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.
(a)  Electrons in an insulator fill all available states in the valence band.  Electrons must jump to the next higher band before they can move freely.  This band where electron motion occurs is called the "conduction band"		(b)  Electrons in a conductor only partially fill the valence band, so electrons can move freely without gaining extra energy.  The conduction band is the same as the valence band.  Click on the image to see an animation of electron motion in the conductor.

Other materials are formed by bonds that only partially fill a valence band, as shown in figure (b) above.  Electrons in this partially filled valence band have plenty of empty states available, so they can move freely from the vicinity of one atom to another.  A partially-filled valence band is also called the conduction band, since electrons in that band can be responsible for conduction.  Not surprisingly, materials with partially-filled valence bands are conductors.

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.
 

Where do semiconductors figure into all this?  What is the band structure of a semiconductor? Go to the next page to find out!