Modeling of GaAs/AlGaAs MODFET inverters and ring oscillators

Abstract

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.