Irradiation and annealing of silicon planar transistors

Abstract

The effects of irradiation by fast neutrons (14 MeV) and by gammas (Co 60) on collector cutoff current I CBO and base current I B at low current densities were investigated on silicon planar transistors. With neutron irradiation I CBO and I B increase linearly, while with gamma irradiation saturation occurs. The temperature dependence of I CBO between 9 and 89°C shows that the recombination level of the induced defects lies in the middle of the forbidden gap. Moreover, gamma irradiation introduces slow surface states, which are responsible for an observed I CBO drift. The measured base currents can be described by the equation I B = const. exp(E b/kT) exp(qV EB/nkT) . It was observed that n = 1·17; E b = 0·95 eV after neutron irradiation and n = 1·8; E b = 0·95 eV after gamma irradiation. The result of neutron irradiation cannot be explained by any of the existing theories. Based upon our measurements we conclude that neutrons induce defects within the volume of the transistors, while the consequences of gamma irradiation are explained by surface effects. All of the observed radiation damages can be completely annealed. Annealing for gamma damage proceeds in one stage at 120°C and that for neutron damage in two stages at 120°C and 270°C respectively. For the first annealing stage of neutron irradiated transistors an activation energy of E a = 1·53+0·07 eV was measured. In many cases certain properties of npn transistors may be improved by irradiation and annealing.