Improved Radiation Storage Time Predictions by a New Approach to Inverted Transistor Characterization

This paper describes and assesses a technique for obtaining those inverted transistor parameters which are required for an accurate prediction of electrical and radiation storage times. The technique, which is based on electrical storage time measurements, was applied to six transistors manufactured by three methods: diffused silicon, epitaxial silicon, and alloyed germanium. The response of these transistors was measured over a range of exposure rates (3 x 107 to 6 x 1010 R/sec) and saturation collector currents (2 to 40 ma). Accurate computer predictions of the radiation responses were obtained and several interesting results were inferred from the study. It was found that the primary photocurrent is insensitive to the amount of forward bias on the collector junction. Another inference was the propriety of ignoring apparent superlinearities in the dependence of photocurrent on rate. A third, and particularly interesting result, was that there appear to be three types of dependence of radiation storage time on the logarithm of dose rate: sublinear, linear, and superlinear according to the degree of saturation being light, moderate, and heavy, respectively. This reconciles the discrepancy between the different theoretical rate dependences developed by Carr and by Gage. An additional result was the apparent insensitivity of the results to certain known approximations of the model used in the computer program.