Modelling weak avalanche multiplication currents in IGFETS and SOS transistors for CAD

A model is developed for the substrate current of an IGET operating in saturation. This current results from the impact ionization of the channel current carriers in the high field region adjacent to the drain. Due to the exponential dependency of the impact ionization process on the electric field, an accurate description of the field near the drain is essential. An expression for the field, accounting for the two-dimensional nature of the drain space charge region, is used. The result is a closed form expression for the substrate current. By using an appropriate p-n junction model for the source diffusion, the floating substrate potential in SOS devices is evaluated.