current density limits in inp dhbts: collector current spreading and effective electron velocity
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Current Density Limits in InP DHBTs: Collector Current Spreading and Effective Electron Velocity
Mattias Dahlström1 and Mark J.W. Rodwell
Department of ECE
University of California, Santa Barbara
USA
mattias@us.ibm.com 802-769-4228
Special thanks to:Zach and Paidi for processing and development work
(1) Now with IBM Microelectronics, Essex Junction, VT
This work was supported by the Office of Naval Research under contracts N00014-01-1-0024 and N0001-40-4-10071, and by DARPA under the TFAST program N66001-02-C-8080.
Introduction
What limits the current density in a HBT?
• Heating– High thermal conductivity InP ☺– Low thermal conductivity InGaAs
– Low Vce ☺
• Kirk effect – Injected electron charge in collector deforms
the conduction band current blocking– thin the collector, increase collector doping
Collector in HBT under current (simulation)and measured effects on ft and Ccb
-2
-1.5
-1
-0.5
0
0.5
0 100 200 300 400
J=0mAJ=1mAJ=2mAJ=3mAJ=4mAJ=5mAJ=6mAJ=7mAJ=8mA
E (
eV
)
Position (A)
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
50 100 150 200 250 300 350
E (
eV
)
Position (A)
Ec
EvEmitter
Collector
Base
InGaAs
InGaAs
InGaAs
InP
InP
InGaAlAs
At some current density Jkirk device performance will degrade due to the Kirk effect
200
220
240
260
280
300
2 2.5 3 3.5 4 4.5 5 5.5 6
f t (G
Hz)
Je (mA/um2)
We=0.5 m
We=0.7 m
We=0.6 m
16
16.5
17
17.5
18
18.5
0 1 2 3 4 5 6 7 8
Ccb
(fF
)
Je (mA/um2)
Vce
=1.5 V
Vce
=1.3 V
eff
ccc qv
JNN
Current blocking and base push-out effects ft and Ccb – the Kirk effect
High current
0
2
4
6
8
10
0 0.2 0.4 0.6 0.8 1 1.2 1.4
J Kirk
mA
/m
2
Web
(m)
Vcb
=0.75 V
Tc=217 nm
Vcb
=0.3 V
Tc=150 nm
Observation: The Kirk current density Jkirk depends on the emitter width
Jkirk extracted from ft and Ccb vs Je, extracted from S-parameter measurements at 5-40 GHz
Collector current spreads laterally in the collector
=0.14 m for Tc=150 nm
=0.19 m for Tc=217 nm
Sources of error:
Coarse Ic
Ohmic losses reduces Jkirk by max 4 %
Device heating not important - low Vcb
Extraction of the current spreading distance Poisson’s equation for the collector
0
1
2
3
4
5
6
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
I Kir
k / L
eb
Web
(m)
Tc=217 nm
Tc=150 nm
2
2'
eb
ebkirkkirk W
WJJ
Averaged data points
22/
222)(2
2
222 ebec
gradesetcc
gradec
gradesetccC
c
bbicbeff
ebeKirkKirk
WLT
TTTTNNqT
TT
TTT
q
EqN
qT
VVv
WLJI
Plot Ikirk/L vs. emitter junction width Web
Current spreading important as emitter width We scales to Jkirk will be much higher !
Poissons equation for the composite collector:
Collector velocity extraction from Vcb
There is no evidence of velocity modulation
0
1
2
3
4
5
-0.4 -0.2 0 0.2 0.4 0.6 0.8
J kirk
(m
A/
m2)
Vcb
(V)
DHBT 17 T
c=217 nm
=190 nm
DHBT 19 T
c=150 nm
=140 nm
∂Jkirk/∂Vcb provides effective electron velocity!
2
2
22/)(2
0,0
222 ebec
cb
cb
effebec
gradesetc
c
bbicbeff
cbcb
Kirk WLqT
V
V
vWLT
TTTT
qT
VVv
VV
I
cbVcbVeffv
Method requires and veff to be constants with regards to Vcb
over measured intervalLinearity of fit indicates this is correct But how can veff be constant with regards to Vcb? -L scattering should lead to velocity modulation!
Tc=150 nm: vsat= 3.2 105 - 3.9 105 m/s
Tc=217 nm: vsat=2.3 105 - 3.2 105 m/s
Why is there no Vcb dependence on veff?
veff is extracted at the Kirk current condition near flat-band at bc interface -L scattering removed from bc interface minimum Vcb influence on veff
-L scattering occurs when electrons in the band scatters to the slower L band veff reducedLarger Vcb -L scattering closer to the bc interface veff reduced
-1
-0.5
0
0.5
1
1.5
0 40 80 120 160
En
erg
y (e
V)
Distance (nm)
EcL
Ec
EcL
Ec
Vcb
= -0.05 V , Je=4 mA/m2
Vcb
=0.2 V , Je=6 mA/m2
Simulated @Je= Jkirk Vcb changes Je= Jkirk(Vcb)
-1
-0.5
0
0.5
1
1.5
0 40 80 120 160
Ene
rgy
(eV
)
Distance (nm)
EcL
Ec
EcL
Ec
Vcb
= -0.05 V , Je=4 mA/m2
Vcb
=0.2 V , Je=4 mA/m2
Simulated @Je<Jkirk Vcb changes Je fixed
Mesa DHBT with 0.6 mm emitter width, 0.5 mm base contact width
Thickness (nm)
MaterialDoping (cm-3)
Description
40 In0.53Ga0.47As 3∙1019 : Si Emitter Cap
80 InP 3∙1019 : Si Emitter
10 InP 8∙1017 : Si Emitter
30 InP 3∙1017 : Si Emitter
30 In0.53Ga0.47As 8-5∙1019 : C Base
20 In0.53Ga0.47As 3∙1016 : Si Setback
24InGaAs/ InAlAs SL
3∙1016 : Si Grade
3 InP 3∙1018 : Si Delta doping
100 InP 3∙1016 : Si Collector
10 InP 1∙1019 : Si Sub Collector
12.5 In0.53Ga0.47As 2∙1019 : Si Sub Collector
300 InP 2∙1019 : Si Sub Collector
Substrate SI : InP
Typical layer composition
DHBT-19 with 150 nm collector
Z. Griffith, M Dahlström
Device results at high current density higher than original Kirk current threshold
0
2
4
6
8
10
12
0 1 2 3 4 5 6
device failure
18 mW/um2
design limit 10 mW/um 2
J max
(m
A/u
m2 )
Vce
(V)
8 m emitter metal length, ~0.6 m junction width
biased without failure (DC-IV)
No RF driftafter 3-hr burn-in ECL
bias points
Low-current breakdown is > 6 Volts
this has little bearing on circuit design
Safe operating area is > 10 mW/um2
these HBTs can be biased ....at ECL voltages
...while carrying the high current densities needed for high speed
0
2
4
6
8
10
12
14
0 0.5 1 1.5 2
J e (m
A/
m2 )
Vce
(V)
Ajbe
= 0.5 x 7 m2 Ib step
= 0.4 mA
Vcb
= 0 V
peak (f, f
max) bias
Tc=150 nm
0
5
10
15
20
25
30
35
1010 1011 1012
Gai
ns
(dB
)
Frequency (Hz)
ft = 369 GHz
fmax
= 460 GHzU
H21
MAG/MSG
Ajbe
= 0.6 x 7 um2
Ic = 35 mA
Jc = 8.3 mA/um2, V
cb= 0.35 V
Conclusions
• Current spreading
0.14 m for Tc=150 nm
0.19 m for Tc=217 nm
(first experimental determination for InP)
• veff=3.2∙105 m/s for both 150 and 217 nm Tc
• Large effect on max collector current for sub- InP HBTs. Jkirk increases drastically
• Must be accounted for in collector isolation by implant or regrowth (provide room for current spreading)
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