Surface Effects of Radiation on Semiconductor Devices

19(57 When a semiconductor is exposed to nuclear radiation, two basically different effects may occur. First, the radiation will cause ionization through one of a number of electronic excitation processes. Second, if the radiation energy exceeds a threshold value which depends on the nature of the irradiating particle, some of the atoms in the semiconductor lattice will be displaced. If the semiconductor exposed to radiation is part of a device, the device characteristics will change; the changes depend on such factors as the nature and energy of the radiation, the materials and geometry of the device, and even the processes used in manufacturing the device. The changes in characteristics which occur when these effects take place in the bulk of a device have been investigated for some time and are quite well understood in terms of the usual physics of solids. However, effects can also occur at the surface of a device, giving rise to the so-called surface effects which have only more recently received attention and which are governed by the less well understood physics of surfaces. The failure of the Telstar® satellite in 19G2 was explained in terms of surface damage to transistors in the command circuits, damage caused by radiation received during transit through the Van Allen belt.1 From the experience gained in analyzing this failure, it is apparent that surface effects of radiation may often control the behavior of solid state devices subjected to nuclear radiation. In present-day semiconductor technology, the effects of radiation damage in the bulk have been reduced in transistors by using very shallow, diffused junctions.