A Consistent Charge Model of GaAs MESFETs for $Ku$-Band Power Amplifiers

In this paper, a consistent gate charge model for GaAs MESFETs based upon charge conservation is proposed for monolithic microwave integrated circuit power amplifier designs. This new model is capable of accurately modeling the transistor under various biasing conditions. The conventional approaches for charge modeling of GaAs MESFETs usually adopt analytical equations to fit nonlinear gate capacitors separately, which might be difficult to implement in circuit simulators whose capacitance is always the derivative of an internal state variable (charge). Moreover, compared with the conventional diode and Statz model, the performance prediction in the linear region, saturation knee region, and subthreshold region is greatly improved. Measured and modeled results of a 2 ×150 μm GaAs MESFET are compared and good agreement has been obtained. Comparisons between the proposed model, diode junction model, and Statz model are also presented in this paper. In addition, a class-AB Ku-band power amplifier using a 0.18-μm GaAs MESFET process was designed with the new model for verification of the new model accuracy.