Neutron Degradation of Ion-Implanted and Uniformly Doped Enhancement Mode GaAs JFET's

Abstract

The degradation of the saturation transconductance of ion-implanted GaAs enhancement mode JFETs by exposure to fast neutron fluences is calculated using simple analytical expressions published by bulk GaAs. It is shown that the saturation transconductance degrades linearly with neutron fluence at about the same rate for devices with widely varying implant profile and junction location, but equal gate geometry and similar threshold voltage. Thus the degradation rate of the relative transconductance (ratio of degraded to initial transconductances) with neutron fluence is decreased by a large initial transconductance. At a fixed gate voltage, and for a given threshold voltage and gate geometry, a large initial transconductance requires high peak dopant concentration and small mean deviation of the implant profile, and a gate junction located near the position of the peak dopant concentration. Experimental data for the degradation of the transconductance by neutron fluence for two ion-implanted transistors have been matched by theory assuming the presence of an undegraded parasitic source resistance. The degradation rate of an uniformly doped epitaxial transistor is found to be 79% of that of ionimplanted devices with equal gate geometry and similar threshold voltage.