The modulation doped field effect transistor (MODFET) represents a most promising high speed low power dissipation device. Although the MODFET demonstrates good performance at 300K the most impressive results are obtained at 77K, at which the MODFET holds a record in speed among existing semiconductor transistors. However, excellent performances of this device at low temperatures are always measured under conditions of white light illumination. We present a study of the low temperature behavior of the MODFET together with the results obtained on specially designed test devices and structures. We discuss the physical model of the observed phenomena.