BOUNDING THE TOTAL-DOSE RESPONSE OF MODERN BIPOLAR-TRANSISTORS

The excess base current in an irradiated BJT increases superlinearly w ith total dose at low-total-dose levels. In this regime, the excess ba se current depends on the particular charge-trapping properties of the oxide that covers the emitter-base junction. The device response is d ose-rate-, irradiation-bias-, and technology-dependent in this regime. However, once a critical amount of charge has accumulated in the oxid e, the excess base current saturates at a value that is independent of how the charge accumulated. This saturated excess base current depend s on the device layout, bulk lifetime in the base region, and the meas urement bias. In addition to providing important insight into the phys ics of bipolar-transistor total-dose response, these results have sign ificant circuit-level implications. For example, in some circuits, the transistor gain that corresponds to the saturated excess base current is sufficient to allow reliable circuit operation. For cases in which the saturated value of current gain is acceptable, and where other ci rcuit elements permit such over-testing, this can greatly simplify har dness assurance for space applications.