Substrate Current due to Impact Ionization in MOS-FET

The substrate leakage current vs. gate voltage characteristics of MOS-FET was examined over a wide range of device parameters and measurement conditions. With the increase in the gate voltage, the substrate current increases until it reaches a maximum value. Then it decreases to the value of the generation-recombination current. The substrate current has a high value at low measurement temperatures, high drain voltages, high impurity concentrations of silicon substrates, thin gate-oxide thicknesses and a large drain current. These experimental results were semi-quantitatively explained on the basis of a model in which the substrate current is caused by the first-order impact ionization of the carriers within the pinched-off region. The observed increase of the substrate current is mainly dominated by an increase of the drain current, and the decrease of the substrate current is mainly dominated by a decrease of the impact ionization coefficient.