Modeling of Ionizing Radiation Effects in Short-Channel MOSFETs

The effect of ionizing radiation on short-channel MOSFETs is modeled using a charge-sheet approach. The primary effect of ionizing radiation is the introduction of oxide trapped charge (OTC) and interface trapped charge (ITC). Using a two-dimensional charge-sheet model, transistors with channel lengths between 4.65μm and 0.27μm were studied. A range of net OTC and ITC values of ±4.0×1011 cm-2 corresponding to a dose of approximately 106 rad (SiO2) was used to study total dose effects. ITC and OTC cause significant effects in each region of operation. In the subthreshold region, the sensitivity of drain current to these charges is exponential. A more realistic model must include the energy distribution of the ITC charge as well as two-dimensional charge sharing effects. In the triode region, the effects of ITC and OTC are indistinguishable from two-dimensional charge sharing effects. This implies that a simple analysis of threshold voltage offsets in short-channel MOSFETs is incapable of providing a physical separation of two-dimensional effects from radiation-induced effects. In the saturation region, the combined OTC and ITC contribute a fixed charge component to the channel charge which can shift the critical field point at the edge of the pinch-off region in the channel. This critical field effect alters the formation of the "knee" region of the output characteristic and can alter the output conductance in the saturation region for short-channel transistors.