Capacitance Characterization and Modeling of Metal - Field-Effect Transistors

GaAs metal-semiconductor field-effect transistors (MESFETs) have been increasingly used for signal amplification in battery-powered wireless communication equipment. A large-signal MESFET model accurate for such low-voltage operations is essential for predicting the amplifier nonlinearity, as well as for optimizing the MESFET structure to minimize the nonlinearity. This work focuses on the capacitance instead of conductance characteristics of the MESFET, because the former has not been extensively studied and is particularly important for low-voltage operations. The modeling work comprises both physical and empirical models. The physical device simulation was performed by solving the Poisson and current continuity equations in two dimensions. Drain -bias-dependent nonlinearity of gate-source and gate-drain capacitances were found when the MESFET transitions from linear to saturation mode of operations. From the potential and charge distributions, it was concluded that this nonlinearity is due to dipole domain formation at the drain end of the channel. Therefore, devices with thin channel thickness, or low-doped region between gate and drain electrodes, or long gate-length, contain less dipole induced nonlinearity. A bias-dependent large-signal capacitance model was empirically developed and implemented in a commercial microwave circuit simulator. This model incorporates the nonlinearity found during the physical device simulation. This model was verified by microwave power and waveform measurements in both frequency and time domain. According to the model, the gate capacitance nonlinearity can cause harmonic distortion at moderate input power levels, under which the amplifier would otherwise be linear.