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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 RodwellECE Department, University of California, Santa Barbara, CA 93106-9560
Miguel UrteagaTeledyne Scientific & Imaging, Thousand Oaks, CA 91360
D Loubychev, A Snyder, Y Wu, J M Fastenau, W K LiuIQE Inc., 119 Technology Drive, Bethlehem, PA 18015
[email protected], 805-893-3273
InP and Related Materials 2011
2
Outline
• Need for high speed HBTs
• HBT Scaling Laws
• Fabrication– Challenges
– Process Development
• DHBT – Epitaxial Design
– Results
• Summary
3
0
5
10
15
20
25
30
35
40
109 1010 1011 1012
Tra
nsi
sto
r P
ow
er G
ain
(d
B)
Freq (Hz)
Why THz Transistors?
High gain at microwave frequencies precision analog design, high resolution ADC & DAC, high performance receivers
THz amplifiers for imaging, sensing, communications
Digital Logic for Optical fiber circuits
4
Ohmic contacts
Lateral scaling
Epitaxial scaling
Bipolar transistor scaling laws
To double cutoff frequencies of a mesa HBT, must:
(emitter length Le)
We
Tb TcWbc
Keep constant all resistances and currentsReduce all capacitances and transit delays by 2
RCf tr
2
1
exitbnbb vTDT 22satcc vT 2
eex AR /contact
contacts
contactsheet 612 AL
W
L
WR
e
bc
e
ebb
cccb /TAC 2
cbmax, /)( cbieeffc TVAvI
effcbeffbb CR
ff
,,max 8
5
Base Access Resistance
contacts
contactgapsh,bcsh,esh, 2612 AL
W
L
W
L
WR
e
gap
e
bc
e
ebb
We
Wgap
Wbc
bcsh,esh,gapsh, ,
• Surface Depletion
• Process Damage
Need for very small Wgap
• Small undercut in InP emitter
• Self-aligned base contact
cbbbCR
ff
8max
6
Base-Emitter Short
Undercut in thick emitter semiconductor
Helps in Self Aligned Base Liftoff
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
Slow etch plane
InP Wet Etch
Fast etch plane
7
Composite Emitter Metal Stack
TiW
W
• W/TiW metal stack
• Low stress
• Refractory metal emitters
• Vertical dry etch profile
W emitter
Erik Lind
Evan LobisserTiW emitter
8
TiW
W
Base Metal
BCB
SiNx
100nm
Vertical etch profile
Low stress
High emitter yield
Scalable emitter process
Vertical EmitterFIB/TEM by E Lobisser
9
InGaAs capMo contact
InP emitter
Dual SiN sidewall
Controlled InP undercut
Narrow BE gap50nm
Narrow Emitter UndercutFIB/TEM by E Lobisser
10
Process flow
Mo contact to n-InGaAs for emitter
W/TiW/SiO2/Cr dep
SF6/Ar etch
SiNx Sidewall
SiO2/Cr removal
InGaAs Wet Etch
Second SiNx Sidewall
InP Wet Etch
Base Contact Lift-off
Base and collector formed via lift off and wet etch
BCB used to passivate and planarize devices
Self-aligned process flow for DHBTs
Mo
base
N- collector
InP substrate
sub collector
emitteremitter cap
n+ InGaAs
n InP
emitter capemitter
base
SiO2/Cr
TiW
MoW
SiN (SW)
emitterbase base base
11
Epitaxial Design
T(nm) Material Doping (cm-3) Description
10 In0.53Ga0.47As 81019 : Si Emitter Cap
20 InP 51019 : Si Emitter
15 InP 21018 : Si Emitter
30 InGaAs 9-51019 : C Base
13.5 In0.53Ga0.47 As 51016 : Si Setback
16.5 InGaAs / InAlAs 51016 : Si B-C Grade
3 InP 3.6 1018 : Si Pulse doping
67 InP 51016 : Si Collector
7.5 InP 11019 : Si Sub Collector
5 In0.53Ga0.47 As 41019 : Si Sub Collector
300 InP 21019 : Si Sub Collector
Substrate SI : InP
Vbe = 1 V, Vcb = 0.7 V, Je = 24 mA/m2
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
0 50 100 150 200
En
erg
y (e
V)
Distance (nm)
Emitter
Collector
Base
Thin emitter semiconductor
Enables wet etching
12
Results - DC Measurements
BVceo = 3.7 V @ Je = 10 kA/cm2
β = 20
Base ρsh = 710 Ω/sq, ρc < 5 Ω·µm2
Collector ρsh = 15 Ω/sq, ρc = 22 Ω·µm2
@Peak f,fmax
Je = 19.4 mA/m2
P = 32 mW/m2
Gummel plot
Common emitter I-V
0
5
10
15
20
25
30
0 1 2 3 4 5
Je (
mA
/m
2)
Vce
(V)
P = 30mW/m2
P = 20mW/m2
BV
Peak f/f
max
Ib,step
= 100 A
10-9
10-7
10-5
10-3
10-1
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1
I c, Ib (
A)
Cu
rrent G
ain
()
Vbe
(V)
Solid line: Vcb
= 0.7V
Dashed: Vcb
= 0VIc
Ib
nc = 1.17
nb = 1.98
13
1-67 GHz RF Data
Ic = 11.5 mA
Vce = 1.66 V
Je = 19.4 mA/m2
Vcb = 0.7 V
Single-pole fit to obtain cut-off frequencies
0
5
10
15
20
25
30
35
109 1010 1011 1012
Ga
in (
dB
)
freq (Hz)
fmax
= 850 GHz
f = 460 GHz
U
H21
MSG
Aje
= 0.22 x 2.7 m2
14
Parameter Extraction
Jkirk = 23 mA/m2 (@Vcb = 0.7V)
0
100
200
300
400
500
0 5 10 15 20 25
f (G
Hz)
Je (mA/m2)
Vcb
= 0.7 V
Vcb
= 0 V
300
400
500
600
700
800
900
0 5 10 15 20 25
f max
(GH
z)
Je (mA/m2)
Vcb
= 0.7 V
Vcb
= 0 V
3
4
5
6
7
8
9
0 5 10 15 20 25
Cc
b (
fF)
Je (mA/m2)
Vcb
= 0.7 V
Vcb
= 0.5 V
Vcb
= 0.3 V
Vcb
= 0 V
15
Equivalent Circuit
Hybrid- equivalent circuit from measured RF data
Rex < 4 m2
Ccb,x = 3.71 fF
Ccb,i = 0.81 fF
Rcb = 34 k
Rc = 4.7
Rex = 6
Rbe = 121
Rbb = 30
Cje + Cdiff = (7.4 + 47.6) fF gmVbee-j
0.197Vbee(-j0.17ps)
Base
Emitter
Col
Ccg = 3.6 fF
Ccb,x = 3.71 fF
Ccb,i = 0.81 fF
Rcb = 34 k
Rc = 4.7
Rex = 6
Rbe = 121
Rbb = 30
Cje + Cdiff = (7.4 + 47.6) fF gmVbee-j
0.197Vbee(-j0.17ps)
Base
Emitter
Col
Ccg = 3.6 fF
freq (1.000GHz to 67.00GHz)
S(1
,1)
S(2
,2)
S(1
,2)*
5S
(2,1
)/10
freq (100.0MHz to 67.00GHz)
S_p
aram
eter
_Dee
mbe
d_P
NA
..S12
d*5
S_p
aram
eter
_Dee
mbe
d_P
NA
..S21
d/10
S_p
aram
eter
_Dee
mbe
d_P
NA
..S11
dS
_par
amet
er_D
eem
bed_
PN
A..S
22d
S21/10S12x5
S11
S22freq (1.000GHz to 67.00GHz)
S(1
,1)
S(2
,2)
S(1
,2)*
5S
(2,1
)/10
freq (100.0MHz to 67.00GHz)
S_p
aram
eter
_Dee
mbe
d_P
NA
..S12
d*5
S_p
aram
eter
_Dee
mbe
d_P
NA
..S21
d/10
S_p
aram
eter
_Dee
mbe
d_P
NA
..S11
dS
_par
amet
er_D
eem
bed_
PN
A..S
22d
S21/10S12x5
S11
S22
--- : Measured x : Simulated
16
TEM – Wide base mesa
0.2 m
High Ac / Ae ratio (>5)
High Rex . Ccb delay
Low f
220 nm
17
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
18
Questions?
Thank You
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