BASICS of Metal Oxide Semiconductor Structures

EEC7102: Electronic Devices & Technology

Internal Assessment

Assignment

Submitted by

Sricharan Ch

M.Tech

Electronics Engineering

Course Instructor:

Samundeswari, M.Tech(Phd).

Dept. of Electronics Engineering

BASICS OF MOS

MOS SURFACES

Generally, semiconductors belong to that category of materials which exhibit conductivity at some particular conditions. MOS Surfaces are the semiconductor surfaces which can be either of the p – type or n – type depending upon the type of doping.

MOS STRUCTURES

MOS structures employ MOS Diode as its basic element. The MOS diode can be thought of as Capacitance under certain conditions which is called as MOS Capacitor. These can be categorised into two categories : Ideal and Real depending upon the conditions we assume.

MOS Capacitor – Ideal

Introduction

Basic bias conditions of MOS capacitor:

Source: Above page, Drain: Below page, Gate: Metal Gate

Fermi levels are different in M and S.Fermi levels are constant within M and within S

Majority-carrier concentration p(x) increases near surface => Accumulation

Surface potential = ϕs

where

Beyond the onset of strong inversion, electrons are induced into the electron channel.The depletion layer thickness does not increase further, i.e. WD = WD, max.

Illustration of band diagram, charge, field, and potential of ideal MOS capacitor:

We use the Depletion Approximation for the acceptor charge in the semiconductor

Threshold voltage of Ideal MOS Capacitor:

Thus the threshold voltage is the sum of a voltage drop in the oxide and in the semiconductor at the onset of strong inversion. Eqn. (26) applies to the ideal MOS structure.

Capacitance of Ideal MOS Capacitor:

CV Curve for Ideal MOS Capacitor:

MOS capacitor – Real

We calculated the threshold voltage of the ideal MOS capacitor. It was given by:

CV curve for Real MOS Capacitor: The various CV Plots for MOS capacitor using different materials is shown below.

References:1. Semiconductor Devices Physics & Technology, S.M. Sze – Pg. no:

170 – 179.2. ECSE‐2210, Microelectronics Technology, Prof. E. F. Schubert :

Chapter 32 – page 1 – 12.3. ECSE‐2210, Microelectronics Technology, Prof. E. F. Schubert :

Chapter 33 – page 1 – 3.