A surface potential and quasi-Fermi potential based drain current model for pocket-implanted MOS transistors in subthreshold regime

Abstract

A subthreshold drain current model for pocket-implanted MOS transistors, incorporating both the drift and diffusion currents, is presented in this paper. In this model, the concept of splitting of the quasi-Fermi energy levels under nonequilibrium condition is used. It is well known that the surface potential based drain current models strongly depend on the potential profile of the channel. For short-channel devices, the end effect at the source and drain ends on the surface potential, and consequently on the drain current, cannot be ignored. The end effect gives rise to a position dependent potential profile, in contrast to a flat 1D profile in a long-channel device; which implies that both the drift and diffusion components are required to be considered for an accurate drain current. The concept of the gradient in the quasi-Fermi level is a convenient way to do so. In this work, a pseudo 2D potential profile which takes into account the vertical field due to the gate and the lateral field due to the source and drain junctions in addition to the difference in the flat-band voltage along the channel is used. Moreover, the mobility and the effective conduction layer depth used are also position dependent since the channel doping varies along the channel. Model predictions are compared with the results predicted by the 2D numerical device simulator DESSIS, and a very good agreement between the two are observed.