Modeling of GaAs/AlGaAs MODFET inverters and ring oscillators

Detailed understanding of the MODFET inverter chains is lacking and present designs are based on ground rules developed by trial and error. We developed a simple and straightforward model for the current-voltage characteristics using results from numerical solutions of the quantum mechanical problem; this model agrees very well with experimental results obtained in our laboratory. High-frequency gate capacitance voltage characteristics for a wide gate voltage range was modeled for the first time in a similar fashion. These models were used to simulate a chain of inverters at 77 and 300 K for a wide range of supply voltage and load current. The maximum device speed is obtained for small supply voltages, ≤ 1 V both at 300 and 77 K, where the effects of transconductance degradation and large gate capacitance are minimized. The devices exhibit under 10-ps switching times both at 300 and 77 K, with 77 K logic swing being much larger. The results are in qualitative agreement with the reported experimental results.