Roles of low field mobility and its carrier-concentration dependences in high electron mobility transistors and other field effect transistors

A model is proposed for high-electron-mobility transistors (HEMTs) and other heterostructure FETs in which the dependence of low field mobility mu on carrier concentration N/sub s/ is taken into account. On the basis of this model, the influence of mu and its N/sub s/ dependence on drain current and transconductance g/sub m/ are clarified, In particular, high mobility (>10/sup 5/ cm/sup 2//V-s) is shown to be effective in achieving and maintaining the intrinsic limit of g/sub m/(= epsilon /sub 2/ nu /sub s//d*) irrespective of bias conditions, where nu /sub s/ is the saturation velocity and epsilon /sub 2/ and d* are the dielectric permittivity and the effective thickness of the gate insulator, respectively. The N/sub s/ dependence of mobility is found to greatly affect the gate-voltage dependence of g/sub m/ and leads, in some cases, to an appreciable increase of g/sub m/ above its intrinsic limit.<>