Substrate bias dependence of the transconductance of deep submicron silicon NMOSFETs

Abstract

Transconductance enhancement due to substrate bias in deep submicron MOSFETs is studied over a large temperature range, from 300 down to 30 K. This effect is explained by Fermi potential gradient enhancement which occurs in short channel devices. For longer MOSFETs, the impact of the substrate bias on the Fermi potential is weak and the degradation of mobility explains the transconductance reduction. In cryogenic operation, numerical simulation has shown that the channel potential gradient increases and, therefore, the transconductance enhancement with substrate bias is larger at low temperature for deep submicron devices.