Impact of the excess base current and the emitter injection efficiency on radiation tolerance of a vertical PNP power transistor in a voltage regulator

Examinations of the minimum dropout voltage with a high load current demonstrated proper operation of voltage regulators L4940V5 for gamma radiation doses of up to 500 Gy(SiO2). An increase in the minimum dropout voltage of approximately 0.5 V was measured. Due to the relatively small dissipation, the thermal protection circuit did not significantly affect the operation of the voltage regulator L4940V5. According to the line regulation characteristics, obtained after absorption of a total ionising dose of 300 Gy(SiO2), the voltage reference was negligibly affected by the irradiation. Variations in the line regulation characteristics after irradiation were less than 1% in comparison with initial values. Despite the operation with a load current of 400 mA, the serial PNP transistor emitter injection efficiency remained high after irradiation, with an estimated value greater than 0.93. The interdigitated structure of the serial transistor's base-emitter junction led to an abrupt increase of the surface recombination in a gamma radiation environment, followed by a great rise in the excess base current. This effect was enabled by the feedback circuit reaction and its influence on a driver transistor, affecting the sharp increase of the serial transistor's base current. On the other hand, the antisaturation circuit prevented a rise in the total voltage regulator's quiescent current above the limit of approximately 40 mA.