High-injection open-circuit voltage decay in pn-junction diodes with lightly doped bases

Abstract

Results of experimental studies of open-circuit voltage decay (OCVD) in a high resistivity (3000 Ω cm) base (i.e. lightly doped) pn-junction diode are reported. For moderate values of applied junction voltage, a rapid initial decay followed by a slow decay is observed. As the applied junction voltage increases to higher values, the initial drop becomes more rapid, it is followed by a plateau and finally the slow decay. At very high applied voltages, a peak is observed in the decay curve after the initial rapid drop. The final slow decay rate remains the same in all cases. The initial rapid decay and the final slow decay can be explained with the help of the high-injection theory of Schlangenotto and Gerlach. According to this theory, the excess carriers recombine not only in the base (as in the low-injection theory of a base dominated diode) but they migrate to the emitter and recombine there also. Mathematically, this emitter base coupling is described by a parameter β. The initial rapid decay and the final slow decay can be explained using a constant value of β as assumed by Schlangenotto and Gerlach. It is, however, necessary to assume that β decreases with time to explain the observed peak. Possible physical reasons for the decrease in β are given. The final slow decay rate is used to obtain a value of 0.4 ms for the minority carrier lifetime in the base.