Both normally on and normally off modulation-doped FETs have been fabricated from MBE grown AlGaAs/GaAs heterojunction structures. The static I-V characteristics of recessed-gate FETs were measured over the temperature range 77-400 K. A large change in the threshold voltage V//t with temperature T was observed. The observed V//t(T) data are explained, in part, in terms of a simple model based on the temperature-dependent occupation of deep donor traps in the AlGaAs layer. The traps were characterized by different energies for emission and capture. The transient capacitance of large area modulation-doped Schottky diodes was also measured. The capacitance data is explained by the time-dependent occupation of the same donor traps. The value of the activation energy for carrier emission was estimated to be 450 mv while the donor level was found to be about 42 mv below the conduction band.