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1HICUM Workshop 2013
Benchmarking Circuits for
Model Verification
Electronics Research Laboratory/DIMESDelft University of Technology, The Netherlands
L. Vera and J.R. Long
May 27, 2013
Benchmarking Circuits for Model Verification 2
HICUM Workshop 2013 L. Vera and J. R. Long
Outline
Motivation
Broadband amplifier (small-signal circuit)- Measurements vs. simulations
- S-parameter verification
- Sensitivity analysis using HICUM
Static frequency divider (large-signal circuit)- Operating above the breakdown voltage
- ECL latch
- Measurement vs. simulation
Conclusions
Benchmarking Circuits for Model Verification 3
HICUM Workshop 2013 L. Vera and J. R. Long
Independent experiments are required to verify models
extracted from single transistor measurements
Design kit (DK) verification and performance benchmarking
of a technology using multi-transistor circuit configurations
– Early verification of model accuracy and device
performance improves DK quality
Hardware performance benchmarks to guide technology
development and improvements
Motivations
Benchmarking Circuits for Model Verification 4
HICUM Workshop 2013 L. Vera and J. R. Long
A benchmarking circuit should be:
1. Easy to test: simple set-up; data can be gathered quickly
2. Sensitive to process variation (i.e., can be used to
characterize spread)
3. More complex than a single device (i.e., not just repeating
single device measurements used for model extraction)
4. Differences between simulation and measurement can be
related or traced to model parameters
Benchmarking Circuit Characteristics
Benchmarking Circuits for Model Verification 5
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier
Potential applications:
Wideband gain block
Fiber-optic transceivers, mm-wave phased-array radar or
radio imaging
Digital radio and satellite transceivers
Benchmarking Circuits for Model Verification 6
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Feedback Amplifier
Simple biasing
2 port S-parameter characterization
Sensitive to processing variations
Possible to correlate measurements to model or processing
variations
Benchmarking Circuits for Model Verification 7
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier Design
S21 = - gmTotal (RF||RL) ≈ -gm2(RF||50Ω)
Rout= Rin = RF/(1-S21)
For a 50Ω input/output system at low frequency:
Benchmarking Circuits for Model Verification 8
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier Design
S21 = - gmTotal (RF||RL) ≈ -gm2(RF||50Ω) → gm2≈96mS
Rout= Rin = RF/(1-S21) → RF=50(5)=250
For a 50Ω input/output system at low frequency:
Benchmarking Circuits for Model Verification 9
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier Transistor Sizing
For maximum bandwidth:
Given IC2, Q2 is biased at peak fT (sets length L2 of Q2)
Length of Q1 (L1) optimized to provide best input/output
match at higher frequencies
Q1 also biased at peak fT (using RE1)
Benchmarking Circuits for Model Verification 10
HICUM Workshop 2013 L. Vera and J. R. Long
Ballast resistors RE1 and RE2 provide thermal stability (e.g.,
counteract rise of IC2 due to self-heating)
Adding RE2 decreases gmTotal; IC2 must be increased to
compensate
Broadband Amplifier Biasing
Benchmarking Circuits for Model Verification 11
HICUM Workshop 2013 L. Vera and J. R. Long
Transistor Bias Voltages
VCB of Q2 = VCE of Q1
Reduced breakdown voltage of newer technologies restricts VCB2
Simulation of Q2 must be accurate between BVCES and BVCEO
Benchmarking Circuits for Model Verification 12
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier Prototype
Feedback amplifier prototype in SiGe:
Low frequency gain of 12dB
Maximum f-3dB determined by the technology
50Ω input/output impedances for simple
characterization
G
S
G
G
S
G
Benchmarking Circuits for Model Verification 13
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier Measurements
Measurements: gain, bandwidth, input/output impedances, etc.
DC supplies
HP 4142B
Agilent 8361A PNA
PicoprobeGSG 110H
PicoprobeGSG 110H
Benchmarking Circuits for Model Verification 14
HICUM Workshop 2013 L. Vera and J. R. Long
Resistor values adjusted to post-fabrication values for simulation
Alpha and Beta models define different gm values: gm2_Alpha is
14% higher than gm2_Beta
Ballast resistor (RE2) reduces difference in gm to just 2%
Measurement vs. Simulations: LF Gain
11.5 dB
11.1 dB
11.35dB
Benchmarking Circuits for Model Verification 15
HICUM Workshop 2013 L. Vera and J. R. Long
Measurement vs. Simulations: HF Gain
Initial comparison of measurements vs. simulations showed a >40GHz
difference in -3dB bandwidth using Alpha model
CBC and CCS in Alpha model are one-half of Beta model values
Simulations using Beta model fit measurements better than Alpha model
55 GHz
54 GHz
95 GHz
Benchmarking Circuits for Model Verification 16
HICUM Workshop 2013 L. Vera and J. R. Long
Input and Output Impedances
|S11| for Alpha and Beta models do NOT agree with measurements
for f > 10GHz
Beta model predicts higher |S11| and |S22| than measured due to:
lower rπ, rb and ro, and higher CBC and CCS
Benchmarking Circuits for Model Verification 17
HICUM Workshop 2013 L. Vera and J. R. Long
Reconstructing the Amplifier
The amplifier was reconstructed using single transistor S-parameter
measurements (up to 50 GHz), ideal resistors and EM-simulation
datasets for the input and output transmission lines
Method does not account for biasing variations
Layout
Q2
RE1 RE2
RF
Q1
PAD + TL TL + PAD
R (ideal)Transistor
S-param.
Cadence Schematic
Benchmarking Circuits for Model Verification 18
HICUM Workshop 2013 L. Vera and J. R. Long
Reconstructed Amplifier Comparison
Reconstructed amplifier agrees better with measured data than
simulations with Alpha model
Differences between simulated and reconstructed |S11| are related
to transistor parameters rπ, rb and CBC
Benchmarking Circuits for Model Verification 19
HICUM Workshop 2013 L. Vera and J. R. Long
Modified Beta Model
Simulated |S11| and |S22| using modified Beta model (higher
CBC and CCS, and lower rπ, rb and ro) approaches
measurements better than original Alpha and Beta models
Benchmarking Circuits for Model Verification 20
HICUM Workshop 2013 L. Vera and J. R. Long
Monte Carlo and Modified Beta Model
Monte Carlo simulations for 100 cases (gray lines) show the circuit
sensitivity to processing variations
Measurements lie within the Monte Carlo simulation bounds
Simulation setting process parameters in the model close to
fabricated values (red lines) approaches measurements best
Benchmarking Circuits for Model Verification 21
HICUM Workshop 2013 L. Vera and J. R. Long
Increasing fmax
fmax may be improved changing rb, CBC , or both:
Lower R: 27% lower base resistance, 4% higher collector-base
capacitance, 14% higher fmax
Lower C: 29% lower collector-base capacitance, 20% higher
base resistance, 8% higher fmax
Lower RC: 20% lower base resistance and 34% lower collector-
base capacitance, 37% higher fmax
(E.2)fT
2⋅π⋅rb⋅CBC
fmax =
Benchmarking Circuits for Model Verification 22
HICUM Workshop 2013 L. Vera and J. R. Long
Broadband Amplifier Analysis
Small-signal amplifier model retains components dominant in the
frequency response
Dominant time constant of the simplified model (eq. E.1) examined
for optimization
(E.1)RF
1+ R’L+RF+ g’mR’LR’Gτp =
[Cin+Cµ(1+g’mR’L)]R’G+(CL+Cµ)R’L
Benchmarking Circuits for Model Verification 23
HICUM Workshop 2013 L. Vera and J. R. Long
Sensitivity Analysis Using HICUM
Four time constants (τi ) dominate the first pole of the amplifier
Reducing τ1 and τ2 benefit amplifier bandwidth the most
8.7
8.7
8.7
8.7
τ4 = CLR’L(%)
1.8
2
3
2.9
τ3 = CµR’L(%)
75.4
95.5
87.5
100
τTOT = Σ τi
(%)
24.4
33.2
37.2
42.4
τ2 = Cµ(1+gmR’L)R’G(%)
Lower RC
Lower C
Lower R
Fabricated
npn option
40.5
51.6
38.6
46
τ1 = CinR’G(%)
Benchmarking Circuits for Model Verification 24
HICUM Workshop 2013 L. Vera and J. R. Long
Simulations predict 19.5GHz higher -3dB bandwidth when CBC and
rb are optimized
1.39
1.06
1.10
1
-3dB BW
simulated
1.33
1.10
1.14
1
-3dB BW
analytical
Lower RC
Lower C
Lower R
fabricated
Process
1.37
1.08
1.14
1
fmax
55.7 GHz
50.5 GHz
53.6 GHz 70 GHz
S21
frequency (GHz)
S21 (d
B)
3dB
Sensitivity Analysis Using HICUM
Benchmarking Circuits for Model Verification 25
HICUM Workshop 2013 L. Vera and J. R. Long
Large-Signal Circuits
Large-signal circuit performance depends on:
Time-varying capacitances
Bias-dependent intrinsic base resistance
Non-quasi-static effects (NQS)
Potential candidates for study are:
Power amplifiers
Switching logic (e.g., frequency divider)
Benchmarking Circuits for Model Verification 26
HICUM Workshop 2013 L. Vera and J. R. Long
Operating Above Breakdown Voltage
SiGe HBTs reach higher speeds at the cost of reduced breakdown voltages
Some circuits may operate properly with VCE above
BVCEO (e.g., common base amplifier), but other configurations do not function properly under this
condition
Precise and accurate modeling above BVCEO
needed to predict circuit behavior when VCE > BVCEO
Benchmarking Circuits for Model Verification 27
HICUM Workshop 2013 L. Vera and J. R. Long
Static Divide-by-2
Large-signal benchmark circuit
Output is at one-half the frequency of the input signal
Metrics: maximum toggle frequency, rise/fall time, input
sensitivity, output voltage swing, etc.
Maximum operating frequency is limited by the delay of the
individual latch stages
Master/Slave D-type Flip-Flop
RF IN+ RF IN-
RF OUT+ RF OUT
GND GND
I1
I2V1
V2
Benchmarking Circuits for Model Verification 28
HICUM Workshop 2013 L. Vera and J. R. Long
ECL Latch
Speed is proportional to RC time
constant at node A as defined by:
CBC, CCS (Q1 and Q3), CBC and CBE
(Q5), and R1
Given an output voltage amplitude,
peak current in Q1,3 goes above
peak fT (i.e., higher diffusion cap.,
smaller R1 – effective RC reduction)
Q7 and Q8 sized to conduct peak
currents around peak fT
VCB of Q3 (and Q4) is VBE of Q5
(Q6), i.e., VCE~2VBE
Key circuit parameters are time
variant
Benchmarking Circuits for Model Verification 29
HICUM Workshop 2013 L. Vera and J. R. Long
Verifying Model Above Breakdown
Measured and simulated results agree in amplitude and output
frequency
Bias-T bandwidth (50GHz) limits measured rise/fall time
Full characterization of divider requires investment in (multiple) test
sources for K, Ka, V and W-bands
Measurement
200mV
133ps
Beta model
200mV
133ps
Alpha model
66ps
Benchmarking Circuits for Model Verification 30
HICUM Workshop 2013 L. Vera and J. R. Long
Conclusions
Benchmarking measurements motivate
modifications to transistor rπ, rb, ro, CBC and CCS
Model parameter accuracy improved by
correlating model simulations with benchmark circuit measurements (e.g., Monte Carlo
simulations of modified Beta model for the BBA)
Optimizing a benchmarking circuit is a quantitative
way of evaluating technology development scenarios (e.g., for fmax/fT)
Accurate models for VCE>BVCEO are required
Recommended