InAlAs/InGaAs/InP DHBTs
with Polycrystalline InAs Extrinsic
Emitter Regrowth
D. Scott, H. Xing, S. Krishnan, M. Urteaga, N.
Parthasarathy and M. Rodwell
University of California, Santa Barbara
dennis@umail.ucsb.edu 805-893-8044, 805-893-3262 fax
Advantages of InP vs. SiGe HBTs
InP HBT Material Properties:
Si/SiGe HBT Material Properties:
•
Available lattice-matched materials
allows for emitter bandgap wider than
base, allowing for higher base doping
and lower base sheet resistance
•
Allowable lattice mismatch limits
Ge:Si alloy ratio resulting in smaller
emitter-base bandgap difference and
higher base sheet resistance
•
Electron velocities reported as high
as 4107 cm/s
•
4:1 lower electron velocity is seen in
silicon
InP HBT Processing Technology :
Si/SiGe Processing Technology :
•
High topography mesa structure
allows for small-scale integration
•
Planar process using silicon CMOS
technology allows for VLSI
•
Base-emitter junctions defined by
etching and depositing a self-aligned
base metal results in low yield and
limits emitter scaling
•
Self-aligned base-emitter junctions
are diffused, extrinsic base and
emitter wider than the active junction
allows for high degree of scaling
Evolution Cbc Reduction in III-V HBTs
Emitter
Emitter
Collector
Base
Collector
Base
Subcollector
Subcollector
S.I. Substrate
S.I. Substrate
Mesa HBT
Cbc Reduction HBT
Emitter
Emitter
Base
Collector
Subcollector
Collector
S.I. Substrate
Transferred Substrate HBT
Highly Scaled HBT
UCSB Highly Scaled HBT
UCSB has demonstrated laterally
scaled HBTs with emitters written
by e-beam lithography.
These HBTs show problems with:
• High emitter resistance, Rex
• Low yield
These devices demonstrated lower
than predicted values of f despite
aggressive thinning of the epitaxial
layers.
1 / 2
f 
 base   collector  RexCcb  kT / qI e C je  Ccb 
Si/SiGe HBT Process Advantages
•Highly scaled
• very narrow active junction
areas
• very low device parasitics
• high speed
• Low emitter resistance using
wide n+ polysilicon contact
• Low base resistance using
large extrinsic polysilicon contact
• High-yield, planar processing
• high levels of integration
Published Si/SiGe HBT f as high as 210 GHz
InP-based HBT f as high as 341 GHz
• LSI and VLSI capabilities
Polycrystalline n+ InAs
Polycrystalline InAs grown on SiNx
Hall measurements as high as:
Poly InAs:Si Doping vs. Temp
01/28/2002
• Doping = 1.3 1019 cm-3,
Mobility = 620 cm2/V•s
Doping
y = 1.13477e+05 * e^(3.35301e-02x) R= 9.99672e-01
2.2 10
19
2 10
19
1.8 10
19
1.6 10
19
1.4 10
19
1.2 10
19
1 10
19
8 10
18
6 10
18
• Results in doping-mobility
product of 81021 (V •s •cm)-1
Compare these numbers to InGaAs
lattice matched to InP:
• Doping = 1.0 1019 cm-3,
Mobility = 2200 cm2/V•s
945
950
955
960
965
Temp
970
975
980
985
• Results in doping-mobility
product of 221021 (V •s •cm)-1
Polycrystalline InAs has potential
as an extrinsic emitter contact!
Base-Collector Template for Regrown
Emitter HBT
Base-collector template
as-grown
Base-collector template
prior to regrowth
Regrown Emitter Fabrication Process
Regrowth
Emitter/cap
etch
Base/collector
etch
Metalization
Large-area Small-emitter HBTs
First Attempt Results
First Attempt Regrown Emitter HBT
Common Emitter Curves, Ib = 500 uA, 6 steps
-3
6 10
Regrown area
-3
5 10
-3
Ic (A)
4 10
-3
3 10
-3
2 10
-3
1 10
SiNx
0 10
0
0
0.5
1
1.5
Vce (V)
Regrown area very rough
Transistor action!!
2
Growth and Process Improvements
Regrown area
SiNx
First attempt at the baseemitter junction without
RHEED or pyrometer
Regrown area
SiNx
Second attempt with improved
pre-regrowth processing and
RHEED/pyrometer features
added to the wafer
Growth and Process Improvements
First attempt at the baseemitter junction without
RHEED or pyrometer
Second attempt with improved
pre-regrowth processing and
RHEED/pyrometer features
added to the wafer
Base-emitter Regrowth SEM Detail
Base-emitter Regrowth SEM
2 μm emitter regrowth
30K magnification
1 μm emitter regrowth
55K magnification
Second Attempt DC Results
Regrown Common-Emitter Curves
A = 0.8 x 15 um
2
I = 100uA/step
10 10
0
8.0 10
0
6.0 10
0
4.0 10
0
2.0 10
0
0.0 10
0
b
c
I (mA)
E
0
0.5
1
1.5
2
V
ce
2.5
3
(V)
Common-emitter gain, β > 15
3.5
4
Unintended InAlAs Layer (>50Å)
• wide-bandgap layer acts as a
current block from emitter to base
• reduces common-emitter gain
• may account for the dip in
common-emitter curves
Base-emitter Current Leakage
2
2
Gummel for 1x15 um Emitter
Regrown Base-Emitter Diode for 1x15 um Emitter
2
-2
10
1.0 10
Ic
1
10
Tight Alignment
-3
8.0 10
Less Tight Alignment
0
10
10-1
Ic, Ib (mA)
Ib (amps)
-3
6.0 10
-3
4.0 10
Ib
10-2
10-3
10-4
-3
2.0 10
-5
10
-6
0
0.0 10
10
0
0.5
1
1.5
2
0
0.2
0.4
0.6
0.8
1
1.2
Vbe (volts)
Vbe (volts)
Evidence of resistance seen in the
base-emitter diode
Evidence of base-emitter leakage
seen in Gummel
Third Attempt DC Results
2
Gummel for 1x15 um Emitter
2
10
101
100
2
E
1.0 10
Ib, Ic (mA)
Regrown Common-Emitter Curves
A = 0.8 x 15 um
I = 100uA/step
b
1
0
6.0 10
0
10
-1
10
-2
10
-3
10
-4
Ib
I (mA)
8.0 10
Ic
0
0.2
0.4
0.6
0.8
1
1.2
c
Vbe (volts)
4.0 10
0
2
1x15 um Base-Emitter Diode
-3
10 10
2.0 10
0
-3
8 10
0
-3
6 10
0
1
2
V
3
ce
4
(V)
IE (A)
0.0 10
-3
4 10
-3
2 10
Common-emitter gain, β > 20
0 10
0
-1
-0.5
0
V
BE
(V)
0.5
1
Base-collector Grade Design Error
InP collector
InGaAs
base
Base-collector band diagram with
the incorrect base-collector grade.
This mistake may account for the
oscillations seen in the HBT I-V curve.
InP collector
InGaAs
base
Base-collector band diagram with
the corrected base-collector grade.
A thin, heavily-doped layer was
inserted between the grade and
collector to pull the conduction band
down at the grade-collector junction.
Regrowth with Buried Base Contact
InP HBTs with polycrystalline InAs extrinsic emitter regrowth
Objective:
• Emulate high-yield 0.2 um SiGe emitter process
• Polycrystalline extrinsic emitter  wide contact for low resistance
Future Work:
• RF devices need to be designed and demonstrated
•
GaAsSb based DHBTs should be demonstrated
•
Higher scaling in the regrown emitters needs to be examined
Growth Related Work:
• A low-resistance p-type polycrystalline contact needs to be verified
• Regrowth of the base will need to be explored to obtain a fully planar
HBT completely analogous to the Si/SiGe HBT
InP HBTs with polycrystalline InAs extrinsic emitter regrowth
Objective:
Emulate high-yield 0.2 um SiGe emitter process
Polycrystalline extrinsic emitter  wide contact for low resistance
Future Work (short-term):
Improve DC characteristics.
Improve base capping layer to lower extrinsic base resistance
GaAsSb base layers for higher carbon incorporation
Deep submicron scaling of regrown emitter.
RF device demonstration
Future work (long-term): full SiGe-like process flow for submicron InP HBT
regrown emitter,
regrown extrinsic base over buried dielectric spacer for Ccb reduction
Future Work
DC Device Work:
• DC characteristics should be demonstrated without the design errors
• Improvements will be made to the base capping layer to lower
extrinsic base resistance
• GaAsSb based DHBTs should be demonstrated
• Higher scaling in the regrown emitters needs to be examined
RF Device Work:
• RF devices need to be designed and demonstrated
Growth Related Work:
• A low-resistance p-type polycrystalline contact needs to be verified
• Regrowth of the base will need to be explored to obtain a fully planar
HBT completely analogous to the Si/SiGe HBT