Current voltage characteristics of single period modulation doped (Al,Ga) As/GaAs heterostructures are reported. The measurements were performed in a lattice temperature range of 10-300 K and up to an electric field of 2 kV/cm. While no nonlinearity was observed at 300 K, a substantial nonlinearity was encountered at lattice temperatures below 170 K. Compared to bulk GaAs FET's, the results indicate that a speed performance improvement of 60% at 300 K and four times at 78 K can be obtained in modulation doped FET's operating at a field strength of 2 kV/cm. Modulation doped GaAs/n- (Al,Ga) As FET's with 1 µm gate lengths exhibited a gain of 12 dB at 10 GHz. This represents the best result reported to date.