Power dependent input impedance of field plate MESFETs

Field plates, when applied to FETs, are known to allow operation at much higher drain voltages and power densities. A gate field plate changes the internal charge and electric field distribution within the MESFET channel and therefore affects the internal capacitances. The average gate capacitance and transconductance are shown to be strongly dependant on the power drive level or time average of the gate voltage. The source matching impedance strongly affects the compression characteristics of the 4mm unit FET cell, for a given fixed output load impedance tuned for maximum delivered power. Proper compression and optimum performance are achieved when the MESFET input impedance is tuned under high power drive conditions. The gate-source capacitance is shown to strongly decrease with power drive and must be properly accounted for to achieve optimum RF performance of a high power amplifier (HPA).