High speed (207 GHz f), Low Thermal

Resistance, High Current Density Metamorphic InP/InGaAs/InP DHBTs grown on a GaAs Substrate

Y.M. Kim, M. Dahlstrǒm, S. Lee, Y. Wei, M.J.W. Rodwell, A.C. Gossard

Department of Electrical Engineering, Materials Department,

University of California, Santa Barbara

Technical Objective

Growth of InGaAs/InAlAs/InP HBTs on GaAs substrates

….with low leakage and high yield

….for low-cost high-volume manufacturing of InP HBT integrated circuits on 6" diameter substrates

Gives basic data for growth which is free of lattice constant limit

Why InP-based HBTs ?better device bandwidth than GaAs or Si bipolar transistors

microwave ADCs, DACs, digital frequency synthesisbetter Emaxvsat than GaAs

millimeter-wave power

Why metamorphic HBTs ?--economic argumentlow cost, high volume processing: wafer size is critical GaAs substrates, processes: 6" diameter now large InP substrates:

high cost, high breakage, only 4" available todaybreakage much worse with 6" wafers

grow InP-based HBTs on GaAs substrates for cost and manufacturability

Metamorphic HBTs

InGaAs/InP or InGaAs/InAlAs HBT on a GaAs substrate

Lattice mismatch between substrate and epitaxial device layersThick intervening buffer layer to capture most defects

InGaAs base

emitter

base

collector

InAlAs or InP emitter

InP or InGaAs collector

InP or InGaAs subcollector

buffer layer: captures defects

GaAs substrate

Why might M-HBTs be harder than M-HEMTs ?

Much thicker depletion regions: base-collector (2kÅ) vs. gate-channel junctions (200 Å)1,000--10,000 times more active device area defect density, thermal resistance: more serious concerns

source drain

GaAs substrate

gate

emitterbase

GaAs substrate

collector

HBT HEMT

What are the potential problems ?

emitter

base

collector

GaAs substrate

Defects collapse in DC gain recombination in e/b junction surface recombination recombination in base generation in collector

Thick (ternary) buffer layer poor thermal conductivity

RHEED of metamorphic layer

AlGaAsSb InAlAs

InP

• Show the streak lines

• Indicate good surface

morphology

Morphology of metamorphic layer

AlGaAsSb InAlAs

InP

AFM image of metamorphic layer

Metamorphic

buffer

Surface roughness

(nm)

AlGaAsSb 4.0

InAlAs 11.7

InP 9.5

AlGaAsSb InAlAs

InP

Thermal Conductivity Measurement

• Pattern a 1x100 μm Pt line – 50 nm thick

• Measure the resistance with varying input power

• As the input power increases, the Pt wire gets hot and the resistance increases.

• Resistance change is determined by the thermal conductivity of underlying layer.

• Extract thermal conductivity of film from finite element simulation.

GaAs subst.

Metamorphic layer

Pt wire

Results and Junction Temperature Calculation

Metamorphic

buffer

Thermal conductivity

(W/mC)

AlGaAsSb 8.4

InAlAs 10.5

InP 16.1

GaAs bulk 44

InP bulk 69

InP buffer has best thermal conductivity though it is smaller than bulk value.

GaAs 350 μm

Metamorphic layer 1.5 μm

HBT 8 μm x 0.5 μm1000 μm

1000 μm

• 30 HBTs with 45 μm device separation

• Solve the 3D Laplace eq. to determine junction temp. as function of thermal conductivity

• power density : 200 kW/cm2

Thermal Conductivity vs. HBT Temp.

0

100

200

300

400

500

600

700

0 10 20 30 40 50

Thermal conductivity of metamorphic buffer layer (W/mK)

AlGaAsSb (128°C)

InAlAs (112°C)

InP (89°C)

Without metamorphic (65°C)

• Power density

: 200 kW/cm2

• 0.5 m x 8 m emitter device

• 30 HBTs with 45 m device seperation

Power density vs. HBT Temp.

0

50

100

150

200

250

300

0 1 2 3 4 5 6

Power density (105 W/cm2)

AlGaAsSb

InAlAs

InP

No metamorphic

• High power density is required for future device.

• Need high thermal conductivity buffer layer

Expected Reliability of HBT

Failture Criterion : 5% increase in VBE

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.5 2 2.5 3

1000/T(K)

MT

TF

(hr

)

InP

InAlAs

AlGaAsSb

Metamorphic

buffer

Life time relative to

AlGaAsSb HBT

AlGaAsSb 1

InAlAs 6.3

InP 119

• Long life time shows that InP buffer is essential in metamorphic HBT from thermal point of view.

Ref) K.Kiziloglu et al. IPRM, 294 (2000)

Mesa structure for RF measurement

Advantage of mesa structure

• Adequate for metamorphic HBT due to the excellent heat flow

• High speed operation

GaAs substrate

Metamorphic buffer (InP, InAlAs,AlGaAsSb)

In0.53Ga0.47As subcollector

InP collector

In0.53Ga0.47As base

InP emitteremitter

base

collector

Structure of metamorphic M-DHBT

Emitter cap In0.53Ga0.47As : Si (2x1019 cm-3) 300 Ǻ

Emitter grade In0.53Ga0.47As/In0.52Al0.48As : Si (2x1019 cm-3) 200 Ǻ

EmitterInP : Si (1x1019 cm-3) 700 Ǻ

InP : Si (8x1017 cm-3) 500 Ǻ

Grade In0.53Ga0.47As/In0.52Al0.48As : Si (4x1017 cm-3) 280 Ǻ

Base In0.53Ga0.47As : Be (4x1019 cm-3) 400 Ǻ

SetBack In0.53Ga0.47As : Si (2x1016 cm-3) 100 Ǻ

Grade In0.53Ga0.47As/In0.52Al0.48As : Si (2x1016 cm-3) 240 Ǻ

Delta doping InP : Si (5.6x1018 cm-3) 30 Ǻ

Collector InP : Si (2x1016 cm-3) 1630 Ǻ

Sub collector In0.53Ga0.47As : Si (1x1019 cm-3) 250 Ǻ

Sub collector InP : Si (1x1019 cm-3) 1250 Ǻ

Buffer InP 1.5 μm

GaAs (100) semi-insulating substrate

• 500Ǻ thick and 8e17/cm3 n-doped emitter1 layer was grown for low Cje

• 400 Ǻ base with 50 meV bandgap grading

• 100 Ǻ setback layer was introduced

• 2000 Ǻ collector

• 1.5 μm InP metamorphic layer was grown at 470oC on GaAs wafer

0

10

20

30

40

0.1 1 10 100 1000frequency (GHz)

h21

U

ft = 207 GHz

fmax

= 140 GHz

InP/InGaAs/InP Metamorphic DHBTon GaAs substrate

Growth: 400 Å base, 2000 Å collector GaAs substrate InP metamorphic buffer layer

(high thermal conductivity)Processing conventional mesa HBT narrow 2 um base mesa, 0.4 um emitterResults 207 GHz ft, 140 GHz fmax,

6 Volt BVCEO, =76

0

2

4

6

8

10

12

14

0

1 105

2 105

3 105

4 105

0 1 2 3 4 5 6J (A

/cm2 )I C

(m

A)

VCE

(V)

Gummel curves

Large area (60m x 60m)

Small area (0.4m x 0.75m)

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

0 0.2 0.4 0.6 0.8 1

I C,

I B (

A)

VBE

(V)

VCB

= 0.3 V

IC

IB

IC

IB

• Small area device shows larger leakage current than large area device.

The leakage current source is not the growth defect.

pad to pad leakage turned out to be the source.

There may be surface leakage through the side wall.

More study is being tried

InP/InGaAs/InP Metamorphic DHBTon GaAs substrate

50

100

150

200

0.5 1 1.5 2 2.5 3VCE

(V)

ft

fmax

80

100

120

140

160

180

200

220

0 1 105 2 105 3 105 4 105 5 105

Current density (W/cm2)

ft

fmax

4 10-12

5 10-12

6 10-12

7 10-12

8 10-12

9 10-12

1 10-11

1.1 10-11

0 100 200 300 400 5001/I

C (1/A)

1/ft

1/fmax

VCE = 1.5V

VCE = 1.5V

J = 3.2e5 A/cm2

Summary

• Several materials were tried for metamorphic

buffer layer

• InP was chosen because of high thermal

conductivity

• Highest speed for MHBT was acquired

• More study is needed for reducing leakage current