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280 GHz fT InP DHBT
with 1.2 m2 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
This work was supported by the DARPA TFAST program and by the Office of Naval Research (ONR)
* RF Micro Devices, Infrastructure Product Line, GaN TechnologyCharlotte, North Carolina 28269, Tel: (704)319-2033; [email protected]
Motivation for Regrown-Emitter HBT: InP vs. Si/SiGe
Advantages of InP ~20:1 lower base sheet resistance ~5:1 higher base electron diffusivity ~3:1 higher collector electron velocity ~4:1 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.
Emitter Resistance is a Key HBT Scaling Limit
rateclock GHz 200for needed m- 7
margin noise logic Degrades
mV. 300 of fraction large
mV 150)mmA/ 01()m- 15(
excessive becomes resistanceemitter of drop Voltage
rateclock GHz 200for needed mmA/ 10
.1 where;
charging isdelay Largest
.or withcorrelated not well Delay ECL
2
logic
22
2
2max,
emitter
logiccollectorlogic
max
ex
eexcex
e
ceeCC
cb
cb
V
JIR
J
/TJAJ
V
T
εA
I
VC
C
ff
Io
RL
Rex
Noise margin
2kT/q+IoRex
Vin
Vout
Vlogic=IoRL
Vlogic
Why Emitter Regrowth ?
N- collector
N+ subcollector
base contact
Si3N
4
regrownInAlAs/InAs emitter*
emitter contact
collector contact
Target Benefits:
Eliminate emitter undercut etch
Eliminate base-emitter metal liftoff
Flared emitter structure → large contact, small junction → low emitter access resistance
Thick, ~2*1020/cm3-doped extrinsic base → low resistance: 250 Ohms/square → tolerant of contact metal migration
Thin, ~3*1019/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
N- collector
N+ subcollector
S.I. substrate
intrinsic base
extrinsic basebase contact
Si3
N4
regrown emitter *emitter contact
collector contact
N+ collectorpedestalimplant
Regrown emitter HBT RF fabrication process
0.3 um Intrinsic emitter0.3 um Intrinsic emitter
0.3 x 4 um2 regrown-emitter InP DHBT
extrinsic baseextrinsic base
base contactbase contactcollector contactcollector contact
extrinsic emitterextrinsic emitter
polyimidepolyimide
emitteremitter
collectorcollector
base plugbase plug
Initial DC/RF results using CSL (graded) InAlAs emitter
* Y. Wei, D. Scott, et al., IEEE EDL, May 2004, pp.232-4.
fT=162 GHzf
max=140 GHz
h21
0
5
10
15
20
25
30
1 10 100 1000
CSL Emitter 0.7 x 8 um2 SA
Gai
n (
dB
)
Frequency (GHz)
MSG/MAG
U
0
2
4
6
8
10
12
0 1 2 3 4 5 6 7J
E (
mA
/um
2 )V
CE (V)
AE=0.7 um x 8 um
AE=0.3 um x 4 um
10-8
10-7
10-6
10-5
10-4
10-3
10-2
0 0.3 0.6 0.9
I C,
I B (
A)
VBE
(V)
IC
IB
Peak fτ = 162 GHz, fmax = 140 GHz Common-emitter current gain, h21 = 20 ηC = 1.2, ηB = 2.2
First DC/RF results with improved surface & InP emitter
0.0 100
5.0 10-3
1.0 10-2
1.5 10-2
2.0 10-2
2.5 10-2
3.0 10-2
3.5 10-2
0 1 2 3 4 5 6 7
I C(A
)V
CE (V)
IB_step
=500 uA
Peak fτ = 183 GHz, fmax = 165 GHz Common-emitter current gain, h21 ~17
Abrupt base-emitter junction and InP emitter
0
5
10
15
20
25
30
1 10 100 1000
h2
1, U
, M
SG
/MA
G (
dB
)K
Frequency (GHz)
fT = 183 GHz
U
h21
MSG/MAG
K
0.7 x 8 um2, Ic=17 mA, Vce=1.3 V
fMAX
= 165 GHz
* D. Scott, Y. Wei, et al., IEEE EDL, June 2004, pp.360~362.
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
intrinsicemitter
extrinsicemitter
SiN
Improving emitter film continuity
intrinsicemitterintrinsicemitter
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
[110][110]
[100][100]
extrinsicemitterextrinsicemitter
SixNySixNy
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 1~2e20 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
Regrown-Emitter InP DHBT with 0.34 µm2 junction: 280 GHz fT
VCE,sat< 0.9 V at JE=11mA/µm2
Peak AC current gain=30
Collector breakdown voltage VCEO=5 V
Peak fτ = 280 GHz, fmax = 148 GHz
Emitter access resistance Rex=11 Ohm, RexAe=13 Ohm-um2
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6
VCE
(V)I C
(mA
)
Ib_initial
=0 uA
Ib_step
=100 uA
10-7
10-6
10-5
0.0001
0.001
0.01
0 0.2 0.4 0.6 0.8 1
I B, I C (
A)
VBE
(V)
IB
IC
0
5
10
15
20
25
30
1 10 100 1000
IC=9.72 mA
VCE
=1.2 V
U,
MS
G/M
AG
, h
21 (
dB
), K
Frequency (GHz)
U
h21MAG/MSG
K
fT=280 GHzf
MAX=148GHz
ηB =3.2 ηC =1.2
Base Dopant Passivation by Hydrogen Degrades Performance
Hydrogen passivation of carbon base dopingincreases 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 sidewallsSiNx coated InGaAs:C Passivation after RTA
Sputtered SiN PECVD SiN
Sheet Res. (Ohm/Sq) 265.333 1360.0
Cont. Resistance (Ohm-um) 81.333 4493.3 Cont. Resistivity (Ohm-um^2) 25.064 14848.9
SiNx coated InGaAs:C Passivation During MBE Regrowth MBE Annealed No MBE Anneal
Sheet Res. (Ohm/Sq) 3710.7 203.2 Cont. Resistance (Ohm-um) 4322.9 60.3 Cont. Resistivity (Ohm-um^2) 5082.4 18.0
extrinsic base
intrinsic base
base refractory
silicon nitride
emitter contact
regrown emitter
collectordepletionregion
subcollector
silicon nitridesidewall
Hydrogen-Free Sputter-Deposited SiN Sidewalls
substrate
WWWW
S. SiN
Sputter-deposited SiN process development:• 4 inch Si wafer uniformity testing• refractive index measurement using ellipsometer: RI=2.06• BHF wet etching rate testing: ~8 Å/min- Stoichiometry controllable by heating, gas ratio and pressure
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 fτ , 148 GHz fmax , peak AC current gain=30 VCE,sat< 0.9 V at JE=11mA/µm2 rex=RexAe=13 Ohm-um2
Remaining improvements needed for 400-GHz-class device: hydrogen-free sputtered SiN sidewall further improvements in regrown emitter film continuity