Influence of substrate freeze-out on the characteristics of MOS transistors at very low temperatures

Abstract

The low temperatures current-voltage characteristics of N-channel MOS transistors have been analysed. An excess drain current is observed for intermediate values of drain voltage. This anomalous drain current is explained in terms of substrate freeze-out, since at very low temperatures the MOS structure has a type of floating substrate potential within the depletion region. Due to the increase of the majority carrier current, flowing through the substrate to the source at increasing drain voltage, this substrate potential increases and causes a change of threshold voltage. This change is observed in the current-drain voltage characteristics of the MOSFET. Various experiments, such as measurements of substrate current, effects of temperature, gate and substrate voltages, support this interpretation. MOS transistors with various geometries and various dopings are analysed.