SYNERGETIC EFFECTS OF RADIATION STRESS AND HOT-CARRIER STRESS ON THE CURRENT GAIN OF NPN BIPOLAR JUNCTION TRANSISTORS

The combined effects of ionizing radiation and hot-carrier stress on t he current gain of npn bipolar junction transistors were investigated. The analysis was carried out experimentally by examining the conseque nces of interchanging the order in which the two stress types were app lied to identical transistors which were stressed to various levels of damage. The results indicate that the hot-carrier response of the tra nsistor is improved by radiation damage, whereas hot-carrier damage ha s little effect on subsequent radiation stress. Characterization of th e temporal progression of hot-carrier effects revealed that hot-carrie r stress acts initially to reduce excess base current and improve curr ent gain in irradiated transistors. PISCES simulations show that the m agnitude of the peak electric-field within the emitter-base depletion region is reduced significantly by net positive oxide charges induced by radiation. The interaction of the two stress types is explained in a qualitative model based on the probability of hot-carrier injection determined by radiation damage and on the neutralization and compensat ion of radiation-induced positive oxide charges by injected electrons. The results imply that a bound on damage due to the combined stress t ypes is achieved when hot-carrier stress precedes any irradiation.