Evaluation of GaAs Power MESFET for Wireless Communication

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Evaluation of GaAs Power MESFET for Wireless
Communication

• Li Xiang

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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

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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

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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

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Linearity

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

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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
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Fabrication Flow
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Frequency Dispersion of Idss

• FETs with setback layer show smaller frequency
  dispersion
• 15-20 improvement at 1 MHz

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RF Power Measurement
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Power Characteristics at 950 MHz

• Test signal ?/4 shift QPSK modulation
• Operational condition class AB
• Improved linearity P1dB 34.5 dBm? 36 dBm

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IM3 Characteristics

• Bias point 10 Idss
• Improved IM3 10 dB smaller

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Distortion Characteristics

• Channel separation 50 kHz
• Improvements 11 dB lower at 31.5 dBm output power

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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.