1 Classification of Metals, Conductors, and Semi-conductors

Classification of Metals, Conductors, and Semi-conductors

• Metals − Possess very low resistivity (or high conductivity)
• Semi-conductors − Possess resistivity or conductivity intermediate to metals and insulators
• Insulators − Possess high resistivity (or low conductivity)

Semi-conductors are of two types:

• Elemental semi-conductor − Example: Si and Ge
• Compound semi-conductor − Example: CdS, GaAs, CdSe, InP, etc.

Energy band diagram of metals or conductors

• Conduction band is partially filled and the valence band is partially empty or the conduction and valence band overlap.
• Due to overlap, electrons can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction.
• When the valence band is partially empty, electrons from their lower levels can move to higher levels making conduction possible.

Energy band diagram for insulators

• Large band gap E_g exists. (E_g > 3 eV)
• Since there are no electrons in the conduction band, no electrical conduction is possible.
• The electron cannot be excited from the valence band to the conduction band by thermal excitation.

Energy band diagram for semi-conductors

• Energy band gap Eg is small. (Eg < 3eV)
• At room temperature, some electrons from valence band cross the energy gap and enter the conduction band.