High velocity N-on and N-off modulation doped GaAs/AlxGal-xAs FETs

The purpose of this work is to determine the optimum design parameters of modulation doped AlxGa1-xAs/GaAs field effect transistors (MODFETs). An analytical model for MODFETs was developed and used to characterize several 1 µm gate transistors. Extremely high transconductances were obtained and are attributed to large electron saturation velocities in the undoped GaAs. At 300 K transconductances of 250 mS/mm and 235 mS/mm have been obtained for normally-off and normally-on devices respectively. At 77 K a transconductance of 400 mS/mm was obtained for a normally-off MODFET. Using our model to characterize these devices requires the electron saturation velocity to be about 2 × 10⁷cm/s at 300 K and 3 × 10⁷cm/s at 77 K.