Surface Effects of Radiation on Transistors

Surface effects caused by ionizing radiation have been found to result in serious parameter changes in certain transistors. These effects include large increases in leakage current, changes in gain, and increases in noise. Studies have been made of the effects of dose, dose rate, bias, transistor environment, and surface protection, in addition to the phenomena of recovery and memory. The greatest changes occur when bias and radiation are applied simultaneously to a gas-encapsulated silicon transistor with unprotected surfaces. Most of these effects can be qualitatively explained through the use of a simple model of the process. Radiation ionizes the encapsulating gas, and the resulting ions and electrons are directed to transistor surfaces by electric fields at the junction surfaces and between the transistor and its can. Inversion layers are produced at the surface which grossly change certain transistor parameters. The great variability among devices of the same type suggests that the gas ions interact with the surface by imparting charge to surface contaminants. Detailed investigation of the influence of electric fields between a transistor structure and its can have been carried out on special gas-encapsulated mesa and planar transistors which are electrically insulated from their cans. Results indicate that inversion layers on both collector and base are affecting ICBO. while, as would be expected, gain is altered principally by an inversion layer on the base. In the gasencapsulated planar devices the effect of can bias on ICBO is greatly reduced and its effect on gain is practically zero.