (PdTiPdAu with 600 A gold)sm=0.5 Ω/sq 3-8 Ω added to Rbb

•TiPdAu 200/400/9000 A•PdTiPdAu 30/200/400/600 A•TiPdAu 200/400/4000 A

Reduces fmax

Thermal stability?

Results

• 2150 A collector high fmax, high Vbr,CEO

IPRM 2002, Electron Device Letters, Jul. 2003; M. Dahlström et al, ''Ultra-Wideband DHBTs using a Graded Carbon-Doped

InGaAs Base''

• 1500 A collector high f, high fmax , high Jc

Submitted to DRC 2003; M. Dahlstrom, Z. Griffith et al.,“InGaAs/InP DHBT’s with ft and fmax over 370 GHz using Graded

Carbon-Doped Base”

InGaAs 3E19 Si 400 Å

InP 3E19 Si 800 Å

InP 8E17 Si 100 Å

InP 3E17 Si 300 Å

InGaAs graded doping 300 Å

Setback 2E16 Si 200 Å

InP 3E18 Si 30 Å

InP 2E16 Si 1700 Å

SI-InP substrate

Grade 2E16 Si 240 Å

InP 1.5E19 Si 500 Å

InGaAs 2E19 Si 500 Å

InP 3E19 Si 2000 Å

• 300 A doping graded base

• Carbon doped 8*10195* 1019 cm-2

• 200 Å n-InGaAs setback

• 240 Å InAlAs-InGaAs SL grade

• Thin InGaAs in subcollector

High fmax DHBT Layer Structure and Band Diagram

Vbe = 0.75 V

Vce = 1.3 V

Emitter Collector

Base

InGaAs 3E19 Si 400 Å

InP 3E19 Si 800 Å

InP 8E17 Si 100 Å

InP 5E17 Si 400 Å

InGaAs graded doping 300 Å

Setback 3E16 Si 200 Å

InP 3E18 Si 30 Å

InP 3E16 Si 1030 Å

SI-InP substrate

Grade 3E16 Si 240 Å

InP 1.5E19 Si 500 Å

InGaAs 2E19 Si 125 Å

InP 3E19 Si 3000 Å

• Thinner InP collector

•Collector doping increased to 3 1016 cm-3

• Thinner InGaAs in subcollector

•Thicker InP subcollector

High f DHBT Layer Structure and Band Diagram

Vbe = 0.75 V

Vce = 1.3 V

Emitter Collector

Base

Results: DC

0

2

4

6

8

10

0 0.5 1 1.5 2 2.5

J c (m

A/

m2 )

Vce

(V)

Ajbe

=0.6 x 7 m2

Vcb

= 0 V

Ib step

= 0.4 mA

0

1

2

3

4

5

0 0.5 1 1.5 2 2.5 3

I C (

mA

)

VCE

(V)

emitter junction area: 0.44 m x 7.4 mIB step = 50 uA

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5 6 7 8

High fmax DHBT High f DHBT

Gain: 23-28nb/nc: 1.05/1.44Vbr,CEO: 7 V

Gain: 8-10nb/nc: 1.04/1.55Vbr,CEO:4 V

No evidence of current blocking or trapping

Results: RF

0

5

10

15

20

25

30

1010 1011 1012

Gai

n (

dB)

Frequency (Hz)

f=282 GHz

fmax

=400 GHzU

H21

Ajbe

= 0.54 x 7.6 um2

Ic = 15 mA

J = 3.6 mA/um2 , Vce

= 1.7 V

High fmax DHBT High f DHBT

0

5

10

15

20

25

30

1010 1011 1012

Ga

ins

(dB

)

Frequency (Hz)

ft= 370 GHz

fmax

=375 GHzU

H21

MAG/MSG

Ajbe

= 0.6 x 7 um2

Ic = 30 mA

J = 7.2 mA/um2 , Vce

= 1.3V

• Highest fmax for mesa HBT • Highest f for mesa DHBT• Highest (f, fmax) for any HBT• High current density

Results: Base width dependence

260

270

280

290

300

310

320

330

f t (G

Hz)

Wb=0.3 m W

b=0.5 m W

b=1.0 m

Je=5.9 mA/m

2

Je=7.2 mA/m

2

260

270

280

290

300

310

320

f max

(G

Hz)

Wb=0.3 m W

b=0.5 m W

b=1.0 m

Je=5.9 mA/m

2

Je=7.2 mA/m

2

Emitter junction 0.6 x 7 m, Vce=1.3 VTb=300 A. Tc=1500 A

bcibbCR

ff

8max

bccexbcjec

Bcb CRRCC

qI

Tk

f

2

1

f maxf

Results: RF - trends

0

50

100

150

200

250

300

0 1 2 3 4 5 6

f t (G

Hz)

Je (mA/um2)

0.9 V

1.0 VV

ce=1.7 V

Vce

=0.75 V

1.25 V

200

220

240

260

280

300

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

f t

fmax

VCE

f

fmax

Variation of f vs. Ic and Vce , of an HBT with a 0.54 m x 7.7 m emitter, and a 2.7 m width base-collector junction.

Variation of f and fmax vs. Vce , of an HBT with a 0.54 m x 7.7 m emitter, and a 2.7 m width base-collector junction. Ic=20 mA.

sdsd

Need higher Vce for high current f drops at high Vce

high Vce for full collector depletion

Results: evolution

100

150

200

250

300

350

400

DHBT3 DHBT6 DHBT9 DHBT18

f t (G

Hz)

f fmax

100

150

200

250

300

350

400

450

DHBT3 DHBT6 DHBT9 DHBT18

f max

(G

Hz)Final grade

Old grade

New grade

0

1 105

2 105

3 105

4 105

5 105

6 105

7 105

8 105

DHBT3 DHBT6 DHBT9 DHBT18

J opt (

mA

/um

2)

DHBT 19B

Strong improvement in f and Jopt

Jopt

f and fmax > 200 GHz at Jc >10 mA/m2

Tc =1500 A

Capacitance vs. current

16

16.5

17

17.5

18

18.5

0 1 2 3 4 5 6 7 8

Ccb

(fF

)

Je (mA/um2)

Vce

=1.5 V

Vce

=1.3 V

Emitter junction 0.5x7.6 umTc= 1500 A, Nc=3 1016 cm-3

DHBT 20Graded emitter base junction

DHBT 17Abrupt emitter base junction

Emitter junction 0.54x7.6 um and 0.34x7.6 um. Tc= 2150 A, Nc=2 1016 cm-3

2max

1

ckirk T

JJ

10

11

12

13

14

15

16

0 1 2 3 4 5 6

Ccb

(fF

)

Je (mA/um2)

Vce

=1.5 V

Vce

=1.7 V

Vce

=1.7 V

Jmax~3 mA/m2 Jmax~6.5 mA/m2

48 % Jmax~3.2 mA/m2 for Tc=2150 A

Area dependence on capacitance reduction

0

5

10

15

20

25

30

0.1 0.15 0.2 0.25 0.3 0.35 0.4

CC

B r

ed

uct

ion

(%

)

Emitter to basemesa ratio

Vce

=1.5-1.7 V

DHBT 17

DHBT 17

DHBT 18

DHBT 19

DHBT 20DHBT 20

DHBT 19

DHBT 20

DHBT 19

DHBT 20DHBT 20

We

BB

C

E

Wbc

bc

e

W

Wratio

Ccb from Y-parametersat 5 GHz

Vce=1.3 V

Vce=1.5 V

Extrapolating with linear fit gives 55 % for r=1

Ccb is reduced where the current flows reduce extrinsic base

Max current density vs. emitter size

The current at which Ccb increases (Jmax) as a function of emitter width for two different HBT

3

4

5

6

7

8

0.2 0.4 0.6 0.8 1 1.2 1.4

Emitter width (m)

Vce

=1.5 V

Tc=1500 A

J C

(m

A/u

m2)

Vce

=1.7 V

Tc=2150 A

• Narrow emitters have higher critical current density• Not necessarily higher ft (due to Rex) - Current spreading

200

220

240

260

280

300

2 2.5 3 3.5 4 4.5 5 5.5 6

f t (G

Hz)

Je (mA/um2)

We=0.5 m

We=0.7 m

We=0.6 m

Calculation of current spreading

cc

r v

xJqN

dx

d )(1E

e

c

de

eee

LT

xLW

LWJxJ

2

)(

1

22

21ln2

21ln

2

d

e

dd

e

de

d

e LW

LL

W

LW

L

W

VV

TqNvJ bcbi

c

rcckirk 2

2

• Poisson’s equation with depth dependant current J(x)

• Solving double integral provides Kirk threshold correction term

• J now has emitter width dependencecced DL ,

ee WL

0)0( xE

at Jkirk

Lateral diffusion

One-dimension

Kirk condition

Summary of delay terms

Tau_ec 503.65 fs

RexCcb 31.4 fs 6.1 %RexClay 11.27 fs 2.2 %tau_f 385.62 fs 75.3 %

kT/qI times Cje 48 fs 9.4 %kT/qI times Ccb 26 fs 5.1 %kT/qI times Clayout 9 fs 1.8 %

SUM 512 fs 100.0 %

ft_corr 311 GHz Rex-related 8.3 %ft_meas 316 GHz

Emitter heat sinking

Emitter interconnect metal 2 μm to 7 μm

Process improvements: local alignment

Machine alignment provides <0.2 μm alignment in good weeks

Process improvements: lift-off

• Improved hardening of top resist surface• 0.4 x 8 μm emitters, ~1 μm thick

What to do in the future: short term

• Have new material with InAs rich emitter cap less Rex increased f

• Doping grade and combined grade less b increased f ?

• Small scale circuits by Z. Griffith and others• Write paper on Kirk effect / collector current spreading

Hålls me slåttern

What to do in the future: long term

• Need a more SiGe like processing technology– Lift-off– Isolation – Emitter regrowth

• Work on HBT design– Emitter design– Base grade

• See circuits come out …

Summary of work

• Linear base doping grade

• New base-collector grade

• Pd based base ohmics

• Narrow base mesa HBT– Record fmax

– Record f

• InAs HEMT’s

Conclusion

• Mesa HBT can achieve superior performance to T.S.

• InAlAs/InGaAs S.L. grade permits use of InGaAs for base and InP for collector

• Excellent transport characteristics in collector

• InGaAs setback layer improves b-c grade

• PdTiPdAu base ohmics can achieve p-type contact resistance as good as n-type

in case of questions

Results: base-collector capacitance

0

10

20

30

40

50

60

70

0 1 2 3 4 5

Ccb

(fF

)

Je (mA/um2)

0.75 V

Vce

=1.7 V

1.1 V1.25 V

1.0 V

0.9 V

Full depletion

Variation of Ccb vs. Ic and Vce. Note that

Vbe=0.85-0.90 volts over the same bias range.

Hole mobility extraction

• With measured base sheet resistance and doping level the base hole mobility can be estimated

30

40

50

60

70

80

90

CBe

SHBT DHBT9 MHBT1 DHBT18 DHBT219 DHBT220

uh (

cm2 /

Vs)

30

40

50

60

70

80

90

2 1019 6 1019 1 1020 1.4 1020

Ba

se h

ole

mob

ility

(cm

2 / V

s)

Base doping cm-3

bbhs TNq

1

Collector velocity from Kirk threshold

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2

J m

A/u

m2

Vce

(V)

200 nm collector, Ae=0.62 m x 7.7 m

cm/s102.4 7

cm/s100.3 7

csat

cgradesetccsat

grade

gradesetc

c

crsat

c

biappliedrsatkirk

Nqv

TTTTTNNTqv

T

TTT

Tq

Eqv

qT

VVqvJ

2

22

2

122

22

)(2

Slope corresponds to collector saturation velocity

Collector velocity from bc

0

100

200

300

400

500

600

0 1000 2000 3000

t c (fs

)

Tc (A)

sat

cc v

T

2

slopedTd

Tv

c

cc

csat

1

2

11

2

1

2

m/s1005.3

m/s1046.45

5

satv

m/s1005.3 5satv

InP-InGaAs and InP-GaAsSb

Base-collector grades necessary

Grades not necessary

H21 at 5 GHz vs. current

19

20

21

22

23

24

25

26

27

0 1 2 3 4 5 6 7

Ga

in

Je (mA/um2)

Vce

=1.25 1.5 1.75 V

Emitter junction 0.5x7.6 umE0.7 B05

Gain does not depend on Vce , but on bias.Max gain around 26.5

Current RF gain vs. voltage

Heating likely cause

Results: Gummel

DHBT 20: Capacitance cancellation data

0

50

100

150

200

250

0.5 1 1.5 2 2.5 3

f t and

fm

ax (

GH

z)

Vce

(V)

I=20 mA

ft

fmax

Not max ft,fmax (current too low for that, but wanted to avoid blowing)cc

Theory: G-L scattering reduces collector transit time and heating

Capacitance cancellation

Previous slide CE

ec

CB

c

VV

f2

1ec

CB

cc

c

Ere,cb dV

dI

T

AC

0

2 10-13

4 10-13

6 10-13

8 10-13

1 10-12

1.2 10-12

0.5 1 1.5 2 2.5 3

t ec

Vce

(V)

I=20 mA

4/I fF/A

4 fF reduction from ft vs. Vce relation, very close to measured

Results: RF validity

0

5

10

15

20

25

10 100 1000

Gai

ns (

dB)

Frequency (Hz)

ft=370 GHz

fmax

>370 GHz

U

H21

W-band measurements one week apart Re-measurements show similar ft and fmax. Roll-off is very close to -20 dB/decade in the 75-110 GHz band.

Resistance vs. doping

InGaAs and InP n-type doping : 1-3 1019 cm-3

InGaAs p-type doping 1.2 1020 cm-3: no p-InP with C doping

Mesa HBT mask set: first iteration

Emitters 0.4, 0.5, 0.6, 0.7, 1.0, 2.0 μm wide, 8 μm long for RF measurementsBase extends 0.25, 0.5 and 1.0 μm on each side of baseBase plug in revision 1Emitter ground metal 2 μm wide

Mesa HBT mask set: second iteration

Emitters 0.4, 0.5, 0.6, 0.7, 1.0, 2.0 μm wide, 8 μm long for RF measurementsBase extends 0.35, 0.5 and 1.0 μm on each side of baseBase plug now on smaller tennis-racquet handleEmitter ground metal extended to 7 μm width

RF measurements: CPW structure

230 m 230 m

RF measurements: air bridges

120m

New m: /4=137 um

117 m 120m

RF measurements: calibration

• LRL calibration using on wafer Open, Zero-length through line, and delay line

• OLTS used to check U in DC-50 GHz band

• Probe pads separated by 460 m to reduce p-p coupling

• RF environment not ideal, need: thinning, air bridges, vias for parasitic mode suppression

RF parameter extraction

0

2

4

6

8

10

0 50 100 150

1/R

e(Y

21)

I-1 (A-1)

Rex

=3.4

n=1.45

)(

1

21Y

RR bb

ex

Emitter resistance

(Error page 101 eq. 5.4)

cbbbcbicb

CjRCRY

2

121

Base collector capacitanceBase collector resistance

Base collector delay time, ideality factor and capacitance

bccexbcjec

Bcb CRRCC

qI

Tnk

f

2

1

I

VC

.

• Switching speed limited by output capacitance

How do we get speed improvement

Design Specifications set ΔV and RL sets I

Reduce C by decreasing AC

Increase in J since I fixed J limited by Kirk Effect Increase in J increase dissipated power density

Formula simplisticinsight

Can we measure Rth (Method of Lui et al )

0.50 0.520.48 0.54

0.002

0.004

0.006

0.008

0.000

0.010

VBE

IC.i

VBE

I_DCSRC2Idc=IB

L8E7B21X1

I_ProbeIC V_DC

SRC1Vdc=VCE

Ramp IB for different VCE

Measure VBE and IC

CCE

BET IV

VR

Depends on current density

2 3 4 5 6

x 10-3

1000

2000

3000

4000

5000

PAve

RT

Large uncertainty in values. Fitting regression curves helps to reduce error

Validation of Model

0

5

10

15

20

25

30

35

40

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

centerEdge

Tem

per

atu

re R

ise

(K)

Distance from substrate (m)

SC ES C B E E Metal

Caused by Low K

of InGaAs

Max T in Collector

Ave Tj (Base-Emitter) =26.20°CMeasured Tj=26°CGood agreement.

Advice Limit InGaAs Increase size of emitter arm

Ultra High Speed InP Heterojunction Bipolar Transistors

Why this title?

• Some recent conference results show transistor f of 130 GHz…

• InP is a brittle semiconductor with a metallic luster. We mix it with GaAs and AlAs. Use Si and C as dopants

• Heterojunction: contains junctions of different materials

DHBT carrier profile

1015

1016

1017

1018

1019

1020

0 50 100 150 200 250 300 350 400-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

N (cm

-3)

Distance (Å)

Base Collector

electrons

Emitter Subcollector

holes

quick comment that this is unbiased....under bias both DR will fill with E