In semiconductor devices it is desirable to be able to control minority carrier lifetime i.e. the time in which an electron will recombine in a P type semiconductor region or a hole will recombine in an N type semiconductor region. A relatively low minority carrier lifetime permits a semiconductor device to have a relatively fast switching speed and in the case of a field effect transistor (FET) reduces the gain of the parasitic bipolar transistor.
To reduce minority carrier lifetime, the literature conventionally teaches such processes as heavy metal doping or irradiation with electrons, x-rays, alpha particles, gamma rays or neutrons. As would be expected, a variety of advantages and disadvantages is associated with each of these lifetime-killing processes. In the case of neutron irradiation many of the practical disadvantages have been overcome. For example, the problem of residual long term radioactivity has been avoided by irradiating at an appropriate point during processing and the problem of inadvertent transmutation doping of the semiconductor material has been overcome by using certain radiation shielding. One of the difficulties that nonetheless remains in the long term stability of the neutron-induced crystalline damage. That is, in some cases the stability of the minority carrier lifetime induced by conventional neutron irradiation is unacceptably low. In an effort to analyze the source of this stability problem the present invention was discovered.