We have investigated the effects of rapid thermal annealing on the electrical and optical properties of planar‐doped AlGaAs/InGaAs/GaAs high electron mobility transistor structures grown by molecular beam epitaxy. Hall effect and photoluminescence measurements on samples with In0.22Ga0.78As and In0.28Ga0.72As channels reveal a temperature‐dependent degradation in sheet charge density, Hall mobility, and photoluminescence response. The structures were essentially stable through the temperature range used in normal device processing. However, annealing temperatures greater than 700 °C resulted in strain relaxation and layer intermixing, especially for the In0.28Ga0.72As sample.