Compact MOSFET Models for VLSI Design

4

Long Channel MOS Transistor

4.1 Introduction

The MOS transistor may be viewed as an extension of the three terminal MOS structure where two diffused regions known as source and drain, acting as electrical probes, establish contacts with the inversion layer formed under the gate. MOS transistors can be broadly classified into four categories:

1. n-channel enhancement.
2. p-channel enhancement.
3. n-channel depletion.
4. p-channel depletion.

For an n-channel transistor the substrate is of p-type with source and drain made of n⁺ diffused regions. The inversion carriers, induced at the Si–SiO₂ interface by the positive gate voltage, form a conducting channel connecting highly conducting source and drain diffused layers. The carriers in the inversion layer forming the channel under the gate are supplied by the source, and collected by the drain which is biased at a higher potential than that at the source. Therefore, for the n-channel MOS transistor, the terminal at higher potential is called the drain and the one which is at lower potential is called the source. Conventionally, the current is considered positive, when the current enters the terminal. Structurally, the MOS device is symmetrical. For a p-channel MOS transistor, which is the complement of the n-channel device and has n-Si as the substrate, the conducting channel formed under the gate due to the negative voltage applied on the gate, has holes as inversion carriers in the channel. The drain is biased at a lower potential than the ...