Current–voltage characteristics of AlGaN/GaN modulation doped field‐effect transistors at elevated temperatures are studied experimentally. The drain–source current and extrinsic transconductance are both found to decrease with increasing temperature. Decrease in mobility with increasing temperatures is considered to be one of the causes of the reduction in the current and transconductance. The capacitance–voltage characteristic reveals the absence of heat activated traps in the modulation doped layer. The physics underlying various high‐temperature operations of current–voltage characteristics is discussed. ©1996 American Institute of Physics.