High Linearity and High Efficiency Power Amplifiers in GaN HEMT Technology

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High Linearity and High Efficiency Power
Amplifiers in GaN HEMT Technology

• Shouxuan Xie
• Department of Electrical and Computer
  Engineering,
• University of California, Santa Barbara
• June 30, 2003

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Outline

• 1. Introduction and motivation
• - Why GaN HEMTs
• - Objectives of the GaN HEMTs PA design
• Class B for high efficiency and high linearity
• - Why single-ended Class B
• - Circuit design and measurement result
• Identify and model nonlinear sources of GaN HEMTs
• - Nonlinear gm
• - Nonlinear Cgs
• - Nonlinear Gds
• Proposed new designs to further improve linearity
• - Common drain Class B (to improve gm
  nonlinearity)
• - Pre-linearization diode (to improve Cgs
  nonlinearity)
• 5. Problems and future works

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Why GaN HEMTs
Standard AlGaN/GaN HEMT structure

• Advantages of GaN
• - High breakdown field 3 MV/cm
• - High Vsat _at_ 2.5 x 107 cm/s
• - Thermal conductivity 3x GaAs
• - Large channel charge gt 1x1013 cm-2
• - Good electron mobility gt1200 cm2/V-s

• Advantages of GaN HEMTs
• - High power density 12W/mm for X-band (8-12GHz)
  
• - High Ft (50GHz) and fmax (80GHz) for 0.25um
  device
• - Linear I-V characteristics

4
GaN HEMT process and device structure
0.25um T-gate for 50GHz ft
Air-bridge for ground connection of CPW
MIM capacitors
SiN passivation for High RF output power
SiC substrate for high heat conducting
5
Device performance
I-V Curve for 600?m SG device
Linear Id-Vgs characteristic on SiC
Idss 1 A/mm _at_Vgs0V
RF Performance 150?m DG device
Device performance summary

• Lg 0.25um,
• Idss 1A/mm
• ft 55GHz (50GHz for DG)
• Vbr 40V (55V for DG)

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Objectives of GaN HEMT PA design
Design RF MMIC power amplifier in GaN HEMT
technology to achieve 1. High linearity (low
IMD3 distortion) 2. High efficiency 3. High
output power 4. Broad bandwidth (High linearity
and high efficiency are primarily concerned here)
Class A Very high linearity and wide bandwidth
but very low efficiency (Ideal PAE 50, feasible
PAE 20-30). Switch mode Amplifiers (Class D,
E) Very high efficiency (Ideal PAE 100,
feasible PAE 60-70 ) but poor linearity and
poor bandwidth. Class B Good efficiency (Ideal
PAE 78.6 feasible PAE 40-50 ) and good
bandwidth, and potentially low distortion.
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Push-pull Class B

• Even harmonics are suppressed by symmetry gt wide
  bandwidth
• Half-sinusoidal current is needed at each drain.
  This requires an even-harmonic short. It can be
  achieved at HF/VHF frequencies with transformers
  or bandpass filters. However,

1. Most wideband microwave baluns can not provide
   effective short for even-mode. Efficiency is
   then poor.
2. They occupy a lot of expensive die area on MMIC.

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Single-ended push-pull
Push-pull Class B
Even harmonics suppressed by symmetry
Single-ended Class B with bandpass filter
Even harmonics suppressed by filter
Conclusion From linearity point of view,
push-pull and single-ended Class B with bandpass
filter B are equivalent same transfer function.
Bandwidth restriction lt 21
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Class B bias for high linearity
Ideal Class B
Bias too low Class C
Bias too high Class AB
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Single-ended Class B Power Amplifier
? - section lowpass filter
Lossy input matching

• Dual gate device is used since it has higher
  Vbr, higher MSG (smaller S12) and higher output
  resistance Rds
• Lossy input matching network to widen the
  bandwidth
• Cds is absorbed into output matching network
  (Low pass filter)

11
Measurement setup
Measurements

• Single tone from 4 GHz to 12 GHz
• Two-tone measurement at f1 8 GHz, f2 8.001
  GHz
• Bias sweep Class A (Vgs -3.1V), Class B (Vgs
  -5.1V), Class C (Vgs - 5.5 V) and AB (Vgs
  -4.5 V).

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Class B PA measurement results
Gain and bandwidth
Class AB
Class B
3 dB bandwidth for Class B 7GHz - 10GHz
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Class B bias _at_Vgs - 5.1V
Single tone performance _at_ f0 8GHz
Saturated output power 36 dBm
PAE (saturated) 34
f1,f2
Two tone performance _at_ f18GHz, f28.001GHz
2f1-f2, 2f2-f1

• Good IM3 performance
• 40dBc at Pin 15 dBm
• gt 35 dBc for Pin lt 17.5 dBm

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Class A bias _at_Vgs - 3.1V
Single tone performance _at_ f0 8GHz
Saturated output power 36 dBm
PAE (saturated) 34
Two tone performance _at_ f18GHz, and f28.001GHz
f1,f2
2f1-f2, 2f2-f1
Good IM3 performance at low power level but
becomes bad rapidly at high power levels
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IM3 suppressions of all Classes

1. Low output power levels (Pout lt 24 dBm), Class A
   and Class B both exhibit good linearity (Class B
   gt 36 dBc, Class A gt 45 dBc).
2. Higher output power levels, Class A behaves
   almost the same as Class B.
3. Class AB and C exhibit more distortion compared
   to Class A and B.

16
Class B vs. Class A
IM3 suppression and PAE of two-tone
PAE of single tone
Class A
Class B
Class B
Class A
Maintaining good IM3 suppression, Class B can get
10 PAE improvement over Class A during low
distortion operation.
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Nonlinear sources of GaN HEMT
1. gm vs. Vgs of 600um SG device
Goal Try to investigate nonlinear sources of the
GaN HEMT device and understand how they affect
the linearity on circuit
Three major sources have been investigated 1.
Nonlinear gm ( or Ids -Vgs characteristic) 2.
Nonlinear Cgs 3. Nonlinear Gds
3. Gds vs. Vgs and Vds of 600um SG device
2. Cgs vs. Vgs of SG device
Vds20V
Vds15V
Vds10V
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Nonlinear sources of GaN HEMT
Input MN (linear, Zs)
Cgs
Cds
RL
Gds
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Nonlinear gm
Input MN (linear, Zs)
Cgs
Cds
RL
Gds
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Nonlinear gm
Modeled as
This term creates IM3 distortion
Dominate at high output power levels more
interesting
Vp
Dominate at low output power levels
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Nonlinear Cgs
Directly effect of Cgs
Input MN (linear, Zs)
Cgs
Cds
-

RL
Gds
Q(Vgs)
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Nonlinear Cgs
Cgs vs Vgs of GaN HEMTs on SiC
If modeled as
direct
Anti-symmetric about VVc
then should be no distortion
Vc ? Vp
Therefore even order component of Cgs(Vgs)
creates IM3 distortion
This term creates IM3 distortion
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Nonlinear Cgs Indirect effect
Input MN (linear, Zs)
Cgs
-

Cds
RL
Q(Vgs)
Gds
24
Nonlinear Cgs Indirect effect
Input MN (linear, Zs)
Cgs
-

Cds
RL
Q(Vgs)
Gds
25
Nonlinear Cgs nonlinear gm
Input MN (linear, Zs)
Cgs
Cds
Gds
direct
Indirect
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Nonlinear Gds
Input MN (linear, Zs)
Cgs
Cds
RL
Rds
Gds vs. Vgs of 600um SG device
Vds20V
Vds15V
Vds10V
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Nonlinear Gds
DC I-V curve of 600um device on SiC
Short channel effect
Vgs 0 V
Vgs -7 V Vds 15V
Vgs -7V Vds 8V
Current through GaN buffer, need more gate
voltage to pinch off
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Vp shift due to short channel effect
1.2mm SG device DC I-V curve at different drain
bias
Vds20V
Vds15V
Vp shift
Vds10V
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Nonlinear Cgs Vp shift
Cgs(Vin)
Cgs(-Vin)
DC
Even order component
VbVpVc
VbltVc
VbgtVc
VbgtgtVc
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Paidis nonlinear model
Nonlinear Gds currently is modeled by shift in
Vp
Cgs is ideal tanH
I-V characteristic currently is linear
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Further improve linearity
1. Common drain Class B to improve gm linearity
CD circuit schematic
Linearization factor
RL also functions as series-series feedback
resistor, which increase gm linearity.
RL
Disadvantage -- Stability problem Since the MSG
is less, the circuit is not unconditionally
stable in order to keep reasonable high
efficiency. Therefore, extra requirement for the
source and load impedance is needed.
32
Simulation result of CD _at_5GHz
Pout and PAE in single tone
Pout 38dBm
Pout
PAE
PAE(sat) 38
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Simulation result of CD vs. CS cont.
Two-tone simulation result of CD vs. CS
Common Drain
10 dB
12 dB
Common Source with 37.6dBm Pout and 42 PAE(sat)
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Common Drain vs. Common Source cont.
Simulation result of IM3 suppression at 1W total
output power as a function of bias point
Class C
Common Drain
Class AB
Common Source
Class B
Class A
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Further improve linearity cont.
2. Pre-linearization diode to improve Cgs
linearity
C_total
Cgs
C_pd
Vc
36
Pre-linearization diode
Vb1Vp-4V
0.25umx100umx12
Vb12Vp-8V
Can be very easily implemented on chip and occupy
very small area
Gate length can be varied and optimum value can
be found since write using E-beam-lithography
0.75umx100umx4
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Simulation result of PD
IM3 simulation result the designed dual gate CS
Class B with pre-linearization diode _at_10GHz
At least 4dBc improvement in IMD3
With PD
Without PD
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Problems and future works
!! Problem Short channel effect for 0.25um
device !!
0.75umx100um device on Sapphire
0.25umx100um device on Sapphire
Vgs0V
Vgs0V
Vgs-10V Vds16V
Vgs-7V

• Nonlinear Gds will affect linearity performance
  directly
• It creates Vp shift, hence generate nonlinear
  Cgs distortion
• Increases DC bias current, hence decreases PAE
• Decreases breakdown voltage, hence decreases the
  output power and also PAE

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Short channel effect
Currently dual gate device is used - Nearly no
Vp shift - Lower Gds (higher Rds) - Higher
maximum stable gain (MSG)
I-V curve of 600um DG device
Gds of 600um devices at Vds20V
Vds 15V
Single gate
Vds 20V
Dual gate
- Number of gates get doubled, hard to yield
all - Little bit lower ft, and higher Vknee,
hence lower PAE - Not easy to model the
nonlinear effect
40
Layouts of the new designed circuits
CD SG Class B _at_5GHz
CS SG Class B _at_5GHz
CS DG Class B _at_10GHz
CS DG Class B _at_10GHz with PD
41
New device structures to improve linearity
Improve short channel effect by - Make the Fe
doping layer closer to the channel - Gate
recess to increase aspect ratio
Add Fe doping layer to decrease leakage current
through the buffer
??? Question How about decrease Al in AlGaN
-Increase breakdown and decrease gm? How about
P-type doping GaN buffer layer?
??? Other ideas to increase breakdown???
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Summary

• Class B bias is good for high linearity and high
  efficiency
• Three main nonlinear sources of the GaN HEMT
  device have been investigated with a new idea of
  nonlinear model
• According to simulation, common drain class B can
  improve linearity by 10dB over CS, and
  pre-linearization diode can improve linearity by
  4dB. Four more circuits are designed and being
  fabricated to prove them
• Short channel effect for 0.25um device has been
  observed. New device structure is proposed to
  solve the problem and better linearity
  performance is expected.

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Proposed future works
1. Fabricate and measure the new designed
circuits (CD and PD) - Need to stabilize the
PECVD passivation process 2. Complete the new
model to understand all the nonlinear effects
- Add gm nonlinearity - More accurate model
for dual gate device 3. Further improve
linearity by new device structures - Work
with Mishras group to improve the short channel
effect 4. Publish paper and write thesis 5. New
ideas on device structure and model to further
increase linearity and efficiency
summer
summer
Fall
Fall
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Publications and references
Publications

1. Vamsi Paidi, Shouxuan Xie, R. Coffie, U. Mishra,
   M J W Rodwell, S. Long, Simulations of High
   linearity and high efficiency of Class B Power
   Amplifiers in GaN HEMT Technology.  Lester
   Eastman Conference, Aug. 2002
2. Shouxuan Xie, Vamsi Paidi, R. Coffie, S. Keller,
   S. Heikman, A. Chini, U. Mishra, S. Long, M.
   Rodwell, High Linearity Class B Power Amplifiers
   in GaN HEMT Technology. Topical Workshop on
   Power Amplifiers, Sept. 2002
3. Shouxuan Xie, Vamsi Paidi, R. Coffie, S. Keller,
   S. Heikman, A. Chini, U. Mishra, S. Long, M.J.W.
   Rodwell, High linearity of Class B Power
   Amplifiers in GaN HEMT technology. Microwave and
   Wireless Components Letters, to be published
4. Vamsi Paidi, Shouxuan Xie, R. Coffie, B. Moran,
   S. Heikman, S. Keller, A. Chini, S. P. DenBaars,
   U. K. Mishra, S. Long and M. J.W. Rodwell, High
   Linearity and High Efficiency of Class B Power
   Amplifiers in GaN HEMT Technology.  IEEE
   Transactions on Microwave Theory and Techniques,
   Vol. 51, No. 2, Feb. 2003

Other references

1. K. Krishnamurthy, R. Vetury, S. Keller, U.
   Mishra, M. J. W. Rodwell and S. I. Long,
   Broadband GaAs MESFET and GaN HEMT Resistive
   Feedback Power Amplifiers. IEEE Journal of Solid
   State Circuits, Vol. 35, No. 9, Sept. 2000.
2. K. Krishnamurthy, S. Keller, C. Chen, R. Coffie,
   M. Rodwell, U. K. Mishra, Dual-gate AlGaN/GaN
   Modulation-doped Field-effect Transistors with
   Cut-Off Frequencies ƒT gt60 GHz, IEEE Electron
   Device Letters, Vol. 21, No. 12, Dec. 2000

45
Publications and references- cont.

• Solid State Radio Engineering, Herbert L. Krauss,
  W. Bostian, Frederick H. Raab/ Wiley, John
  Sons, Nov. 1980
• Raab, F.H. Maximum efficiency and output of
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  Microwave Theory and Techniques, vol.49, (no.6,
  pt.2), IEEE, June 2001. p.1162-6.
• Kobayashi, H. Hinrichs, J.M. Asbeck, P.M.
  Current-mode class-D power amplifiers for
  high-efficiency RF applications. IEEE
  Transactions on Microwave Theory and Techniques,
  vol.49, (no.12), IEEE, Dec. 2001. p.2480-5.
• Eastman, L.F. Green, B. Smart, J. Tilak, V.
  Chumbes, E. Hyungtak Kim Prunty, T. Weimann,
  N. Dimitrov, R. Ambacher, O. Schaff, W.J.
  Shealy, J.R. Power limits of polarization-induced
  AlGaN/GaN HEMT's. Proceedings 2000 IEEE/ Cornell
  Conference on High Performance Devices,
  Piscataway, NJ, USA IEEE, 2000. p.242-6. 274
  pp..
• Wu, Y.-F. Kapolnek, D. Ibbetson, J. Zhang,
  N.-Q. Parikh, P. Keller, B.P. Mishra, U.K.
  High Al-content AlGaN/GaN HEMTs on SiC
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  International Electron Devices Meeting 1999,
  Piscataway, NJ, USA IEEE, 1999. p.925-7. 943 pp.
• Joseph, J. Teaching design while constructing a
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Thank you!
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Does Vc change?
Vc
Consider Cgs nonlinearity only simulate IM3
result at 1W output power level Vp -5V, Vc
-5V, without PD 46.3dBc, with PD 57.4dBc Vp
-5.5V, Vc -5V, without PD 40.1dBc, with PD
57.6dBc
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GaN HEMT Model Vp shift
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Advantages of GaN
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Class B two-tone output spectrum
Medium input power 1
Low input power
Pout 18 dBm IM3 39 dBc
Pout 4 dBm IM3 43 dBc
Medium input power 2
High input power
Pout 26 dBm IM3 25 dBc
Pout 22 dBm IM3 40 dBc
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Class A two-tone output spectrum
Low input power
Medium input power 2
Pout 10 dBm IM3 gt 50 dBc
Pout 23 dBm IM3 42 dBc
Medium input power 2
High input power
Pout 27 dBm IM3 31 dBc
Pout 31 dBm IM3 15 dBc