Analysis of an anomalous subthreshold current in a fully recessed oxide MOSFET using a three-dimensional device simulator

This paper describes a three-dimensional device simulator, TOPMOST, and its use in the analysis of the anomalous subthreshold current "hump" in a fully recessed oxide MOS structure. TOPMOST solves Poisson\´s and current continuity equations using the finite-difference method and the box-integration technique, which has been extended to suit an arbitrary three-dimensional structure. The device simulator can be optionally coupled with a two-dimensional process simulator for investigating the influence of process conditions on device performances. Using TOPMOST, the subthreshold current characteristics of a fully recessed oxide structure are examined. The mechanism underlying the hump is clarified, and the dependence on structure parameters, such as channel width, gate oxide thickness, and gate extension, are studied. It is shown that there is a worst case where the current hump becomes most conspicuous. It is also shown that the hump can be suppressed by a side-wall implantation.