Displacement Damage in MOS Transistors

The changes in MOS device characteristics produced by neutron irradiation from the Northrop TRIGA reactor have been observed. Three damage mechanisms have been identified: an increase in net surface state density, a decrease in substrate resistivity, and a decrease in carrier mobility in the channel. The surface effect is usually dominant, although the bulk resistivity effect becomes increasingly important as the resistivity of the substrate is decreased. An exact closed form expression for the turn-on voltage has been derived by obtaining a solution to Poisson's equation in the gate region. The open circuit gate to substrate capacitance as afunction of gate to substrate voltage has been obtained by numerical integration techniques in terms of the charge density and dielectric constants present in the SiO2 layer and Si substrate. For both enhancement and depletion devices, an increase in positive charge density was apparent in the oxide region. It is postulated that the observedincrease in positive charge density is due to creation of mobile positive ions, and that the decreasing net accumulation rate with increasing flux is due to a diffusion or recombination process competing with the creation process. On this basis, a rate effect is expected.