Investigations on the effect of drift-field-dependent mobility on MOST characteristics—Part II: QBfield dependent

Abstract

The MOST <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</tex> static characteristics were investigated in Part I [1], taking the mobility µ only as variable with the drift field. In this part, the analysis is extended to include the simultaneous variations of the mobility µ and the bulk charge Q <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> along the channel. From the mobility point of view, the analysis follows that of Part I with the two approaches of piecewise and continuous mobility variations. However, in this case the resulting equations are only solvable numerically, and the computed results are plotted. The results show that effects due to the additional variation in Q <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> along the channel are in the same sense and enhance those due to variation in mobility alone. Also, higher doping and greater oxide thickness show themselves in a way as though the mobility were approaching the limit of velocity saturation. The overall effects show more reduction in current levels as well as output and mutual conductances, and increased tendency of earlier saturation. As far as these effects are concerned, the results favor a MOST of small oxide thickness and long channel. However, since a long channel impairs the gain and high-frequency characteristics, the results given in this paper help to show how a compromise may be obtained.