TABLE OF CONTENTS In Chapter 3, we considered the MOSFET to be long and wide. We also assumed the mobility to be independent of the electric field. In this chapter we will include the effects of both the mobility variation with the transverse field and velocity saturation on the drain current. We will also consider edge effects and the electrostatic coupling between drain and source. Small-geometry effects account for short and narrow-channel effects on the threshold voltage and drain-induced barrier lowering (DIBL). The common approach to modeling these second-order effects in MOSFETs, also employed here, is to decorate the long-channel model with a list of corrections related to these effects [1]. We conclude the chapter with some benchmark tests concerning compact transistor modeling.
The mobility is determined by several scattering mechanisms through which the carriers exchange momentum (and kinetic energy) with the semiconductor. In MOS transistors, the carriers flow near the semiconductor interface with the oxide, and extra scattering mechanisms at the interface lower the mobility of the carriers of the inversion layer (surface mobility) to values of the order of one half of the bulk mobility.
Roughly speaking, the scattering mechanisms are due to the imperfections of the semiconductor crystal, namely lattice vibrations, ionized impurity atoms, and interface related imperfections, such as interface trapped charges and surface roughness. Since lattice vibrations (phonons) depend on the temperature, so does the mobility. The mobility depends also on the doping, and on the electric field component perpendicular to the current flow...
