Influence of the substrate bias on the transconductance of deep submicron NMOSFETs.

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