Semiconductors

                                            Resistivity of three types of materials at 0 degrees Celsius

                  A material's ability to conduct electricity is related to it's resistivity. Resistivity is a measure of the resistance to electric current. The Table above provides samples of three types of materials with their repective resistivities.


                 Most of the insulators and metals are crystalline solids. Theri properties are not affected by many small crytals that make them up. in all crystals the atoms are arranged in a regularly spaced 3 dimensional array called lattice.

                Semi conducting solids for electronic application are prepared as a single large crystals. In semi conductors, the fact that atoms are in a 3 dimensional array of large size makes the atom appear invisible to electrons moving within the crystal. This property allows electron to be quite mobile in semi conductors.



                                        CONDUCTION IN SEMICONDUCTORS


INTRINSIC CONDUCTION - a natural conduction of semiconductor materials which could be increased as the temperature of the material is increased.

IMPURITY CONDUCTION - an artificial extra conduction due to impurities added to them during manufacture.


INTRINSIC CONDUCTION 

    In a pure semiconductor such as silicon, all outer electrons of the atoms form bonds with neighboring atoms. At absolute zero, all valence electrons are locked in covalent bonds between the atoms, so none is available for conduction.

    when the temperature of the pure silicon crystal increases, the thermal energy of the atom increases. Increasing the temperature moves some electrons from the valence band into the conduction band. That is freeing and enabling them to move in the crystal lattice.




IMPURITY CONDUCTION

                 During manufacture, small amounts of different elements can be added to semi conductor material to change its conduction properties. Conduction caused by the presence of these impurities, rather than by thermal electrons, is called impurity conduction or extrinsic conduction.
                Adding impurities to a semi conductor material is called doping. A silicon atom has four electrons in its outer shell and all of these are shared with its four nearest neighbors in the crystalline lattice.
               Adding impurity atoms form group V of the periodic table produces negative electrons as charge carriers so that the dope material is called an an N type conductor.

     http://en.wikipedia.org/wiki/N-type_semiconductor

                 Impurity atoms form the group III of the periodic table provide a positive hole for conduction. a semiconductor material with extra holes is called a P-type semiconductor.

http://en.wikipedia.org/wiki/P-type_semiconductor

                 Combinations of P and N type semi conductors are used to make diodes and transistors. The solid state semi conductor components that replaced vacuum tubes in an electronic circuits.