Gain degradation rates were obtained for various transistor operating conditions of current and voltage during Cobalt 60 irradiation and compared with the degradation rate for a radiation exposure in the passive state. The effects of emitter current alone, collector voltage alone, and normal transistor operating modes were studied. The results showed that increasing levels of carrier injection from the emitter caused decreasing rates of damage. For a 2N1613 type silicon transistor, the maximum reduction observed was approximately a factor of 2. This behavior is believed to be due to an alteration of the charge state of the radiation induced defect at the time the defect is formed. A lesser dependency with collector voltage was also observed. The effects of temperature in the range 105 F to 140 F, during gamma irradiation, were found to be small in comparison with the effects of active-passive current variations. Tests conducted in a neutron environment, where normal transistor operating modes were compared with the passive irradiation state, indicated very little influence upon damage rate.
Transistor gain is reduced by ioizing radiation, which affects the transistor surface, and by displacement radiation, which causes lattice defects. The investigation of ionizing radiation damage described in this paper was accomplished with X-rays of photon energy less than 150 kev, which is below the energy necessary for displacement damage in silicon. Experiments were performed on silicon dioxide passivated silicon planar transistors with open leads, in a normal amplifying mode and with other junction bias conditions. AC and DC gain measurements at various injection levels showed a gain degradation dependence on the operating bias conditions. Open leads and back-biasing of each junction during irradiation resulted in considerable damage, with almost complete recovery occurring in most transistors tested when the base emitter junction was forward-biased. Many of the characteristics of the radiation damage observed can be explained by Atalla’s model that charge collection at the surface causes a widening of the space charge region, thus increasing the recombination-generation current.
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