1. Evaluation of GaAs Power MESFET for Wireless Communication
Li Xiang

2. Outline Introduction on power amplifiers for wireless communications
Specific requirements for power amplifier design
How to evaluate efficiency and linearity
Design example of a low-distortion power MESFET
Summary

3. Introduction on RF Power Amplifiers
Technologies suitable for RF power amplifiers
Si BJT, MOSFET
GaAs MESFET, HFET, HBT
SiGe HBT
InP HFET
Wide bandgap materials
Specific requirements for power application
Thermal conductivity
Breakdown voltage
Efficiency
Linearity
Reliability

4. Efficiency Two normally used definitions:
Drain efficiency:
Power-added efficiency
Methods to improve the efficiency
Suppress leakage
Schottkey gate leakage
Substrate leakage
Enhancement mode operation

5. Linearity Definitions of linearity 1 dB gain compression point
Third-order intermodulation distortion
Adjacent channel leakage power

6. Design Example:* Low-distortion Power MESFET
Goal:
To design a low-distortion GaAs MESFET suitable for digital communication system using /4 shift QPSK modulation
Origination of distortion:
Frequency dispersion of transconductance/drain current originated from electron trapping at the gate surface
New structure to improve the distortion performance:
Form semi-insulating setback layer under the gate
* H. Furukawa et. al. IEEE Transactions on Electron Devices, vol. 43, No. 2, 1996

7. Fabrication Flow

8. Frequency Dispersion of Idss
FETs with setback layer show smaller frequency dispersion
15-20% improvement at 1 MHz

9. RF Power Measurement

10. Power Characteristics at 950 MHz
Test signal: /4 shift QPSK modulation
Operational condition: class AB
Improved linearity: P1dB 34.5 dBm 36 dBm

11. IM3 Characteristics
Bias point: 10% Idss
Improved IM3: 10 dB smaller

12. Distortion Characteristics
Channel separation: 50 kHz
Improvements: 11 dB lower at 31.5 dBm output power

13. Summary
A brief introduction has been given on how to evaluate GaAs power FETs.
A GaAs MESFET design with a semi-insulating setback layer has been presented, and the distortion characteristics have been evaluated.