6.1 The MOS Capacitor - Introduction

Table of Contents - Glossary - Study Aids - ¬?®

In this section:

Introduction
The MOS structure

Next: 6.2 MOS Energy band diagram

6.1.1 Introduction

The primary reason to study the MOS capacitor is to understand the principle of operation as well as the detailed analysis of the MOSFET. In this chapter we introduce the Metal-Oxide- Silicon (MOS) structure and its four different modes of operation, namely accumulation, flatband, depletion and inversion. We then consider the flatband voltage in more detail and present the the exact analytical solution which we use to discuss the issue of the inversion layer charge and compare it to the simple MOS capacitance model.

6.1.2 The MOS structure

The MOS capacitor consists of a Metal-Oxide-Semiconductor structure as shown in the figure below. Electrical contacts are made to the metal gate and the back contact to the semiconductor. Also shown is the charge distribution in the structure under acccumulation, depletion and inversion conditions.

mos.gif

Fig.6.1.1

Charges in a Metal-Oxide-Semiconductor structure under accumulation, depletion and inversion conditions

Accumulation occurs typically for negative voltages where the negative charge on the gate attracts holes from the substrate to the oxide-semiconductor interface. Depletion occurs for positive voltages; The positive charge on the gate pushes the mobile holes into the substrate, thereby depleting the semiconductor of the mobile carriers and leaving a negative charge in the space charge region which is due to the ionized acceptor ions. The voltage separating the accumulation and depletion regime is refered to as the flatband voltage. Inversion occurs at more positive voltages which are larger than the threshold voltage. In addition to the depletion layer charge a negatively charged inversion layer forms at the oxide-semiconductor interface.

6. ¬?® 6.2