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Characteristics of Submicron HBTs in the 140-220 GHz Band
M. Urteaga, S. Krishnan, D. Scott, T. Mathew, Y. Wei, M. Dahlstrom, S. Lee, M. Rodwell.
Department of Electrical and Computer Engineering,
University of California, Santa Barbara
urteaga@ece.ucsb.edu 1-805-893-8044 DRC, June 2001, South Bend, IN
Ultra-high fmax Transferred-Substrate HBTs
• Substrate transfer provides access to both sides of device epitaxy
• Permits simultaneous scaling of emitter and collector widths
• Maximum frequency of oscillation
• Sub-micron scaling of emitter and collector widths has resulted in record values of extrapolated fmax
• Extrapolation begins where measurements end
• New 140-220 GHz Vector Network Analyzer (VNA) extends device measurement range
0
5
10
15
20
25
30
10 100 1000
Gai
ns,
dB
Frequency, GHz
fmax = 1.1 THz ??
f = 204 GHz
Mason's gain, U
H21
MSG
Emitter, 0.4 x 6 m2
Collector, 0.7 x 6 m2
Ic
= 6 mA, Vce
= 1.2 V
3000 Å collector400 Å base with 52 meV gradingAlInAs / GaInAs / GaInAs HBT
cbbbCRff 8/max
High Frequency Device Characterization
MotivationCharacterize transistors to highest measurable frequency Develop an accurate measurement methodology Measure Transistor Power Gains
ResultsMeasured submicron transistors DC-45 GHz, 75-110 GHz, 140-220 GHz bands
Observed singularity in Unilateral Power Gain
Submicron HBTs have very high power gain,
but fmax can’t be determined
InGaAs 1E19 Si 1000 Å
Grade 1E19 Si 200 Å
InAlAs 1E19 Si 700 Å
InAlAs 8E17 Si 500 Å
Grade 8E17 Si 233 Å
Grade 2E18 Be 67 Å
InGaAs 4E19 Be 400 Å
InGaAs 1E16 Si 400 Å
InGaAs 1E18 Si 50 Å
InGaAs 1E16 Si 2550 Å
InAlAs UID 2500 Å
S.I. InP
400 Å base, 4* 1019/cm3 3000 Å collector
InGaAs/InAlAs HBT Material System
Layer StructureAlInAs/GaInAs graded base HBT
Band diagram under normal operating voltagesVce = 0.9 V, Vbe= 0.7 V
• 500 Å 5E19 graded base (Eg = kT), 3000 Å collector
-2
-1.5
-1
-0.5
0
0.5
0 1000 2000 3000 4000 5000 6000
Distance, Å
Gradedbase
Collector depletion regionEmitter
Schottkycollector
2kT base bandgap grading
Band diagram at Vbe = 0.7 V, Vce = 0.9 V
Transferred-Substrate Process Flow
• Emitter metal• Emitter etch• Self-aligned base• Mesa isolation
• Polyimide planarization• Interconnect metal• Silicon nitride insulation• Benzocyclobutene, etch vias• Electroplate gold• Bond to carrier wafer with solder
• Remove InP substrate • Collector metal• Collector recess etch
Ultra-high fmax Submicron HBTs
• Electron beam lithography used to define submicron emitters and collectors
• Minimum feature sizes 0.2 m emitter stripe widths 0.3 m collector stripe widths
• Improved collector-to-emitter alignment using local alignment marks
• Aggressive scaling of transistor dimensions predicts progressive improvement of fmax
As we scale HBT to <0.4 m, fmax keeps increasing,measurements become very difficult
0.3 m Emitter before polyimide planarization
Submicron Collector Stripes(typical: 0.7 um collector)
Stabilize transistor and simultaneously match input and output of device
Approximate value for hybrid- model
To first order, MSG does not depend on f or Rbb
Simultaneously match input and output of device
K = Rollet stability factor
How do we measure fmax?
1KKS
S 2
12
21 MAG
cexcb
12
21
12
21
qIkTRωC
1
Y
Y
S
SMSG
For Hybrid- model, MSG rolls off at 10 dB/decade, MAG has no fixed slopeCANNOT be used to accurately extrapolate fmax
Maximum Available Gain
Transistor must be unconditionally stable or MAG does not exist
Maximum Stable Gain
g e n e r a t o r
lo s s le s sm a t c h in gn e tw o rk
R g e n
V g e n
lo s s le s sm a tc h in gn e tw o rk
R L
lo a d
g e n e r a t o r
lo s s le s sm a t c h in gn e tw o rk
R g e n
V g e n
lo s s le s sm a tc h in gn e tw o rk
R L
lo a d
r e s is t iv elo s s
( s ta b iliz -a t io n )
Use lossless reactive feedback to cancel device feedback and stablize the device, then match input/output.
12212211
2
1221
GGGG4
YY
U
Unilateral Power Gain
Mason’s Unilateral Power Gain
0
5
10
15
20
25
30
35
40
1 10 100
Gai
ns, d
B
Frequency, GHz
MAG/MSGcommon base
U: all 3
MAG/MSGcommon collector
MAG/MSGcommon emitter
For Hybrid- model, U rolls off at 20 dB/decade
ALL Power Gains must be unity at fmax
U is not changed by pad reactances
g e n e ra to r
lo s s le s sm a tc h in gn e two rk
R g e n
Vg e n
lo s s le s sm a tc h in gn e two rk
R L
lo a d
s e rie sfe e d b a c k
s h un tfe e d b a c k
Negative Unilateral Power Gain ???
YES, if denominator is negative
This may occur for device with a negative output conductance (G22) or some positive feedback (G12)
12212211
2
1221
GGGG4
YY
U
1221L2211
2
1221
GGGGG4
YY
U
2-portNetwork G L
Select GL such that denominator is zero:
Can U be Negative?
What Does Negative U Mean?
Device with negative U will have infinite Unilateral Power Gain with the addition of a proper source or load impedance
AFTER Unilateralization• Network would have negative output resistance
• Can support one-port oscillation
• Can provide infinite two-port power gainU
Simple Hybrid- HBT model will NOT show negative U
0
5
10
15
20
25
30
35
1 10 100Frequency, GHz
MSG
h21
Mason'sGain, U
• Submicron HBTs have very low Ccb (< 5 fF)
• 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
Place calibration reference planes at transistor terminals
Line-Reflect-Line Calibration
Standards easily realized on-wafer
Does not require accurate characterization of reflect standards
Characteristics of Line Standards are well controlled in transferred-substrate microstrip wiring environment
Accurate Transistor Measurements Are Not Easy
Transistor Embedded in LRL Test Structure
230 m 230 m
Corrupted 75-110 GHz measurements due toexcessive probe-to-probe coupling
• HP8510C VNA used with Oleson Microwave Lab mmwave Extenders
• Extenders connected to GGB Industries coplanar wafer probes via short length of WR-5 waveguide
• Internal bias Tee’s in probes for biasing active devices
• Full-two port T/R measurement capability
• 75-110 GHz measurement set-up uses same waveguide-to-probe configuration with internal HP test set
140-220 GHz On-Wafer Network Analysis
UCSB 140-220 GHz VNA Measurement Set-up
140 150 160 170 180 190 200 210 220
freq, GHz
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
freq (75.00GHz to 110.0GHz)
Can we trust the calibration ?
freq (140.0GHz to 220.0GHz)
S11 of throughAbout –40 dB
140-220 GHz calibration looks OK75-110 GHz calibration looks Great
S11 of openAbout 0.1 dB / 3o error
dBS21 of through line is off by less than 0.05 dB
S11 of openS11 of short S11 of through
75 80 85 90 95 100 105 110
freq, GHz
-70
-65
-60
-55
-50
-45
-40
Probe-Probe couplingis better than –45 dB
1E10 1E11 1E12
Freq.
-5
0
5
10
15
20
25
30
35
40
RF G
ains
U
MAG/MSG
h21 S11
S22
-6 -4 -2 0 2 4 6
freq (6.000GHz to 45.00GHz)freq (75.00GHz to 110.0GHz)freq (140.0GHz to 220.0GHz)freq (6.000GHz to 45.00GHz)freq (75.00GHz to 110.0GHz)freq (140.0GHz to 220.0GHz)
S12*20
S21Emitter: 0.3 x 18 m2, Collector: 0.7 x 18.6 m2
Ic = 5 mA, Vce = 1.1 V
RF Gains
0.3 m Emitter / 0.7 m Collector HBTs: Negative U
Gains are high at 200 GHzbut fmax can’t be determined
Negative U
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
-0.0015
-0.0010
-0.0005
0.0000
0.0005
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
0.00
0.02
0.04
0.06
0.08
0.10
Real (Y11)
0.3 m Emitter / 0.7 m Collector HBTs: Negative Output Conductance
Real (Y21)
Real (Y12)
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Real (Y22)
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
-0.0035
-0.0030
-0.0025
-0.0020
-0.0015
-0.0010
-0.0005
0.0000
0.0005
Negative Y22
Emitter: 0.3 x 18 m2, Collector: 0.7 x 18.6 m2
Ic = 5 mA, Vce = 1.1 V
1E10 1E11 1E12
Freq.
-5
0
5
10
15
20
25
RF G
ains
U
MAG/MSG
h21
S11
S22
-4 -3 -2 -1 0 1 2 3 4
freq (6.000GHz to 45.00GHz)freq (75.00GHz to 110.0GHz)freq (140.0GHz to 220.0GHz)freq (6.000GHz to 45.00GHz)freq (75.00GHz to 110.0GHz)freq (140.0GHz to 220.0GHz)
S12*20S21
RF Gains
0.4 m Emitter / 1.0 m Collector HBTs
Emitter: 0.4 x 6 m2, Collector: 1.0 x 6.6 m2
Ic = 3 mA, Vce = 1.1 V
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
-0.0006
-0.0004
-0.0002
0.0000
0.0002
0.0004
0 20 40 60 80 100 120 140 160 180 200 220
Freq. GHz
-0.0014
-0.0012
-0.0010
-0.0008
-0.0006
-0.0004
-0.0002
0.0000
Real (Y11) Real (Y21)
Real (Y12) Real (Y22)
0.4 m Emitter / 1.0 m Collector HBTs
Negative Y22
Emitter: 0.4 x 6 m2 Collector: 1.0 x 6.6 m2
Ic = 3 mA, Vce = 1.1 V
-10 -8 -6 -4 -2 0 2 4 6 8 10
freq (6.000GHz to 45.00GHz)freq (75.00GHz to 110.0GHz)freq (6.000GHz to 45.00GHz)freq (75.00GHz to 110.0GHz)
S21
S12*30
RF Gains
Less Scaled Devices show expected Power Gain Rolloff
S11S22
1E10 1E11 1E12
Freq.
-5
0
5
10
15
20
25
RF G
ains
U
MAG/MSG
h21
Emitter: 0.5 x 8.0 m2, Collector: 1.2 x 8.6 m2
Ic = 4 mA, Vce = 1.8 V
Submicron HBTs have Extremely Low Parasitics
Extremely High Power Gains
High fmax HBTs are hard to measure
Probe-to-Probe coupling can cause errors in S21
Highly scaled transistors show a negative unilateral power gain
coinciding with a negative output conductance
Cannot extrapolate fmax from measurements of U but…
Device has ~ 8 dB MAG at 200 GHz
Single-stage amplifiers with 6.3 dB gain at 175 GHz have been fabricated(To be presented 2001 GaAs IC Conference Baltimore, MD)
Possible sources of Negative Output Conductance
Dynamics of capacitance cancellation
Dynamics of base-collector avalanche breakdown
Measurement Errors (We hope we’ve convinced you otherwise)
Conclusions
This work was supported by the ONR under grant N0014-99-1-0041
And the AFOSR under grant F49620-99-1-0079
Acknowledgements