1

1.0-THz fmax InP DHBTs in a refractory emitter and self-aligned base process for reduced base access resistance

Vibhor Jain, Johann C. Rode, Han-Wei Chiang, Ashish Baraskar, Evan Lobisser, Brian J Thibeault, Mark RodwellECE Department, University of California, Santa Barbara, CA 93106-9560

Miguel UrteagaTeledyne Scientific & Imaging, Thousand Oaks, CA 91360

D Loubychev, A Snyder, Y Wu, J M Fastenau, W K LiuIQE Inc., 119 Technology Drive, Bethlehem, PA 18015

vibhor@ece.ucsb.edu, 805-893-3273

Device Research Conference 2011

2

Outline

• Need for high speed HBTs

• Fabrication

– Challenges

– Process Development

• DHBT

– Epitaxial Design

– Results

• Summary

3

0

5

10

15

20

25

30

35

40

109

1010

1011

1012

Tra

nsis

tor

Po

wer

Ga

in (

dB

)

Freq (Hz)

Why THz Transistors?

High gain at microwave frequencies precision analog design, high resolution ADC & DAC, high performance receivers

THz amplifiers for imaging, sensing, communications

Digital Logic for Optical fiber circuits

4

HBT process requirements

• Refractory emitter contact and metal stack

– To sustain high current density operation

• Low stress emitters

– For high yield

• Low base access resistance

– For improved device fmax

• Thin emitter semiconductor

– To enable a wet etched emitter process for reliability and scalability

5

Fabrication Challenges – Stable refractory emitters

Emitter yield drops during base contact, subsequent lift-off steps

High stress in emitter metal stack

Poor metal adhesion to InGaAs

Need for low stress, high yield emitters

Fallen emitters

6

Fabrication Challenges – Base-Emitter Short

Undercut in thick emitter semiconductor

Helps in Self Aligned Base Liftoff

For controlled semiconductor undercut

Thin semiconductor

To prevent base – emitter short

Vertical emitter profile and line of sight metal deposition

Shadowing effect due to high emitter aspect ratio

Slow etch plane

InP Wet Etch

Fast etch plane

7

Fabrication Challenges – Base Access Resistance

contacts

contact

gapsh,bcsh,esh,2612 AL

W

L

W

L

WR

e

gap

e

bc

e

e

bb

We

Wgap

Wbc

bcsh,esh,gapsh,,

• Surface Depletion

• Process Damage

Need for very small Wgap

• Small undercut in InP emitter

• Self-aligned base contact

cbbbCR

ff

8max

8

Composite Emitter Metal Stack

TiW

W

• W/TiW metal stack

• Low stress

• Refractory metal emitters

• Vertical dry etch profile

W emitter

Erik Lind

Evan LobisserTiW emitter

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TiW

W

Base

Metal

BCB

SiNx

100nm

Vertical etch profile

Low stress

High emitter yield

Scalable emitter process

Vertical EmitterFIB/TEM by E Lobisser

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InGaAs capMo contact

InP emitter

Dual SiN

sidewall

Controlled

InP undercut

Narrow BE gap50nm

Narrow Emitter UndercutFIB/TEM by E Lobisser

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Epitaxial Design

T(nm) Material Doping (cm-3) Description

10 In0.53Ga0.47As 81019 : Si Emitter Cap

20 InP 51019 : Si Emitter

15 InP 21018 : Si Emitter

30 InGaAs 9-51019 : C Base

13.5 In0.53Ga0.47 As 51016 : Si Setback

16.5 InGaAs / InAlAs 51016 : Si B-C Grade

3 InP 3.6 1018 : Si Pulse doping

67 InP 51016 : Si Collector

7.5 InP 11019 : Si Sub Collector

5 In0.53Ga0.47 As 41019 : Si Sub Collector

300 InP 21019 : Si Sub Collector

Substrate SI : InP

Vbe = 1 V, Vcb = 0.7 V, Je = 24 mA/mm2

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 50 100 150 200

En

erg

y (

eV

)

Distance (nm)

Emitter

Collector

Base

Thin emitter semiconductor

Enables wet etching

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Results - DC Measurements

BVceo = 3.7 V @ Je = 0.1 mA/cm2

β = 17

@Peak f,fmax

Je = 20.4 mA/mm2

P = 33.5 mW/mm2

Gummel plot

Common emitter I-V

0

5

10

15

20

25

30

0 1 2 3 4 5

Je (

mA

/mm

2)

Vce

(V)

P = 30 mW/mm2

Ib,step

= 200 mA

BV

P = 20 mW/mm2

Aje

= 0.22 x 2.7 mm2

10-9

10-7

10-5

10-3

10-1

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1

I c, I b

(A

)

Vbe

(V)

Solid line: Vcb

= 0.7V

Dashed: Vcb

= 0V

nc = 1.19

nb = 1.87

Ic

Ib

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TEM – Wide, misaligned base mesa

0.5 mm

50 nm

Small EB gap

Misalignment

FIB/TEM by H. Chiang

• 220 nm emitter-base junction

• 1.1 mm wide base-collector mesa

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RF Data

Ic = 12.1 mA

Je = 20.4 mA/mm2

Vcb = 0.7 V

P = 33.5 mW/mm2

0

5

10

15

20

25

30

35

109

1010

1011

1012

Gain

s (

dB

)

freq (Hz)

Aje

= 0.22 x 2.7 mm2

f = 480 GHz

fmax

= 1.0 THz

H21

U

W-Band measurements to verify f/fmax

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Base Post Cap

Ccb,post does not scale with Le

Adversely effects fmax as Le ↓

Need to minimize the Ccb,post value

c

postr

postcbT

AC

0

,

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Base Post Cap

Ccb,post does not scale with Le

Adversely effects fmax as Le ↓

Need to minimize the Ccb,post value

c

postr

postcbT

AC

0

,

Undercut below base post

0

2

4

6

0 1 2 3 4 5

Ccb (

fF)

Le (mm)

y = 1.09x - 0.02

No contribution of Base post to Ccb

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Base Metal Resistance

bc

e

metalbbW

LR

6metalsh,,

BaseIb

BP

• Rbb,metal increases with emitter length

fmax decreases with increase in emitter length

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Base Metal Resistance

bc

e

metalbbW

LR

6metalsh,,

BaseIb

BP

• Rbb,metal increases with emitter length

fmax decreases with increase in emitter length

200

300

400

500

0 5 10 15 20 25

Le = 3mm

Le = 4mm

Le = 5mm

f (

GH

z)

Je (mA/mm

2)

200

400

600

800

1000

0 5 10 15 20 25

Le = 3mm

Le = 4mm

Le = 5mm

f ma

x (

GH

z)

Je (mA/mm

2)

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Parameter Extraction

Jkirk = 23 mA/mm2 (@Vcb = 0.7V)

200

600

1000

200

300

400

500

600

0 5 10 15 20 25 30

f

f max (

GH

z) f

(GH

z)

Je (mA/mm

2)

Vcb

= 0V

Vcb

= 0.7Vfmax

2

3

4

5

6

0 5 10 15 20 25 30

Ccb (

fF)

Je (mA/mm

2)

Vcb

= 0.7 V

Vcb

= 0.5 V

Vcb

= 0 V

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Equivalent Circuit

Hybrid- equivalent circuit from measured RF data

Rex ~ 4.2 Wmm2

freq (1.000GHz to 67.00GHz)

S(1

,1)

S(2

,2)

S(1

,2)*

5S

(2,1

)/10

freq (100.0MHz to 67.00GHz)

S_para

mete

r_D

eem

bed_P

NA

..S

12d*5

S_para

mete

r_D

eem

bed_P

NA

..S

21d/1

0S

_para

mete

r_D

eem

bed_P

NA

..S

11d

S_para

mete

r_D

eem

bed_P

NA

..S

22d

S21/10S12x5

S11

S22

--- : Measured

x : Simulated

freq (1.000GHz to 67.00GHz)

S(1

,1)

S(2

,2)

S(1

,2)*

5S

(2,1

)/10

freq (100.0MHz to 67.00GHz)

S_para

mete

r_D

eem

bed_P

NA

..S

12d*5

S_para

mete

r_D

eem

bed_P

NA

..S

21d/1

0S

_para

mete

r_D

eem

bed_P

NA

..S

11d

S_para

mete

r_D

eem

bed_P

NA

..S

22d

S21/10S12x5

S11

S22

--- : Measured

x : Simulated

Ccb,x = 2.72 fF

Ccb,i = 0.52 fF

Rcb = 27 kW

Rc = 3.4 W

Rex = 7 W

Rbe = 86 W

Rbb = 27 W

Cje + Cdiff = 8.8 + 59.2 fF gmVbee-j

0.234Vbee(-j0.14ps)

Base

Emitter

Col

Ccg = 3.2 fF

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Summary

• Demonstrated DHBTs with peak f / fmax = 480/1000 GHz

• Small Wgap for reduced base access resistance High fmax

• Undercut below the base post to reduce Ccb

• Narrow sidewalls, smaller base mesa and better base ohmicsneeded to enable higher bandwidth devices

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Questions?

Thank You

This work was supported by the DARPA THETA program under HR0011-09-C-006.

A portion of this work was done in the UCSB nanofabrication facility, part of NSF funded NNIN network and MRL Central Facilities supported by the MRSEC Program of the NSF under award No. MR05-20415