High speed (207 GHz ft), 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 synthesis
better Emaxvsat than GaAs
 millimeter-wave power
Why metamorphic HBTs ?--economic argument
low cost, high volume processing: wafer size is critical
GaAs substrates, processes: 6" diameter now
large InP substrates:
high cost, high breakage, only 4" available today
breakage 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 layers
Thick intervening buffer layer to capture most defects
emitter
InAlAs or InP emitter
base
InGaAs base
InP or InGaAs collector
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
HBT
emitter
base
collector
GaAs substrate
HEMT
source
gate
drain
GaAs substrate
What are the potential problems ?
Defects  collapse in DC gain
recombination in e/b junction
surface recombination
recombination in base
generation in collector
emitter
base
collector
GaAs substrate
Thick (ternary) buffer layer
poor thermal conductivity
RHEED of metamorphic layer
AlGaAsSb
InAlAs
• Show the streak lines
• Indicate good surface
morphology
InP
Morphology of metamorphic layer
AlGaAsSb
InP
InAlAs
AFM image of metamorphic layer
AlGaAsSb
InP
InAlAs
Metamorphic
buffer
Surface roughness
(nm)
AlGaAsSb
4.0
InAlAs
11.7
InP
9.5
Thermal Conductivity Measurement
Pt wire
Metamorphic layer
GaAs subst.
• 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.
Results and Junction Temperature Calculation
Metamorphic Thermal conductivity
buffer
(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.
1000
μm
1000
μm
HBT 8 μm x 0.5 μm
Metamorphic layer 1.5 μm
GaAs 350 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.
• Power density
: 200 kW/cm2
AlGaAsSb (128°C)
• 0.5 m x 8 m
emitter device
InAlAs (112°C)
InP (89°C)
Without
metamorphic
(65°C)
• 30 HBTs with
45 m device
seperation
Power density vs. HBT Temp.
• High power density
is required for future
device.
• Need high thermal
conductivity buffer
layer
Expected Reliability of HBT
Failture Criterion : 5% increase in V BE
1.E+09
InP
1.E+08
InAlAs
MTTF (hr)
1.E+07
AlGaAsSb
1.E+06
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.
1.E+05
1.E+04
1.E+03
1.E+02
1.5
2
2.5
1000/T(K)
3
Ref) K.Kiziloglu et al. IPRM, 294
(2000)
Mesa structure for RF measurement
emitter
base
InP emitter
In0.53Ga0.47As base
InP collector
collector
In0.53Ga0.47As
subcollector
Metamorphic
buffer (InP,
InAlAs,AlGaAsSb)
GaAs
substrate
Advantage of mesa structure
• Adequate for metamorphic HBT
due to the excellent heat flow
• High speed operation
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
InP : Si (1x1019 cm-3)
700 Ǻ
InP : Si (8x1017 cm-3)
500 Ǻ
Emitter
(4x1017
Grade
In0.53Ga0.47As/In0.52Al0.48As : Si
Base
In0.53Ga0.47As : Be (4x1019 cm-3)
SetBack
In0.53Ga0.47As : Si
(2x1016
cm-3)
cm-3)
280 Ǻ
400 Ǻ
100 Ǻ
In0.53Ga0.47As/In0.52Al0.48As : Si (2x1016 cm-3)
240 Ǻ
Delta doping
InP : Si (5.6x1018 cm-3)
30 Ǻ
Collector
(2x1016
1630 Ǻ
Grade
InP : Si
cm-3)
Sub collector
In0.53Ga0.47As : Si (1x1019 cm-3)
250 Ǻ
Sub collector
InP : Si (1x1019 cm-3)
1250 Ǻ
InP
1.5 μm
Buffer
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
InP/InGaAs/InP Metamorphic DHBT
on GaAs substrate
14
12
3 10
5
2 10
5
1 10
5
8
6
4
2
C
5
J (A/cm )
I (mA)
10
4 10
2
0
0
0
1
2
3
V
CE
4
5
6
(V)
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 emitter
Results
207 GHz ft, 140 GHz fmax,
6 Volt BVCEO, b=76
Gummel curves
Large area (60m x 60m)
• Small area device shows larger
leakage current than large area
device.
Small area (0.4m x 0.75m)
 The leakage current source is
not the growth defect.
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
-8
I
C
I
C
I
V
CB
B
= 0.3 V
 pad to pad leakage turned out
to be the source.
I
B
C
B
I , I (A)
10
 There may be surface leakage
through the side wall.
0
0.2
0.4
0.6
V
BE
(V)
0.8
1
 More study is being tried
InP/InGaAs/InP Metamorphic DHBT
on GaAs substrate
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