Modeling the response of channel-substrate interface traps to the ixcitation of the substrate of GaAs MESFETs

Abstract

The backgating (or sidegating) phenomenon observed in n-channel GaAs FETs is simulated with the finite difference method by introducing deep levels (donors, acceptors) to the substrate which is initially n or p type. It is observed that if the substrate is initially slightly p-type and contains deep acceptors then the channel-substrate interface can be regarded as a simple n-p junction. The current in the substrate is dominated by the generation component, which saturate because of saturation velocity. The substrate in this case can be regarded as a relaxation like semiconductor. A threshold voltage in the channel conductance is not observed in this case. If the substrate is initially slightly n-type compensated by deep acceptors, then the substrate current has a space charge, trap fill, type of current. In this case the substrate becomes semi-insulating (good relaxation material ) and the diode cannot be regarded as a simple n-p junction. There is a threshold for backgating if the substrate contains a high donors density. But this does not necessarily coincide with the onset of TFL current which depends on the types and energy level of traps; on thickness of the substrate; and the deep acceptor and donors density and the substrate type.