Application of Non Application of Non - - Linear Models in a Linear Models in a range of challenging GaN HEMT Power range of challenging GaN HEMT Power Amplifier Designs Amplifier Designs Ray Pengelly, Brad Ray Pengelly, Brad Millon Millon , Don Farrell, , Don Farrell, Bill Bill Pribble Pribble and Simon Wood and Simon Wood WMC: Challenges in Model-Based HPA Design Cree Inc., Research Triangle Park, NC 27709 Cree Inc., Research Triangle Park, NC 27709
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Application of NonApplication of Non--Linear Models in a Linear Models in a
range of challenging GaN HEMT Power range of challenging GaN HEMT Power
Amplifier Designs Amplifier Designs
Ray Pengelly, BradRay Pengelly, Brad MillonMillon, Don Farrell, , Don Farrell,
BillBill PribblePribble and Simon Woodand Simon Wood
WMC: Challenges in Model-Based HPA Design
Cree Inc., Research Triangle Park, NC 27709Cree Inc., Research Triangle Park, NC 27709
OutlineOutline
• Attributes of GaN HEMTs
• Cree GaN HEMT Models
• Design Examples
– Broadband CW Amplifiers
– Linear WiMAX Amplifier
• Future Model Improvements
• Conclusions
Attributes of GaNAttributes of GaN HEMTsHEMTs
• High Voltage Operation
• High power densities – 4 to 8 watts/mm at 28 and
50 volt operation respectively
• High Frequency Performance – present Cree process has fT of 25 GHz
• High Efficiency
• Low Quiescent Current
• High Native Linearity
• Low capacitance per peak watt (12% of LDMOS and 21% ofGaAs MESFET) – supports broad bandwidths
• Enable new amplifier architectures
• Highly correctable under DPD
• Almost constant CDS as a function of VDS – great for Drain Modulation
WideBan
dgap
Models for GaNModels for GaN HEMTsHEMTs
• Equivalent-circuit based approach
– Relatively simple extraction
– Process sensitive based on individual elements
– Simple implementation using commercial harmonic balance simulators
• Significant historical information for model basis and validation
• Non-linearity introduced as required by element
– Drain current source is dominant non-linearity
– Gate current formulation includes breakdown and forward conduction
– Voltage variations of parasitic capacitances derived from charge formulations
• Model data fit extends over drive, frequency, bias, and temperature
• Many hundreds of successful hybrid and MMIC designs
Model SchematicModel Schematic
vd1g
vg1
Port
P2
Num=2
SRL
SRL4
L=(ld/sc+ld1/(sc*scf)) nH
R=(rd/(sc*scf)) Ohm
R
R6
R=1e6 Ohm
pncap3X15
vgg0=-21 V
cg3=0.1
cg2=0.6
cg1=cgd pF
scf=scf
sc=sc
R
R3
R=0.01 Ohm
SRL
SRL1
L=(lg/sc+lg1/(scf*sc)) nH
R=(rg/sc+rg1/(sc*scf)) Ohm
pncap3
X12
vgg0=vgg0 V
cg3=cg3
cg2=cg2
cg1=cgs pF
scf=scfsc=sc
Port
P1
Num=1
R
R1
R=(ri/(sc*scf)) Ohm
Port
P5
Num=5
SRLSRL3
R=(rs/(sc*scf)) Ohm
L=(ls/sc) nH
Port
P4
Num=4
C
C8
C=30.0 nF
R
R2
R=rth Ohm
VCVS
SRC1
G=1
Port
P8
Num=8
C
C7
C=1.0 uF
R
R5R=1e6 Ohm
C
C9
C=(cds*sc*scf) pF
SDD6P
SDD6P1
Cport[1]=
C[1]=I[6,0]=(vdf)/5e4
I[5,0]=-vd*idt
I[4,0]=(_v4)*gmc
I[3,0]=(_v3)*gdsc
I[2,0]=id1
I[1,0]=(vg)/5e8
• Based on 13-element MESFET model (H. Kondoh – 1986 MTT-S)
• ADS version shown using non-linear equation-based elements
– Easily changed during design process
– Speed comparable to C-coded version
• AWR version uses C code with “model wizard”
Drain current
Thermal resistance
More details on More details on GaNGaN HEMT ModelHEMT Model
• Most FET models implement a gate current-control characteristic that transitions from the sub-threshold region to the linear gate control region directly, without treating the intermediate region, called the quadratic region. Fager et al. implemented an equation and new parameters to fit the quadratic region. This leads to better agreement with measured IMD and other nonlinear characteristics.
• Gate charge is partitioned into gate-source and gate-drain charge. Each charge expression is a function of both VDS and VGS. Using charge partitioning, it is possible to fit most GaN HEMT capacitance functions and observed charge conservation.
-15 -10 -5 0 5 8
Voltage (V)
gm and Ids
0
5
10
15
20
25
30
0
666.7
1333
2000
2667
3333
4000
p1
|S(2,1)|[1,X] (L)Schematic 1
IDC(I_METER.AMP1) (R, mA)Schematic 1
p1: Freq = 0.05 GHz
Sub
Threshold
Blue is DC transconductance
Red is drain current
Quad Linear Compression
Ids, mA
Gate voltage, volts
Gm, mS500
250
0-4 -3 -2 -1 0 0.5
• The model includes new capacitance functions as well as modeling of the drain-source breakdown and self heating.
• The model has four ports, with the extra port providing a measure of the temperature rise. The voltage between the external thermal circuit port and the source node is numerically equal to the junction temperature rise in degrees C. This occurs because the current source in the thermal circuit is numerically equal to the instantaneous power dissipated in the FET and the resistance, R_TH is numerically equal to the thermal resistance. The RC product of the thermal circuit is the thermal time constant.
• The model addresses the sharp turn-on knee in GaN HEMTs leading to the accurate prediction of IMD sweet spots in Class A/B operation.
More details on More details on GaNGaN HEMT ModelHEMT Model
Drain Current ModelDrain Current Model
-3 -2 -1 0 1-4 2
0.05
0.10
0.00
0.15
Vlow
gm
5 10 15 20 25 30 35 400 45
0.1
0.2
0.3
0.4
0.0
0.5
Vhigh
perm
ute(Is_high.i)
• Transconductance curve fit to Gm from small-signal model fits over bias range
• Output conductance dispersion model
• Peak current and knee voltage fit from load-pull - includes trap effects
• Pinch-off fit from DC IV-characteristics – gives model of drain current
• IV function similar to Fager-Statz formulation – good model of pinch-off needed to accurately predict intermodulation distortion
Temperature Dependence Temperature Dependence –– SelfSelf--heatingheating
34
34.5
35
35.5
36
36.5
0 50 100 150 200
chuck temp
output power
• Drain current is only temperature dependent model element
• Drain current scales to provide -0.1 dB/10oC reduction in power for current-limited load-line
• Self-heating included using a thermal resistance – calculated from finite element analysis of die and package.
• Thermal performance due to package needs to be included where appropriate
• Feedback capacitance is a strong function of drain voltage
• Inclusion of this effect necessary to fit small-signal data
• Non-linearity changes harmonic generation from the model – effects efficiency and linearity predictions
• Output Capacitance CDS is linear – no voltage dependence (weak anyway)
Input Capacitance Input Capacitance -- CCGSGS
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
Gate Voltage (V)
Input Capacitance Cgs (pF)
• Input capacitance is a strong function of gate voltage
• CGS is also a function of drain voltage, but this non-linearity is not included at present
• The gate-voltage non-linearity also effects model’s harmonic generation
GaN HEMT Model GaN HEMT Model -- SmallSmall--SignalSignal
• On-wafer S-parameters of 0.5 mm HEMT – 25OC baseplate
• Major challenge of modeling for high power circuits – scaling from reasonable test cell to large periphery output stages – successfully implemented for scaling factors >100:1
• Non-linear model fits small-signal parameters over a range of bias voltages
• All measurements performed using 1% duty cycle, 20µs pulsed bias to control thermal effects
ModelGmax
MeasuredGmax
measured
model
freq (1.000GHz to 14.00GHz)
a(1,1)
b(1,1)
b(2,2)
a(2,2)
1E101E9 2E10
15
20
25
10
30
freq, Hz
ag
bg
GaN HEMT Model GaN HEMT Model -- LargeLarge--SignalSignal
measured
model
• On-wafer load-pull of 0.5 mm HEMT
• Measured at 3.5 GHz, VDS=48V, Id~25%IDSS, 25OC chuck temperature
• PAE contours not used for modeling due to sensitivity to harmonic loading –PAE verified using hybrid amplifier measurements
Power Contour Levels:
36 dBm
35 dBm
34 dBm
Basic Thermal Features of High Voltage GaNBasic Thermal Features of High Voltage GaN
• 240 Watts CW RF from a 28.8mm HEMT operating at 50 volts drain voltage
• Assume 60% DC to RF conversion efficiency
• 160 watts dissipated heat
• Active chip area is 2.5 sq. mm so heat density is
> 40 kilowatts per square inch !
• Much emphasis on new amplifier architectures to improve drain efficiencies