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
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 N000140-4-10071, and by DARPA under the TFAST program N66001-02-C-8080.
mattias@us.ibm.com 802-769-4228
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
0.5
-0.5
Nc  Nc 
InGaAs
E
InGaAs
-1
c
InGaAlAs
InP
-1.5
InP
Base
-2
0
J=0mA
J=1mA
J=2mA
J=3mA
J=4mA
J=5mA
J=6mA
J=7mA
J=8mA
-0.5
-1
InGaAs
E
Emitter
Jc
qveff
E (eV)
0
E (eV)
0.5
High current
v
-1.5
-2.5
Collector
-2
-3
50
100
150
200
250
300
0
350
100
200
300
400
Position (A)
Position (A)
Current blocking and base push-out effects ft and Ccb – the Kirk effect
300
18.5
W =0.6 m
280
e
18
W =0.5 m
C (fF)
260
e
220
V =1.3 V
17.5
cb
W =0.7 m
240
t
f (GHz)
e
ce
17
V =1.5 V
ce
16.5
200
16
2
2.5
3
3.5
4
4.5
2
J (mA/um )
e
5
5.5
6
0
1
2
3
4
5
6
2
J (mA/um )
e
At some current density Jkirk device performance will degrade due to the Kirk effect
7
8
Observation:
The Kirk current density Jkirk depends on the emitter width
10
V =0.3 V
cb
T =150 nm
8
6
4
J
Kirk
mA/m
2
c
V =0.75 V
2
cb
T =217 nm
c
0
0
0.2
0.4
0.6
0.8
W (m)
1
1.2
1.4
eb
Jkirk extracted from ft and Ccb vs Je,
extracted from S-parameter
measurements at 5-40 GHz
Collector current spreads laterally in the collector
Extraction of the current spreading distance D
Poisson’s equation for the collector
I Kirk  J Kirk Le Web  2D 
Poissons equation for the composite collector:
 2 (V    DV )
DEc
cb
bi
b
 veff 

qN

C
q
qTc2

J
'
kirk
 2Tc  2Tset  Tgrade 
 T  Tset  Tgrade  T / 2 

  2qT N   N c  c
 Le Web  2D
2
2



T
T
T
c grade
c




Web
 J kirk
Web  2D
Plot Ikirk/L vs. emitter junction width Web
D=0.14 m for Tc=150 nm
6
D=0.19 m for Tc=217 nm
5
T =150 nm
c
4
3
I
Kirk
/L
eb
Current spreading important as
emitter width We scales to D !
Jkirk will be much higher !
2
2D
T =217 nm
1
0
-0.6
c
Sources of error:
Coarse Ic
-0.4
-0.2
Averaged data points
0
0.2
0.4
W (m)
eb
0.6
0.8
1
Ohmic losses reduces Jkirk by max 4 %
Device heating not important - low Vcb
Collector velocity extraction from Vcb
I Kirk
Vcb
 Vcb
 2 V

   2 (Vcb   bi  DVb )  Tc  Tset  Tgrade  T / 2 
cb

 Le Web  2D   veff 

veff 
2
2
2


Vcb  
qTc
Tc
 qTc









veff
Vcb
Le Web  2D 
D
0 ,
0
Vcb
∂Jkirk/∂Vcb provides effective electron velocity!
There is no evidence of velocity modulation Tc=150 nm: vsat= 3.2 105 - 3.9 105 m/s
5
Tc=217 nm: vsat=2.3 105 - 3.2 105 m/s
DHBT 19
Tc=150 nm
Jkirk (mA/m2)
4
Method requires D and veff to be
constants with regards to Vcb
over measured interval
Linearity of fit indicates this is
correct
D=140 nm
3
2
DHBT 17
Tc=217 nm
1
0
-0.4
But how can veff be constant with
regards to Vcb? G-L scattering should
lead to velocity modulation!
D=190 nm
-0.2
0
0.2
V (V)
cb
0.4
0.6
0.8
Why is there no Vcb dependence on veff?
1.5
E
cL
Energy (eV)
1
E
0.5
cG
E
cL
2
0
V = -0.05 V , J =4 mA/m
cb
e
E
2
cG
V =0.2 V , J =4 mA/m
-0.5
cb
e
Simulated @Je<Jkirk
changes Je fixed
-1
0
40
80
120
Distance (nm)
160
1.5
E
cL
1
Energy (eV)
G-L scattering occurs when
electrons in the G band scatters to
the slower L band  veff reduced
Larger Vcb  G-L scattering closer
to the bc interface  veff reduced
E
0.5
cG
E
cL
2
0
V = -0.05 V , J =4 mA/m
cb
e
E
2
V =0.2 V , J =6 mA/m
-0.5
cb
veff is extracted at the Kirk current
condition
near flat-band at bc interface  GL scattering removed from bc
interface
 minimum Vcb influence on veff
e
-1
0
cG
Vcb
40
80
120
Distance (nm)
160
Simulated @Je= Jkirk Vcb changes Je=
Jkirk(Vcb)
Typical layer composition
Mesa DHBT with 0.6 mm emitter
width, 0.5 mm base contact width
Z. Griffith, M Dahlström
Thicknes
s (nm)
Material
Doping
(cm-3)
Description
40
In0.53Ga0.47A
s
3∙1019 : Si
Emitter Cap
80
InP
3∙1019 : Si
Emitter
10
InP
8∙1017 : Si
Emitter
30
InP
3∙1017 : Si
Emitter
30
In0.53Ga0.47A
s
8-5∙1019 : C
Base
20
In0.53Ga0.47A
s
3∙1016 : Si
Setback
24
InGaAs/
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
In0.53Ga0.47A
s
2∙1019 : Si
Sub Collector
300
InP
2∙10 : Si
Substrate
SI : InP
DHBT-19 with 150 nm collector
19
Sub Collector
Device results at high current density higher
than original Kirk current threshold
V =0V
14
35
cb
A = 0.5 x 7 m
2
I
b step
jbe
12
= 0.4 mA
MAG/MSG
f = 369 GHz
t
30
f
10
25
Gains (dB)
peak (f, fmax) bias
8
6
e
J (mA/m2)
U
max
= 460 GHz
H
20
21
15
4
10
2
5
A
2
jbe
= 0.6 x 7 um
I = 35 mA
c
2
J = 8.3 mA/um , V = 0.35 V
0
0
0
0.5
1
1.5
ce
device failure
J
design limit 10 mW/um
No RF drift
after 3-hr
ECL
burn-in
bias points
4
0
8 m emitter metal length,
~0.6 m junction width
0
1
12
10
this has little bearing on circuit design
18 mW/um 2
8
max
(mA/um 2)
10
2
11
10
Frequency (Hz)
Low-current breakdown is > 6 Volts
biased without
failure (DC-IV)
6
cb
10
10
2
V (V)
12
c
2
3
V (V)
ce
4
5
6
2
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
Tc=150 nm
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)