Evaluation of GaAs Power MESFET for Wireless Communication

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21 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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Evaluation of GaAs Power MESFET for
Wireless Communication

Li Xiang


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

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


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





Linearity


Definitions of linearity


1 dB gain compression point




Third
-
order intermodulation distortion





Adjacent channel leakage power

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

Fabrication Flow

Frequency Dispersion of I
dss


FETs with setback layer show smaller frequency dispersion


15
-
20% improvement at 1 MHz


RF Power Measurement

Power Characteristics at 950 MHz


Test signal:

/4 shift QPSK modulation


Operational condition: class AB


Improved linearity: P
1dB

34.5 dBm


36 dBm

IM3 Characteristics


Bias point: 10% I
dss


Improved IM3: 10 dB smaller

Distortion Characteristics


Channel separation: 50 kHz


Improvements: 11 dB lower at 31.5 dBm output power

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.