Chapter 4

Modification of The Physical AlphaPower Law MOSFET Model

4.1 Introduction

The modifications of the physical alphapower law MOSFET model was obvious due to the incorporation of the fast interface states in the model. Although, the model was quite right in its manners to determine the MOS characteristics, the interface capacitance

determines the current response in a quite realistic manner.

4.2 Interface trapped charges

A detailed study of the MOS interface states is presented in chapter 5 in the following. But for the convenience of our discussion we would like to have a bird’s eye view on it.

Generally, an interfacetrapped charge (also called fast interfacestate charge) exists at the oxidesemiconductor

interface. It is caused by the defects at the interface, which gives rise to charge “traps”; these can exchange mobile carriers with the semiconductor, acting as donors or acceptors [14]. The interface trapped charges are very negligible in effect in

case of strong inversion but if we consider the case of very thin gate oxides then we find the trapped charges playing a vital role in case of determining MOS capacitance.

The energy band diagram for a MOS structure at positive voltage is as follows:

From the above figure, we see the existence of interface trapped charges in the oxide region of the MOSFET. Usually in case of significantly thick gate oxides these charges are not of any significance in calculating the MOS capacitance. But, as we see, when the oxide thickness is very thin then these charges existing near to the edge of the oxide surface strongly take part during the application of electric field [5]. So, we see that

negligence about the existence of these charges is not quite always right to determine a better current response from a MOSFET operation.