The (electrical) conductivity
of a material represents how easily charges will flow through the material.
Materials with high conductivity are called conductors.
Materials that do not readily conduct electricity are called insulators.
From these definitions, one might deduce that semiconductors form a third
category of material with conductivities somewhere between conductors and
insulators, but that is not exactly the case. Semiconductors, despite
the name, form a subgroup of insulators and have properties that differ
greatly from the properties of conductors. Pure crystalline silicon,
in fact, is a rather poor conductor. To understand how the term semiconductor
arose, we return to the concepts of electron states and energy bands.
|Electric current is generally due to the motion of valence electrons. An electron can move through a material only by moving from one allowed energy state to another. But most materials are formed by bonds that completely fill a valence band, as shown in figure (a) below. Electrons in this filled valence band have no empty states to move into, unless they somehow gain enough energy to jump across the forbidden band gap into the empty conduction band above. Conduction is therefore very difficult. As you might imagine, this energy band diagram represents an insulator.|
Word of Warning:
The motion of electrons in solids is complex. The diagrams and animations
included in these materials display an infintesimal percentage of the number
of electrons actually present in a physical device and show motion only
in one dimension. The vertical axis in band diagrams represents energy;
vertical motion of electrons in these diagrams does not represent physical
motion in space but represents a change in energy.
Go to the next page to find out!
Copyright © 2003 Doris Jeanne Wagner and Rensselaer Polytechnic Institute. All Rights Reserved.