Ultra Wideband DHBTs using a
Graded Carbon-Doped InGaAs Base
Mattias Dahlström, Miguel Urteaga,Sundararajan Krishnan,
Navin Parthasarathy, Mark Rodwell
Department of Electrical and Computer Engineering,
University of California, Santa Barbara
mattias@ece.ucsb.edu 805-893-8044, 805-893-3262 fax
UCSB
Wideband InP/InGaAs/InP Mesa DHBT
Mattias Dahlström
Objectives:
fast HBTs → mm-wave power, 160 Gb fiber optics
desired: 440 GHz ft & fmax, 10 mA/mm2, Ccb/Ic<0.5 ps/V
better manufacturability than transferred-substrate HBTs
Approach:
narrow base mesa → moderately low Ccb
very low base contact resistance required
→ carbon base doping, good base contact process
high ft through high current density, thin layers
Bandgap engineering: small device transit time with wide
bandgap emitter and collector
DHBT Layer Structure and Band Diagram UCSB
M Dahlstrom
InGaAs 3E19 Si 400 Å
InP 3E19 Si 800 Å
InP 8E17 Si 100 Å
InP 3E17 Si 300 Å
InGaAs graded doping 300 Å
Setback 2E16 Si 200 Å
Grade 2E16 Si 240 Å
InP 3E18 Si 30 Å
InP 2E16 Si 1700 Å
InP 1.5E19 Si 500 Å
InGaAs 2E19 Si 500 Å
InP 3E19 Si 2000 Å
SI-InP substrate
Emitter
Collector
Vbe = 0.8 VBase
Vce = 1.5 V
• 300 A doping graded base
• Carbon doped 8*10195* 1019 cm-2
• 200 Å n-InGaAs setback
• 240 Å InAlAs-InGaAs SL grade
• Thin InGaAs in subcollector
InP/InGaAs/InP Mesa DHBT
Base contact resistance
UCSB
Mattias Dahlström
• Carbon doping 6E19 cm-3
• Pd-based p-contacts
• Careful ashing and oxide etch
• RTP @ 300 C, 1 minute
The size of the base contacts
must be minimized due to Ccb
160
Pc is immeasurably
low: below 10 –7 cm-2
R=5.3+30.1*X (Ohm)
140
Resistance (Ohm)
120
100
80
s=722 /sq
60
40
 c  8 10 8 cm 2 ???
20
0
0
1
2
3
Length (mm)
4
5
Critical for narrow base
mesa HBT
InP/InGaAs/InP Mesa DHBT
Device Results
b=20-25
B
No evidence of current
blocking or heating
4
C
Mattias Dahlström
emitter junction: 0.44 mm x 7.4 mm
I step = 50 mA
5
I (mA)
UCSB
3
2
1
0
0
4
0.5
1
V
1.5
(V)
2
2.5
3
CE
10
IB step=200 mA
Emitter 1x8 mm
emitter junction: 0.44 mm x 7.4 mm
BVCEO=7.5 V
I step = 50 mA
B
2
I (mA)
J=3.5 mA/um2
C
2
JC (mA/um )
8
3
6
BVCEO=7.5 V
4
1
2
0
0.0
0.5
1.0
1.5
VCE (V)
2.0
2.5
3.0
0
0
1
2
3
V
4
(V)
CE
5
6
7
8
Accurate Transistor Measurements
Are Not Easy
UCSB
Miguel Urteaga
Mattias Dahlstrom
• Submicron HBTs have very low Ccb
Characterization requires accurate measure of
very small S12
• Standard 12-term VNA calibrations do not
correct S12 background error due to
probe-to-probe coupling
Solution
Embed transistors in sufficient length of
transmission line to reduce coupling
230 mm
230 mm
Transistor in Embedded in LRL Test Structure
Place calibration reference planes at transistor
terminals
Line-Reflect-Line Calibration
Standards easily realized on-wafer
Does not require accurate characterization of
reflect standards
CPW lines suffer from substrate TE, TM mode
coupling: thin wafer, use Fe absorber !
lateral TEM mode on CPW ground plane…
present above 150 GHz ,
must use narrower CPW grounds
Corrupted 75-110 GHz measurements due to
excessive probe-to-probe coupling
InP/InGaAs/InP Mesa DHBT
Device Results
21
Gain (dB) H , U, MAG/MSG
30
20
Mattias Dahlström
• 2.7 mm base mesa,
• 0.54 mm emitter junction
• 0.7 mm emitter contact
• Vce=1.7 V
• J=3.7E5 A/cm2
U
25
UCSB
MAG/MSG
H21
15
10
5
0
ft = 282 GHz;
fmax=480 GHz
b = 25; BVCEO = 7.5 V
10
10
10
11
frequency (Hz)
10
12
InP/InGaAs/InP Mesa DHBT
Device Results
350
600
300
500
UCSB
Mattias Dahlström
600
300
500
280
250
400
400
Vcb=0.9 V
50
100
0
0
0
1
2
3
2
J (mA/um )
4
5
6
t
f GHz
t
f GHz
300
240
200
um2
Aej=3.4
J=4.4 mA/mm2
220
0
200
1
1.2
1.4
1.6
1.8
V
• Emitter contact sizes
0.5-2.0 um, 8 um long.
•Base extends 0.25-1.0 um
on each side of the contact
•Maximum current density
>10 mA/um2
100
2
2.2
2.4
2.6
CE
fmax measurement above 500 GHz
currently not reliable in CPW
environment
• Vce >1.5 V for best performance
• Best ft found at
current density of 3-5 mA/mm2
GHz
100
260
max
200
f
150
GHz
300
max
f
200
InP/InGaAs/InP Mesa DHBT
Conclusions
UCSB
Mattias Dahlström
Doping-graded base InGaAs/InP
Mesa DHBT:
• High current density
Operates up to 10 mA/mm2 without destruction
…Kirk threshold 4.4 mA/mm2 at 1.5 V
• ft of 280 GHz with a 220 nm collector
• fmax is 450 GHz or higher
• Rbb is no longer a major factor - excellent base ohmics
• fmax no longer a good measure of Ccb or circuit performance
• Ccb reduction a priority
• 87 GHz static frequency divider circuit already demonstrated
Narrow-mesa DHBT:base design
UCSB
Mattias Dahlström
Many approximate methods for determining Ef such
as Boltzmann, Joyce-Dixon are
insufficient
Fermi-Dirac, Boltzman, Joyce-Dixon and Selberherr
0.8
Selberherr
Boltzmann
Joyce-Dixon
Fermi-Dirac numerically
0.7
0.6
0.5
Energy (eV)
Energy (eV)
0.4
0.3
0.2
0.1
0
-0.1
-0.2
15
10
16
10
17
10
18
10
Base doping
(cm-3)
Acceptor concentration
19
10
20
10
Doping graded base:
At degenerate doping levels (>1E19) the variation of the Fermi level in the base is
very rapid
Exponential doping roll-off not needed, linear roll-off good enough!
Narrow-mesa DHBT:base design
UCSB
Mattias Dahlström
Base transit time
0.7
Base transit time calculation:
-Bandgap narrowing
-Fermi-Dirac statistics
- doping and bandgap
dependent mobility
Base transit time
Constant structure case
0.6
0.4
b
time (ps)
Transit
t (ps)
0.5
0.3
0.2
0.1
0
100
200
300
Base400
width (A)
500
Base width (A)
600
700
800
The exit term (electron velocity in top of collector) important
for thin bases: use InGaAs, not InP, close to base