Shaillee Kaushal is teaching live on Unacademy Plus
XII -01 Solid State Part 14
CONDUCTION BAND THEORY In solids where the conduction is by movement of electrons, the electrical conductivity depends upon the number of electrons available to participate in the conduction theory In metals conductivity depends upon the number of valence electrons o The difference between conductors, insulators and semiconductors can be explained on the basis of conduction band theory or band model The atomic orbitals of metal atoms form a large number of molecular orbitals which are very close in energy. As a result they form bonds. The bonds may be partially filled or overlapping as in case of metals such that the electrons can easily move under the influence of electric fiel The electrical conductance of solids depends upon the energy gap between the filled valance bands and next higher vacant energy band. The outermost filled energy band is called valance band and the next empty band is called o o conduction band The spaces between valance band and conduction band are forbidden to electrons and are called energy gaps or forbidden zone. o Band structure of metals may be explained as follows:
Conduction band Valence band Conduction band Valence band
SEMICONDUCTORS o Intrinsic semiconductors o Extrinsic seminconductor or impurit semiconductors: These are insulators at room temperature and become semiconductors when the temperature is increased. For example silicon and Germanium o These are formed due to addition of impurities and the process is called doping o Both of these have a network or lattice of four strong bonds. When energy is supplied in the form of temperature some of the bonds break and electrons are released o When a voltage is applied these electrons leave their site and a positive hole is created o The electrons move in the interstitial sites whereas the positive holes jump from one bond to the other o Width of forbidden zone in semiconductors i very low, thus on heating electrons easily jum from the valence band to the conduction Ban
EXTRINSIC SEMICONDUCTORS n-type semicoductors: P tvpe semiconducto Impurity of lower group in introduced An acceptor level is created just above o In this higher group impurity is o introduced o o For example if in case Germanium or the valance bond silicone is doped with group 15 impurity then the impurity is pentavalent. o When electron jump from valance bond to acceptor level, a p-hole (positively charged hole) in created in valence o 'Ge' has four electrons and these form bonds with the four electrons of 'As'. The fifth electron is rendered free which helps in the electrical conductivity On applying potential difference, these p-holes conduct electric current. Current carriers are negatively charged, hence called n-type. o o Impurity dopping creates donor level just below the conduction band. On applying potential difference electron jumps from donor level to the conduction bond and conduct electric current.
Examples Metallic Vo Tio Cr02 Semiconductors Fe0% 05 Fe203, Mn203 COg 04 Insulators MnO 304, Mn CuO,Cu,0