Investigations on the effect of drift-field-dependent mobility on MOST characteristics—Part I: QBconstant

Abstract

A detailed analysis of MOST <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</tex> static characteristics is given, showing clearly the effect of mobility variation along the channel. In this analysis, the widely used formulas for the mobility field dependence are considered and compared. The analysis follows two approaches. In the first approach, a critical field ε <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> , is defined beyond which the mobility is no longer constant. Since the drift field increases towards the drain, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</tex> plane is divided into three regions by two loci representing the two extreme cases of whether the drift field at the drain or its value at the source is just equal to the critical value. The derived equations are plotted for different values of a parameter <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</tex> that gives flexibility on the way the mobility is expected to vary along the channel. In the second approach, the mobility formula is that proposed by Trofimenkoff and Caughey, and is valid for any value of the drift field. It gives one current equation with the restriction that it is only valid up to a certain drain voltage below pinchoff. The results in both cases show clear tendency of current saturation before pinchoff, as well as substantial overall reduction in the current levels and mutual conductance. This trend is more pronounced in short channel MOST's.