Semi Conductors

Semiconductors

INTRODUCTION

A semiconductor is a material whose ability to conduct electrical current is in between conductors and insultors.

 A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. It may consist of Single-element like antimony (Sb), arsenic (As), astatine (At), boron (B), polonium (Po), tellurium (Te), silicon (Si), and germanium (Ge) or may be Compound semiconductors like gallium arsenide, indium phosphide, gallium nitride, silicon carbide(SiC), and silicon germanium(SiGe). The single-element semiconductors are characterized by atoms with four valence electrons. Silicon is the most commonly used semiconductor.

BAND GAP

In solid materials, interactions between atoms “smear” the valence shell into a band of energy levels called the valence band. Valence electrons are confined to that band. When an electron acquires enough additional energy, it can leave the valence shell, become a free electron, and exist in what is known as the conduction band. The difference in energy between the valence band and the conduction band is called an energy gap or band gap. This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band. Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom. The energy gap or band gap is the difference between two energy levels and electrons are “not allowed” in this energy gap based on quantum theory. Although an electron may not exist in this region, it can “jump” across it under certain conditions. For insulators, the gap can be crossed only when breakdown conditions occur—as when a very high voltage is applied across the material. In semiconductors the band gap is smaller, allowing an electron in the valence band to jump into the conduction band if it absorbs a photon. The band gap depends on the semicon ductor material. In conductors, the conduction band and valence band overlap, so there is no gap. This means that electrons in the valence band move freely into the conduction band, so there are always electrons available as free electrons.

 

 

Download Files

semiconductors.pdf (0.32 MB )