pMOS Dosimeters: Long-Term Annealing and Neutron Response
When a metal-oxide-semiconductor (MOS) device is irradiated, charge builds up in the oxide and new interface states are also created. The effect has been shown to constitute a useful dosimetric principle. The trapped charges may relax in various ways but, in the precision pMOS devices described here, the "fading" is no greater than that found in other dosimeters. Results for some 10-year room-temperature anneals are given. Recent results are given for exposure of pMOS dosimeters to accelerator neutrons of energies 3 and 15 MeV. The response to both 3 MeV and 15 MeV neutrons is about 65mV per Gy (silicon) (650mV per krad (silicon)) for an irradiation bias of +20V across the gate oxide. The energy dissipated by the neutrons in silicon is much less than that dissipated in tissue, because of the high hydrogen content of the latter. This response is thus equivalent to 1.8mV per Gy (tissue) for 3 MeV neutrons and 12.14mV per Gy (tissue) for 15 MeV neutrons. The higher neutron energy is above the threshold for the onset of Si(n,¿) and Si(n,p) reactions at which there occurs a large increase in the absorbed dose per incident neutron, and this affects the MOSFET response correspondingly. The response to gamma rays is about 150 mV per Gy (silicon or tissue) (1500 mV per krad).