Characteristics of Submicron HBTs in the 140-220 GHz Band

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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_at_ece.ucsb.edu 1-805-893-8044
DRC, June 2001, South Bend, IN
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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

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Mason's
3000 Å collector 400 Å base with 52 meV
grading AlInAs / GaInAs / GaInAs HBT
gain, U
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20
Gains, dB
MSG
H
f
1.1 THz ??
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max
f
204 GHz
Emitter, 0.4 x 6 mm2
t
Collector, 0.7 x 6 mm2
I
6 mA, V
1.2 V
c
ce
10
100
1000
Frequency, GHz
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High Frequency Device Characterization
Motivation Characterize transistors to highest
measurable frequency Develop an accurate
methodology for ultra-high frequency
transistor measurements Results Measured
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 cant be determined

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InGaAs/InAlAs HBT Material System
Layer Structure
Band diagram at Vbe 0.7 V, Vce 0.9 V
2kT base bandgap grading
400 A base, 4 1019/cm3 3000 A collector
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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

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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 lt0.4 um, fmax keeps
  increasing,measurements become very difficult

0.3 ?m Emitter before polyimide planarization
Submicron Collector Stripes(typical 0.7 um
collector)
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How do we measure fmax?
Maximum Available Gain
Simultaneously match input and output of
device K Rollet stability factor
Transistor must be unconditionally stable or MAG
does not exist
Maximum Stable Gain
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
For Hybrid- ? model, MSG rolls off at
10 dB/decade, MAG has no fixed
slopeCANNOT be used to accurately extrapolate
fmax
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Unilateral Power Gain
Masons Unilateral Power Gain
Use lossless reactive feedback to cancel device
feedback and stabilize the device, then match
input/output.
U is not changed by pad reactances
For Hybrid- ? model, U rolls off at 20
dB/decadeALL Power Gains must be unity at fmax
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Negative Unilateral Power Gain ???
Can U be Negative?
YES, if denominator is negative This may
occur for device with a negative output
conductance (G22) or some positive feedback (G12)
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 gain

Select GL such that denominator is zero
Simple Hybrid- ? HBT model will NOT show negative
U
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Accurate Transistor Measurements Are Not Easy

• Submicron HBTs have very low Ccb (lt 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

Transistor in Embedded in LRL Test Structure
Corrupted 75-110 GHz measurements due
to excessive probe-to-probe coupling
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140-220 GHz On-Wafer Network Analysis

• 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 Tees 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

UCSB 140-220 GHz VNA Measurement Set-up
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Can we trust the calibration ?
140-220 GHz calibration looks OK
75-110 GHz calibration looks Great
S11 of open About 0.1 dB / 3o error
S11 of through About 40 dB
S11 of short
S11 of open
S11 of through
Probe-Probe coupling is better than 45 dB
S21 of through line is off by less than 0.05 dB
dB
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0.3 ?m Emitter / 0.7 ?m Collector HBTs Negative U
Negative U
Emitter 0.3 x 18 ?m2, Collector 0.7 x 18.6
?m2Ic 5 mA, Vce 1.1 V
Gains are high at 200 GHz but fmax cant be
determined
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0.3 ?m Emitter / 0.7 ?m Collector HBTs Negative
Output Conductance
Real (Y11)
Real (Y12)
Real (Y21)
Real (Y22)
Negative Y22
Emitter 0.3 x 18 ?m2, Collector 0.7 x 18.6
?m2Ic 5 mA, Vce 1.1 V
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0.4 ?m Emitter / 1.0 ?m Collector HBTs
Emitter 0.4 x 6 ?m2, Collector 1.0 x 6.6 ?m2Ic
3 mA, Vce 1.1 V
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0.4 ?m Emitter / 1.0 ?m Collector HBTs
Real (Y12)
Real (Y11)
Real (Y22)
Real (Y21)
Negative Y22
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Less scaled devices show expected power gain
rolloff
Emitter 0.5 x 8.0 ?m2, Collector 1.2 x 8.6
?m2Ic 4 mA, Vce 1.8 V InP/InGaAs/InP DHBT
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Conclusions
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 weve convinced you otherwise)

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Acknowledgements
This work was supported by the ONR under grant
N0014-99-1-0041 And the AFOSR under grant
F49620-99-1-0079