Design and performance of GaAs normally-off MESFET integrated circuits

DC and transient analyses of GaAs normally-off MESFET integrated circuits are described. The design tradeoffs between device parameters and logic characteristics are discussed for an inverter with a resistive load. By increasing the supply voltage to several times that of the built-in voltage, the propagation delay time can be lowered similar to that when using an active load (current source). To investigate the speed-power performance of the IC's, ring oscillators with different fan-in and fan-out configurations were fabricated. A binary frequency divider which uses a master-slave flip-flop was tested. The maximum counting frequency of the divider was 610 MHz at a supply voltage of 1.5 V. This coincides with the results obtained from the ring oscillators with fan-in/fan-out = 2/2. Comparing the experimental results with the theory, the effective electron mobility in the thin channel layer is expected to be very low. By improving the mobility and shortening the gate length to half a micrometer, practical functioning circuits should operate with an average propagation delay time of less than 100 ps.