An experimental study of backgating effects in GaAs MESFETs

This paper presents the results of a series of experiments concerning the backgating effect in GaAs MESFETs, and high-field current behavior of backgate diode structures. The devices tested were selectively ion-implanted and mesa-etched structures fabricated on semi-insulating (SI) undoped and Cr-doped LEC substrates. These experiments were performed to separate the effects of the substrate bias and the backgate bias and to identify the carrier emission processes using bias variations and optical excitations. The main objective was to determine the effects of surface condition by comparing the behaviors of devices with untreated and nitride-a and Al-deposited SI surfaces. A model for the backgating effect is proposed that involves trapping of electrons near the surface in the backgate structure, and space charge build-up beneath the MESFET channel region by deep-level trapping and free-carrier distribution. The temperature dependence of the backgating high-field current suggests that surface trapping by shallow levels is often significant and this causes the backgating threshold voltage to increase rather rapidly, as the temperature is lowered.