Frequently the system designer wishing to utilize semi-conductor devices in a particular radiation environment finds that the only test results available were obtained in a significantly different environment, and that circumstances beyond his control prohibit an empirical test under the new conditions (1). The problem then arises, how to successfully extrapolate to the new environment of interest. A determination of the effect of neutron and neutron spectra on silicon field effect transistors would allow the performance of such devices to be correlated from existing data, and subsequently to be extrapolated to new environments as required.