Bipolar Transistor Screening Methods for Neutron Hardness Assurance

A detailed bipolar transistor model has been developed to determine the dependence of post-irradiation electrical parameters on preirradiation electrical measurements and physical parameters. Some of the features of the model are the following: (a) Postirradiation lifetimes vary with injection level. (b) Electric fields in base and emitter regions increase the emitter efficiency and normal base transport factor. (c) Collector resistance decreases when minority carriers are injected into the collector. (d) Collector resistance decreases when the collector-base junction is reverse biased and the depletion region extends into the collector. (e) Junction doping profiles are modeled as exponential-constant. (f) Junction efficiency degrades as injection level increases. (g) Base width varies with collector-base voltage. (h) All parameters except emitter width and junction areas are determined from electrical measurements, and radiation damage constants are determined from postirradiation parameters. The radiation damage constants are utilized in calculating sensitivity parameters for preirradiation parameters to be used for neutron hardness screening. A computer program varies selected preirradiation model parameters one at a time and the resulting change in an electrical parameter of interest at a specified neutron fluence is determined. The postirradiation performance of other devices of the same type at the same operating point and irradiation level can be predicted as follows: (1) Multiply the appropriate sensitivity parameter by the normalized deviation of each screening parameter from the corresponding reference unit value: (2) add the sum of these products to unity; (3) multiply this quantity by the post-irradiation electrical parameter of the reference unit.