280 GHz fT InP DHBT

with 1.2 m2 base-emitter junction area in MBE Regrown-Emitter Technology

Yun Wei*, Dennis W. Scott, Yingda Dong, Arthur C. Gossard, Mark Rodwell

University of California at Santa Barbara

This work was supported by the DARPA TFAST program and by the Office of Naval Research (ONR)

* RF Micro Devices, Infrastructure Product Line, GaN TechnologyCharlotte, North Carolina 28269, Tel: (704)319-2033; ywei@rfmd.com

Motivation for Regrown-Emitter HBT: InP vs. Si/SiGe

Advantages of InP ~20:1 lower base sheet resistance ~5:1 higher base electron diffusivity ~3:1 higher collector electron velocity ~4:1 higher breakdown at same f

Disadvantages of InP Production devices: large ~ 0.7 µm emitters High emitter resistance: scaling limit Large excess collector capacitance Non-planar device → low IC yield Low integration scales

The advantages of InP-based HBTs lie in the material system. The disadvantages lie in the device structure and fabrication technology.

Emitter Resistance is a Key HBT Scaling Limit

rateclock GHz 200for needed m- 7

margin noise logic Degrades

mV. 300 of fraction large

mV 150)mmA/ 01()m- 15(

excessive becomes resistanceemitter of drop Voltage

rateclock GHz 200for needed mmA/ 10

.1 where;

charging isdelay Largest

.or withcorrelated not well Delay ECL

2

logic

22

2

2max,

emitter

logiccollectorlogic

max

ex

eexcex

e

ceeCC

cb

cb

V

JIR

J

/TJAJ

V

T

εA

I

VC

C

ff

Io

RL

Rex

Noise margin

2kT/q+IoRex

Vin

Vout

Vlogic=IoRL

Vlogic

Why Emitter Regrowth ?

N- collector

N+ subcollector

base contact

Si3N

4

regrownInAlAs/InAs emitter*

emitter contact

collector contact

Target Benefits:

Eliminate emitter undercut etch

Eliminate base-emitter metal liftoff

Flared emitter structure → large contact, small junction → low emitter access resistance

Thick, ~2*1020/cm3-doped extrinsic base → low resistance: 250 Ohms/square → tolerant of contact metal migration

Thin, ~3*1019/cm3 -doped intrinsic base → low transit time → high current gain (less Auger)

Passivated base-emitter junction → reliability

Polycrystalline InAs has low resistivity, can play same role in InP as the polysilicon extrinsic emitter in Si/SiGe

N- collector

N+ subcollector

S.I. substrate

intrinsic base

extrinsic basebase contact

Si3

N4

regrown emitter *emitter contact

collector contact

N+ collectorpedestalimplant

Regrown emitter HBT RF fabrication process

0.3 um Intrinsic emitter0.3 um Intrinsic emitter

0.3 x 4 um2 regrown-emitter InP DHBT

extrinsic baseextrinsic base

base contactbase contactcollector contactcollector contact

extrinsic emitterextrinsic emitter

polyimidepolyimide

emitteremitter

collectorcollector

base plugbase plug

Initial DC/RF results using CSL (graded) InAlAs emitter

* Y. Wei, D. Scott, et al., IEEE EDL, May 2004, pp.232-4.

fT=162 GHzf

max=140 GHz

h21

0

5

10

15

20

25

30

1 10 100 1000

CSL Emitter 0.7 x 8 um2 SA

Gai

n (

dB

)

Frequency (GHz)

MSG/MAG

U

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7J

E (

mA

/um

2 )V

CE (V)

AE=0.7 um x 8 um

AE=0.3 um x 4 um

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0 0.3 0.6 0.9

I C,

I B (

A)

VBE

(V)

IC

IB

Peak fτ = 162 GHz, fmax = 140 GHz Common-emitter current gain, h21 = 20 ηC = 1.2, ηB = 2.2

First DC/RF results with improved surface & InP emitter

0.0 100

5.0 10-3

1.0 10-2

1.5 10-2

2.0 10-2

2.5 10-2

3.0 10-2

3.5 10-2

0 1 2 3 4 5 6 7

I C(A

)V

CE (V)

IB_step

=500 uA

Peak fτ = 183 GHz, fmax = 165 GHz Common-emitter current gain, h21 ~17

Abrupt base-emitter junction and InP emitter

0

5

10

15

20

25

30

1 10 100 1000

h2

1, U

, M

SG

/MA

G (

dB

)K

Frequency (GHz)

fT = 183 GHz

U

h21

MSG/MAG

K

0.7 x 8 um2, Ic=17 mA, Vce=1.3 V

fMAX

= 165 GHz

* D. Scott, Y. Wei, et al., IEEE EDL, June 2004, pp.360~362.

Breaks in Emitter Growth Increase Emitter Resistance

Narrowing or breakage of emitter regrowth - due to facet-dependent growth - due to high surface mobility of indium

intrinsicemitter

extrinsicemitter

SiN

Improving emitter film continuity

intrinsicemitterintrinsicemitter

Suppress indium migration on the regrowth facets by: orienting abrupt InP emitter 60o off [110] inserting alloy-graded InGaXAs1-X layers between the InP emitter and InAs cap

[110][110]

[100][100]

extrinsicemitterextrinsicemitter

SixNySixNy

HBT layer structure

Layer Material Doping (cm-3) Thickness (Å)

Emitter cap InAs 3e19 Si 800

Cap grade InGaXAs1-X 3e19 Si 500

N+ Emitter InP 3e19 Si 800

N- Emitter InP 8e17 Si 100

N-- Emitter InP 3e17 Si 300

Extrinsic base InGaAs 1~2e20 C 500

Etch stop InP 4e19 Be 20

Intrinsic base InGaAs 4e19 C 400

Set-back InGaAs 2e16 Si 200

Grade InGaAlAs 2e16 Si 240

Delta doping InP 3e18 Si 30

Collector InP 2e16 Si 1030

Regrown-Emitter InP DHBT with 0.34 µm2 junction: 280 GHz fT

VCE,sat< 0.9 V at JE=11mA/µm2

Peak AC current gain=30

Collector breakdown voltage VCEO=5 V

Peak fτ = 280 GHz, fmax = 148 GHz

Emitter access resistance Rex=11 Ohm, RexAe=13 Ohm-um2

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6

VCE

(V)I C

(mA

)

Ib_initial

=0 uA

Ib_step

=100 uA

10-7

10-6

10-5

0.0001

0.001

0.01

0 0.2 0.4 0.6 0.8 1

I B, I C (

A)

VBE

(V)

IB

IC

0

5

10

15

20

25

30

1 10 100 1000

IC=9.72 mA

VCE

=1.2 V

U,

MS

G/M

AG

, h

21 (

dB

), K

Frequency (GHz)

U

h21MAG/MSG

K

fT=280 GHzf

MAX=148GHz

ηB =3.2 ηC =1.2

Base Dopant Passivation by Hydrogen Degrades Performance

Hydrogen passivation of carbon base dopingincreases base sheet resistance

source of Hydrogen: PECVD-deposited SixNy

Solution: process uses hydrogen-free sputtered SixNy for surfaces present process still uses PECVD SixNy for sidewalls

need to also used sputtered SixNy for sidewallsSiNx coated InGaAs:C Passivation after RTA

Sputtered SiN PECVD SiN

Sheet Res. (Ohm/Sq) 265.333 1360.0

Cont. Resistance (Ohm-um) 81.333 4493.3 Cont. Resistivity (Ohm-um^2) 25.064 14848.9

SiNx coated InGaAs:C Passivation During MBE Regrowth MBE Annealed No MBE Anneal

Sheet Res. (Ohm/Sq) 3710.7 203.2 Cont. Resistance (Ohm-um) 4322.9 60.3 Cont. Resistivity (Ohm-um^2) 5082.4 18.0

extrinsic base

intrinsic base

base refractory

silicon nitride

emitter contact

regrown emitter

collectordepletionregion

subcollector

silicon nitridesidewall

Hydrogen-Free Sputter-Deposited SiN Sidewalls

substrate

WWWW

S. SiN

Sputter-deposited SiN process development:• 4 inch Si wafer uniformity testing• refractive index measurement using ellipsometer: RI=2.06• BHF wet etching rate testing: ~8 Å/min- Stoichiometry controllable by heating, gas ratio and pressure

Summary

InP HBT with emitter regrowth wide emitter contact, submicron emitter junction→ potential for reduced Rex

junction formation by regrowth, not mesa etching thick extrinsic base for reduce base resistance

Performance still limited by immature process technology breaks in emitter regrowth→ increased Rex

hydrogen passivation from sputtered SiN sidewalls → increased Rbb

Present results: 0.3 um x 4 um regrown-emitter InP HBT 280 GHz fτ , 148 GHz fmax , peak AC current gain=30 VCE,sat< 0.9 V at JE=11mA/µm2 rex=RexAe=13 Ohm-um2

Remaining improvements needed for 400-GHz-class device: hydrogen-free sputtered SiN sidewall further improvements in regrown emitter film continuity