Current—Voltage and capacitance—Voltage characteristics of modulation-doped field-effect transistors

A model describingI-VandC-Vcharacteristics of modulation doped FET's is developed and used to predict the performance of AlxGa1-xAs/GaAs FET's in good agreement with our experimental results. It is shown that the change in the Fermi energy with the gate voltage changes the effective separation between the gate and the two-dimensional electron gas by about 80 Å. Current-voltage characteristics were calculated using a two piece as well as a three piece linear approximation for the electron velocity and compared with experimental results. At 300 K, the two piece model overestimates the current predicted by the three piece model only by approximately 10-20 percent. At 77 K, however, the three piece linear approximation for the velocity field characteristic should be used since the electron mobility decreases very abruptly at about 200 V/cm. The effect of the nonlinear source resistance is also discussed along with the gate-to-source and gate-to-drain capacitances, parameters of paramount importance in determining device performance. These capacitances are calculated as functions of gate-to-source and drain-to-source voltages below saturation.