Parasitic MESFET in (Al, Ga) As/GaAs modulation doped FET's and MODFET characterization

A charge-control model for n-channel modulation doped FET's (MODFET's) is extended to include the drain-to-source current through the doped (Al, Ga)As layer which becomes important for large positive gate voltages. This parasitic conduction leads to decreased device transconductances at high gate voltages. A unified and complete characterization technique for deducing the parameters of our model is introduced and used for the device characterization. Parameters, e.g., the saturation velocity, two-dimensional gas concentration at equilibrium, thickness of the doped (Al, Ga)As layer, etc., deduced using the model, are in good agreement with the independent calculations and measurements. However, the deduced values of the room-temperature low field mobility of the two-dimensional electron gas are considerably smaller than those measured by Hall effect and in long-gate MODFET's. This model is in good agreement with the characteristics of high-current normally on MODFET's. The maximum measured current swing of 300 mA/mm gate is reported.