We present a new room-temperature model for modulation-doped field-effect transistors and heterojunction insulated gate field-effect transistors that accounts for effects induced by the gate current at large gate voltages. For large gate currents the quasi-Fermi levels in the GaAs channel and the AlGaAs barrier layers are different because of the limited electron exchange imposed by the heterojunction band offset. As a consequence, the channel electron concentration for large gate voltages exceeds the value expected for the n-AlGaAs/GaAs material system in thermal equilibrium. The channel electron capacitance can have two peaks as a function of gate voltage leading to similar structure in the gate voltage dependence of the transconductance. We find good agreement between our calculated and measured gate currents as a function of the gate voltage.