1.2 Classification of Semiconductor

There is a large variety of semiconductors available today，although for applications Si completely dominates the market. Nevertheless，other semiconductors can have quite different properties. For example，Si cannot be used for light emitting diodes or lasers，while semiconductors such as GaAs can.

Two general classifications of semiconductors are the elemental semiconductormaterials，found in group Ⅳ of the periodic table，and the compound semiconductormaterials，most of which are formed from special combinations of group Ⅲ and group Ⅴ elements. Tab.1.3 shows a portion of the periodic table in which the more common semiconductors are found and Tab.1.4 lists a few of the semiconductor materials.

The elemental materials，those that are composed of single species of atoms，are silicon and germanium. Silicon is by far the most common semiconductor used in integrated circuits and will be emphasized to a great extent.

Tab.1.3 A portion of the periodic table

Tab.1.4 A list of some semiconductor materials

The two-element，or binary，compounds such as gallium arsenide or gallium phosphide are formed by combining one group III and one group V element. Gallium arsenide is one of the more common of the compound semiconductors. Its good optical properties make it useful in optical devices. GaAs is also used in specialized applications in which，for example，high speed is required.

We can also form a three-element，of ternary，compound semiconductor. An example is AlxGa1－xAs，in which the subscript x indicates the fraction of the lower atomic number element component. More complex semiconductors can also be formed that provide flexibility when choosing material properties.