280 GHz fT InP DHBT with 1.2 mm2 base-emitter junction area in MBE Regrown-Emitter Technology

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280 GHz fT InP DHBT with 1.2 mm2 base-emitter
junction area in MBE Regrown-Emitter Technology
Yun Wei, Dennis W. Scott, Yingda Dong, Arthur
C. Gossard, Mark Rodwell University of California
at Santa Barbara
RF Micro Devices, Infrastructure Product
Line, GaN Technology Charlotte, North Carolina
28269, Tel [phone] ywei_at_rfmd.com
This work was supported by the DARPA TFAST
program and by the Office of Naval Research (ONR)
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Motivation for Regrown-Emitter HBT InP vs.
Si/SiGe
Advantages of InP 201 lower base sheet
resistance 51 higher base electron
diffusivity 31 higher collector electron
velocity 41 higher breakdown at same f?
Disadvantages of InP Production devices large
0.7 µm emitters High emitter resistance
scaling limit Large excess collector
capacitance Non-planar device ? low IC yield
Low integration scales
The advantages of InP-based HBTs lie in the
material system. The disadvantages lie in the
device structure and fabrication technology.
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Emitter Resistance is a Key HBT Scaling Limit
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Why Emitter Regrowth ?
Target Benefits Eliminate emitter undercut
etch Eliminate base-emitter metal
liftoff Flared emitter structure ? large
contact, small junction ? low emitter access
resistance Thick, 21020/cm3-doped extrinsic
base ? low resistance 250 Ohms/square
? tolerant of contact metal migration Thin,
31019/cm3 -doped intrinsic base ? low
transit time ? high current gain (less
Auger) Passivated base-emitter junction ?
reliability
Polycrystalline InAs has low resistivity, can
play same role in InP as the polysilicon
extrinsic emitter in Si/SiGe
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Regrown emitter HBT RF fabrication process
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0.3 x 4 um2 regrown-emitter InP DHBT
base plug
emitter
polyimide
collector
extrinsic emitter
base contact
collector contact
extrinsic base
0.3 um Intrinsic emitter
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Initial DC/RF results using CSL (graded) InAlAs
emitter
Common-emitter current gain, h21 20 ?C 1.2,
?B 2.2
Peak ft 162 GHz, fmax 140 GHz
Y. Wei, D. Scott, et al., IEEE EDL, May 2004,
pp.232-4.
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First DC/RF results with improved surface InP
emitter
Peak ft 183 GHz, fmax 165 GHz
Common-emitter current gain, h21 17 Abrupt
base-emitter junction and InP emitter
D. Scott, Y. Wei, et al., IEEE EDL, June 2004,
pp.360362.
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Breaks in Emitter Growth Increase Emitter
Resistance
Narrowing or breakage of emitter regrowth -
due to facet-dependent growth - due to high
surface mobility of indium
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Improving emitter film continuity
100
extrinsic emitter
110
intrinsic emitter
SixNy
Suppress indium migration on the regrowth facets
by orienting abrupt InP emitter 60o off
110 inserting alloy-graded InGaXAs1-X
layers between the InP emitter and InAs cap
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HBT layer structure

• Layer Material Doping (cm-3)
  Thickness (Å)
• Emitter cap InAs 3e19 Si
  800
• Cap grade InGaXAs1-X 3e19 Si
  500
• N Emitter InP 3e19 Si
  800
• N- Emitter InP 8e17 Si
  100
• N-- Emitter InP 3e17 Si
  300
• Extrinsic base InGaAs 12e20 C
  500
• Etch stop InP 4e19 Be
  20
• Intrinsic base InGaAs 4e19 C
  400
• Set-back InGaAs 2e16 Si
  200
• Grade InGaAlAs 2e16 Si
  240
• Delta doping InP 3e18 Si
  30
• Collector InP 2e16 Si
  1030

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Regrown-Emitter InP DHBT with 0.3?4 µm2 junction
280 GHz fT
?B 3.2 ?C 1.2
VCE,satlt 0.9 V at JE11mA/µm2 Peak AC current
gain30 Collector breakdown voltage VCEO5
V Peak ft 280 GHz, fmax 148 GHz Emitter
access resistance Rex11 Ohm, RexAe13 Ohm-um2
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Base Dopant Passivation by Hydrogen Degrades
Performance
Hydrogen passivation of carbon base
doping increases base sheet resistance
source of Hydrogen PECVD-deposited
SixNy Solution process uses hydrogen-free
sputtered SixNy for surfaces present process
still uses PECVD SixNy for sidewalls need to
also used sputtered SixNy for sidewalls
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Hydrogen-Free Sputter-Deposited SiN Sidewalls
S. SiN
W
substrate

• Sputter-deposited SiN process development
• 4 inch Si wafer uniformity testing
• refractive index measurement using ellipsometer
  RI2.06
• BHF wet etching rate testing 8 Å/min
• - Stoichiometry controllable by heating, gas
  ratio and pressure

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Summary
InP HBT with emitter regrowth wide emitter
contact, submicron emitter junction? potential
for reduced Rex junction formation by
regrowth, not mesa etching thick extrinsic
base for reduce base resistance Performance still
limited by immature process technology breaks
in emitter regrowth? increased Rex hydrogen
passivation from sputtered SiN sidewalls ?
increased Rbb Present results 0.3 um x 4 um
regrown-emitter InP HBT 280 GHz ft , 148
GHz fmax , peak AC current gain30
VCE,satlt 0.9 V at JE11mA/µm2 rexRexAe13
Ohm-um2 Remaining improvements needed for
400-GHz-class device hydrogen-free
sputtered SiN sidewall further improvements
in regrown emitter film continuity