Ultra Wideband DHBTs using a Graded Carbon-Doped InGaAs Base

Mattias Dahlström, Miguel Urteaga,Sundararajan Krishnan, Navin Parthasarathy, Mark Rodwell

Department of Electrical and Computer Engineering, University of California, Santa Barbara

mattias@ece.ucsb.edu 805-893-8044, 805-893-3262 fax

Wideband InP/InGaAs/InP Mesa DHBTMattias Dahlström

UCSB

Objectives: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 HBTs

Approach: narrow base mesa → moderately low Ccb

very low base contact resistance required→ carbon base doping, good base contact process high ft through high current density, thin layersBandgap engineering: small device transit time with wide bandgap emitter and collector

DHBT Layer Structure and Band Diagram

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

Vbe = 0.8 VVce = 1.5 V

M Dahlstrom

Grade 2E16 Si 240 Å

InP 1.5E19 Si 500 Å

InGaAs 2E19 Si 500 Å

InP 3E19 Si 2000 Å

UCSB

• 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

Emitter Collector

Base

Pc is immeasurably low: below 10 –7 cm-2

Critical for narrow base mesa HBT

• Carbon doping 6E19 cm-3

• Pd-based p-contacts• Careful ashing and oxide etch• RTP @ 300 C, 1 minute

InP/InGaAs/InP Mesa DHBTBase contact resistance Mattias Dahlström

UCSB

0 1 2 3 4 50

20

40

60

80

100

120

140

160

R=5.3+30.1*X (Ohm)

Res

ista

nce

(Ohm

)

Length (m)

The size of the base contacts must be minimized due to Ccb

s=722 /sq

???28108 cmc

InP/InGaAs/InP Mesa DHBTDevice Results

UCSBMattias Dahlström

0

1

2

3

4

5

0 0.5 1 1.5 2 2.5 3

I C (

mA

)

VCE

(V)

emitter junction: 0.44 m x 7.4 mIB step = 50 A=20-25

0.0 0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

J C (

mA

/um

2 )

VCE

(V)

IB step=200 A

Emitter 1x8 m

0

2

4

6

8

10

0 1 2 3 4 5 6 7 8

I C (

mA

)

VCE

(V)

emitter junction: 0.44 m x 7.4 mBVCEO=7.5 VIB step = 50 A

J=3.5 mA/um2 BVCEO=7.5 V

No evidence of current blocking or heating

0

5

10

15

20

25

30

35

1 10 100Frequency, GHz

MSG

h21

Mason'sGain, U

• Submicron HBTs have very low Ccb Characterization requires accurate measure of very small S12

• Standard 12-term VNA calibrations do not correct S12 background error due to probe-to-probe coupling

SolutionEmbed transistors in sufficient length of transmission line to reduce coupling

Place calibration reference planes at transistor terminals

Line-Reflect-Line CalibrationStandards easily realized on-wafer

Does not require accurate characterization of reflect standards

CPW lines suffer from substrate TE, TM mode coupling: thin wafer, use Fe absorber !lateral TEM mode on CPW ground plane… present above 150 GHz , must use narrower CPW grounds

Accurate Transistor Measurements Are Not Easy

Transistor in Embedded in LRL Test Structure

230 m 230 m

Corrupted 75-110 GHz measurements due toexcessive probe-to-probe coupling

Miguel UrteagaMattias Dahlstrom

UCSB

InP/InGaAs/InP Mesa DHBTDevice Results Mattias Dahlström

UCSB

0

5

10

15

20

25

30

1010 1011 1012

frequency (Hz)

Ga

in (

dB

) H

21,

U,

MA

G/M

SG

• 2.7 m base mesa, • 0.54 m emitter junction• 0.7 m emitter contact

• Vce=1.7 V

• J=3.7E5 A/cm2

f = 282 GHz; fmax=480 GHz = 25; BVCEO = 7.5 V

MAG/MSG

U

H21

InP/InGaAs/InP Mesa DHBTDevice Results Mattias Dahlström

UCSB

200

220

240

260

280

300

0

100

200

300

400

500

600

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

f t GH

z

fmax G

Hz

VCE

Aej=3.4 um2

J=4.4 mA/m2

0

50

100

150

200

250

300

350

0

100

200

300

400

500

600

0 1 2 3 4 5 6

f t GH

z

fmax G

Hz

J (mA/um2)

Vcb=0.9 V

• Emitter contact sizes

0.5-2.0 um, 8 um long.•Base extends 0.25-1.0 um

on each side of the contact•Maximum current density

>10 mA/um2

• Vce >1.5 V for best performance

• Best ft found at

current density of 3-5 mA/m2

fmax measurement above 500 GHzcurrently not reliable in CPW environment

InP/InGaAs/InP Mesa DHBTConclusions Mattias Dahlström

UCSB

Doping-graded base InGaAs/InP Mesa DHBT:

• High current density Operates up to 10 mA/m2 without destruction …Kirk threshold 4.4 mA/m2 at 1.5 V • ft of 280 GHz with a 220 nm collector• fmax is 450 GHz or higher• Rbb is no longer a major factor - excellent base ohmics• fmax no longer a good measure of Ccb or circuit performance• Ccb reduction a priority• 87 GHz static frequency divider circuit already demonstrated

Narrow-mesa DHBT:base design

1015

1016

1017

1018

1019

1020

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8Fermi-Dirac, Boltzman, Joyce-Dixon and Selberherr

Acceptor concentration

Ene

rgy

(eV

)

SelberherrBoltzmannJoyce-DixonFermi-Dirac numerically

Doping graded base:At degenerate doping levels (>1E19) the variation of the Fermi level in the base is very rapidExponential doping roll-off not needed, linear roll-off good enough!

Many approximate methods for determining Ef such as Boltzmann, Joyce-Dixon areinsufficient

Mattias Dahlström

UCSBE

ne

rgy

(eV

)

Base doping (cm-3)

100 200 300 400 500 600 700 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Base width (A)

b (

ps)

Base transit time

Base transit timeConstant structure case

Narrow-mesa DHBT:base design

Base transit time calculation:-Bandgap narrowing-Fermi-Dirac statistics- doping and bandgap dependent mobility

Mattias Dahlström

UCSBT

ran

sit

time

(p

s)

Base width (A)

The exit term (electron velocity in top of collector) important for thin bases: use InGaAs, not InP, close to base