InP and Related Materials 2011
InGaAs/InP DHBTs demonstrating
simultaneous ft/fmax ~ 460/850 GHz in a
refractory emitter process
Vibhor Jain, Evan Lobisser, Ashish Baraskar, Brian J Thibeault,
Mark Rodwell
ECE Department, University of California, Santa Barbara, CA 93106-9560
Miguel Urteaga
Teledyne Scientific & Imaging, Thousand Oaks, CA 91360
D Loubychev, A Snyder, Y Wu, J M Fastenau, W K Liu
IQE Inc., 119 Technology Drive, Bethlehem, PA 18015
vibhor@ece.ucsb.edu, 805-893-3273
1
Outline
• Need for high speed HBTs
• HBT Scaling Laws
• Fabrication
– Challenges
– Process Development
• DHBT
– Epitaxial Design
– Results
• Summary
2
Why THz Transistors?
Transistor Power Gain (dB)
40
Digital Logic for
Optical fiber circuits
35
30
25
THz amplifiers for
imaging, sensing,
communications
20
15
10
5
0 9
10
10
10
11
10
Freq (Hz)
12
10
High gain at microwave frequencies  precision analog design,
high resolution ADC & DAC, high performance receivers
3
Bipolar transistor scaling laws
1
  tr  RC
2f
f max 
f
8Rbb,eff Ccb ,eff
We
Tb
Wbc
Tc
To double cutoff frequencies of a mesa HBT, must:
Keep constant all resistances and currents
Reduce all capacitances and transit delays by 2
 b  Tb2 2 Dn  Tb vexit
 c  Tc 2v sat
Ccb  Ac /Tc
(emitter length Le)
Epitaxial scaling
I c,max  veff Ae (Vcb  bi ) / T
Rex  contact /Ae
2
c
 We Wbc  contact
 
Rbb  sheet 

 12 Le 6 Le  Acontacts
Lateral scaling
Ohmic contacts
4
Base Access Resistance
f max
f

8RbbCcb
We
Wgap  contact
We
Wbc
Rbb   sh,e 
  sh,bc 
  sh,gap 

12 Le
6 Le
2 Le Acontacts
Wbc
 sh,g ap   sh,e ,  sh,b c
• Surface Depletion
• Process Damage
 Need for very small Wgap
Wgap
• Small undercut in InP emitter
• Self-aligned base contact
5
Base-Emitter Short
Undercut in thick emitter semiconductor
Helps in Self Aligned Base Liftoff
InP Wet Etch
Slow etch plane
Fast etch plane
For controlled semiconductor undercut
 Thin semiconductor
To prevent base – emitter short
 Vertical emitter profile and line of sight metal deposition
 Shadowing effect due to high emitter aspect ratio
6
Composite Emitter Metal Stack
W emitter
TiW
W
Erik Lind
TiW emitter
Evan Lobisser
• W/TiW metal stack
• Low stress
• Refractory metal emitters
• Vertical dry etch profile
7
FIB/TEM by E Lobisser
Vertical Emitter
Base
Metal
SiNx
TiW
Vertical etch profile
BCB
100nm
W
Low stress
High emitter yield
Scalable emitter process
8
FIB/TEM by E Lobisser
Narrow Emitter Undercut
Dual SiN
sidewall
Controlled
InP undercut
Mo contact
InGaAs cap
InP emitter
50nm
Narrow BE gap
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Process flow
SiO2/Cr
Mo
SiN (SW)
n+ InGaAs
TiW
W
Mo
emitter cap
n InP
emitter
base
N- collector
sub collector
InP substrate
Mo contact to nInGaAs for emitter
emitter cap
emitter
base
emitter
base
W/TiW/SiO2/Cr dep
SiO2/Cr removal
SF6/Ar etch
InGaAs Wet Etch
SiNx Sidewall
base
Second SiNx
Sidewall
base
Base Contact
Lift-off
InP Wet Etch
Base and collector formed via lift off and wet etch
BCB used to passivate and planarize devices
Self-aligned process flow for DHBTs
10
Epitaxial Design
Material
Doping (cm-3)
Description
10
In0.53Ga0.47As
81019 : Si
Emitter Cap
20
InP
51019 : Si
Emitter
15
InP
21018 : Si
Emitter
30
InGaAs
9-51019 : C
Base
13.5
In0.53Ga0.47 As
51016 : Si
Setback
16.5
InGaAs / InAlAs
51016 : Si
B-C Grade
3
InP
3.6 1018 : Si
Pulse doping
67
InP
51016 : Si
Collector
7.5
InP
11019 : Si
Sub Collector
5
In0.53Ga0.47 As
41019 : Si
Sub Collector
300
InP
21019 : Si
Sub Collector
Substrate
SI : InP
1.5
1
0.5
Energy (eV)
T(nm)
0
-0.5
-1
-1.5
Emitter
Base
-2
Collector
-2.5
0
50
100
Distance (nm)
150
200
Vbe = 1 V, Vcb = 0.7 V, Je = 24 mA/mm2
Thin emitter semiconductor
 Enables wet etching
11
Results - DC Measurements
e
2
J (mA/mm )
30
P = 20mW/mm
Common emitter I-V
2
25
P = 30mW/mm
20
Peak f /f
2
@Peak f,fmax
 max
15
I
b,step
Je = 19.4 mA/mm2
= 100 mA
P = 32 mW/mm2
10
5
BV
0
0
1
2
V
4
10
5
Dashed: V
I , I (A)
10
-3
β = 20
Base ρsh = 710 Ω/sq, ρc < 5 Ω·µm2
Ω·µm2
cb
cb
= 0.7V
25
I
c
= 0V
20
n = 1.17
10
15
c
-5
n = 1.98
c
b
BVceo = 3.7 V @ Je = 10 kA/cm2
Collector ρsh = 15 Ω/sq, ρc = 22
Solid line: V
-1
10
b
10
-7
10
-9
I

b
5
Current Gain ()
ce
3
(V)
0
0
0.2
Gummel plot
0.4
0.6
V (V)
be
0.8
1
12
1-67 GHz RF Data
35
MSG
U
30
Gain (dB)
25
H
f
= 850 GHz
max
21
20
Ic = 11.5 mA
15
Vce = 1.66 V
f = 460 GHz

10
5
0 9
10
A = 0.22 x 2.7 mm
Je = 19.4 mA/mm2
Vcb = 0.7 V
2
je
10
10
11
10
freq (Hz)
10
12
Single-pole fit to obtain cut-off frequencies
13
Parameter Extraction
500
V
cb
400
9
= 0.7 V
8
cb
cb
(fF)
=0V
200
V
7
cb
6
=0V
V
C

f (GHz)
V
300
cb
= 0.3 V
5
V
100
4
0
cb
0
5
10
15
2
J (mA/mm )
20
25
e
900
(GHz)
max
3
0
5
= 0.5 V
= 0.7 V
10
15
2
J (mA/mm )
20
25
e
V
800
f
V
cb
cb
= 0.7 V
700
V
600
cb
Jkirk = 23 mA/mm2 (@Vcb = 0.7V)
=0V
500
400
300
0
5
10
15
2
J (mA/mm )
e
20
25
14
S(1,1)
Equivalent Circuit
Ccb,x = 3.71 fF
Rex < 4 Wmm2
Rcb = 34 kW
Rbb = 30 W
Ccb,i = 0.81 fF
Rc = 4.7 W
Base
Col
Rbe = 121 W
S21/10
Ccg = 3.6 fF
Cje + Cdiff = (7.4 + 47.6) fF
S12x5
Rex = 6 W
gmVbee-j
0.197Vbee(-j0.17ps)
Emitter
--- : Measured
x : Simulated
S11
S22
Hybrid- equivalent circuit from measured RF data
15
TEM – Wide base mesa
High Ac / Ae ratio (>5)
High Rex . Ccb delay
Low f
0.2 mm
220 nm
16
Summary
• Demonstrated DHBTs with peak f / fmax = 460/850 GHz
• Small Wgap for reduced base access resistance  High fmax
• Narrow sidewalls, smaller base mesa and better base ohmics
needed to enable higher bandwidth devices
17
Thank You
Questions?
This work was supported by the DARPA THETA program under HR0011-09-C-006.
A portion of this work was done in the UCSB nanofabrication facility, part of NSF funded NNIN network and MRL
Central Facilities supported by the MRSEC Program of the NSF under award No. MR05-20415
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