A Study of the Neutron-Induced Base Current Component in Silicon Transistors

In a previous paper a neutron-induced component of base current was reported that increases in proportion to integrated neutron flux and varies with base-to-emitter voltage, VBE, as exp(q/nkT VBE), n being approximately 1.5. A component of base current which varies similarly with base-to-emitter voltage has previously been reported to be of surface-perimeter origin. For this reason it is significant to unambiguously locate the origin of the neutron-induced component. For this, a detailed study of the current-voltage characteristics, an analysis of the deviation of the characteristics from an exponential caused by emission concentration for "ring-dot" structures, and an analysis for the transverse bias dependence of the base current components for a special "tetrode-type" test structure were performed. The results indicate that the small "1.5 component" of current initially present is indeed of surface-perimeter origin while the added "1.5 component" of current induced by neutron bombardment is of bulk space charge region origin. To determine whether this effect is universal, an investigation was made of many different types of silicon transistors, and all were found to exhibit a similar neutron-induced current component. The number n in the exponential term is found to be a function of temperature decreasing with increasing temperature. For example at -50°C, n = 1.6, and at +100°C, n = 1.3. Annealing studies reveal an apparent difference in annealing rates for the neutral bulk base region and the high-field region of the emitter-base junction.