Dependence of decay time of the photocurrent induced by transient ionizing radiation on TID in NPN bipolar transistors

When an NPN bipolar junction transistor (BJT) is exposed to the pulsed gamma (or X) irradiation, photocurrent is generated, containing the primary photocurrent and the secondary photocurrent. TID experiments and transient irradiation experiments were carried out. The results show that, as accumulating total ionizing dose (TID), the secondary photocurrent decays more severely, whereas the primary photocurrent is almost impervious. Moreover, the influence of TID on the secondary photocurrent decay is related to the BJT’s collector current. If the BJT is working under the condition of high injection, the influence of TID is quite weak, but it’s evident when the BJT is working under the medium or low injection condition. The charge control model is used to analyze the decay time of the secondary photocurrent. As a result, the base minority lifetime determines the decay time, and a method how to obtain the equivalent lifetime after TID accumulation is proposed to predict the change of the decay time. Consequently, the deductive results give good agreement with the experimental results.