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Effects of ionizing radiation on small-signal microwave bipolar transistors

Microwave bipolar transistors are shown in this paper to be sensitive to high-energy electron andX-ray ionizing radiation at doses comparable to sensitive MOS devices. The most sensitive parameter is the dc current gain, hFE, which-in small-signal applications-leads to microwave performance sensitivity. Experimental results are presented for three device types which show that this radiation sensitivity can lead to severe device application limitations. An analysis is presented which accounts for the dependence of hFEon starting value, cumulative dose, and application bias levels. Evidence is presented from both degradation and annealing studies to show that the dominant ionization effect is the creation of positive space charge in the oxide. The model which is presented for this effect is consistent with current MOS device radiation effects models. It is concluded that future microwave transistor radiation sensitivity studies will necessitate detailed measurements on the basis of individual device technology as is presently the practice with MOS devices.