Physical basis of short-channel MESFET operation II: Transient behavior

The large-signal switching behavior of planar short-channel metal-semiconductor field-effect transistors (MESFET's) is simulated numerically. First, the intrinsic response of the MESFET is simulated in two space dimensions and time, using measured electric-field-dependent drift velocities and diffusivities in the conventional semiconductor equations; results of the intrinsic device simulations are then used to study the circuit behavior of Si and GaAs MESFET's in two-input NOR circuits. Although the simulated 1-µm-gate Si and GaAs MESFET's have intrinsic response times of 11 and 9 ps to a gate pulse of - 2 V, for fan-in and fan-out = 2, the Si and GaAs NOR gates have average gates delays of 318 and 118 ps, respectively, for 1-µm gate lengths. The power-delay products for these 1-µm-gate Si and GaAs circuits are 1.8 and 1.5 pJ, respectively. These results are compared with measured data and their physical basis is discussed.