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By irradiation of the surface structure of silicon planar transistors with a narrow electron beam adjusted parallel to the pn-junctions of the devices it was possible to show in a very direct manner that the increase in base current occurs only if the beam is directed to the immediate vicinity of the emitter-base pn-junction. Subjecting the collector-base pn-junction and the remainder of the transistor surface to radiation has no effect on the current gain. A comparison of the radiation damage to bipolar silicon planar transistors in metal packages and plastic envelopes shows that the packaging method has a strong influence on the degradation in current gain if the pn-junctions are reverse-biased during the irradiation. The resistance of bipolar planar transistors to ionizing radiation can be improved considerably if the device is subjected to a high dose of ionizing radiation under conditions at which the damage anneals out immediately. The cause for this phenomenon mainly is a decrease of the concentration of interface states effected by this treatment. Reductions of the increase in base/collector current ratio of one order of magnitude have been obtained.