Transient Response of Epitaxial GaAs JFET Structures to Ionizing Radiation

The transient response of epitaxial n-channel GaAs junction field effect transistors (JFETs) to ionizing radiation has been investigated. The devices were fabricated from epitaxial layers of n-type GaAs grown on a Cr-doped semi-insulating GaAs substrate. Prior to neutron exposure, the devices were observed to respond to and recover from the ionizing radiation very rapidly, indicating very short characteristic response times. The peak photocurrents in both the gate and drain circuits were observed to vary in proportion to radiation flux at low flux levels, but varied as the half power of flux at higher ionizing radiation rates. The magnitude of the photocurrents indicated that the previously insulating substrate was contributing a diffusion component to the photocurrent. The participation of the substrate in providing photocurrents was confirmed by application of a substrate bias which served to enhance or inhibit the photocurrent response, depending on bias polarity. To aid in interpretation of the complex phenomena observed in the epitaxial JFET structures, a number of more simple device structures were fabricated and their transient response to ionizing radiation was investigated. Neutron irradiation served to reduce the drain and gate photocurrents at all dose rates in the range 109 to 2 × 1011 rad (GaAs)/s but the observed prompt pulse shape was not altered appreciably by neutron exposure. Exponentially decaying photocurrent tails observed in the response of some structures were not present after neutron irradiation.