Conductor, semiconductor and insulator

*)  The electrons are filled in shells according to formula 2n^2, where (n) is the number of shell. The outermost shell can shell can have maximum of 8 electrons. 

The electrons present in the outer most shell belongs to valance band or valance energy band, the outermost band of atom. The electrons present in the outermost shell have maximum energy and as we move closer to nucleus the energy of electrons in coming shells keep on decreasing, that is the reason why the e in the innermost shell are strongly bound to the nucleus.

Suppose a sheet of metal, having millions of atoms. And if we give energy to electrons, than that e's detaches from their parent atoms but moves freely within that sheet, now the band in which they move is called as conduction band or conduction energy band. In conduction band e moves as free charge carrier. The energy can be given as thermal energy or by applying voltage. after loosing energy it again falls into valance band.

*)Insulator, forbidden zone or energy gap is about 5MeV. At 0K they also show insulating nature. On increasing the temperature the conductivity of  insulators increases, as the band gap decreases.

 Semiconductor, Eg is 0.67 MeV in Germanium, 1.1 MeV in Si, 1.43 MeV in GeAs

At 0K or -273C the semiconductor behaves as pure insulator, but at room temperature the some outermost electrons get sufficient amount of energy to move to conduction band and acts as free charge carriers and conducts electricity. On increasing the temperature the conductivity of  insulators increases, as the band gap decreases.

Conductors, Eg 0 MeV. At 0K all metals like copper, iron show conductivity. At room temperature they show conductivity(gold>silver>copper), copper is economically feasible. On inc. temp. the conductivity of conductors decreases, because the number of free charge carriers increases in very large amount as a result the no. of collisions increases and the conductivity decreases. 

*) Semiconductors 

    1) Single crystal(indirect band gap)                2) Compound semiconductor(direct band gap)

    a)Intrinsic                      b)Extrinsic 

p-type 13(less e so holes positive)+14  trivalent impiurity

n-type 15(more e so electrons negative)+14 pentavalent impurity

Doped- The process of increasing the conductivity of semiconductors by adding impurity of 13 or 15 group elements.

If impurity level is less than 1 part per 100 million, than semiconductor is considered intrinsic.

Applications- Heat or light sensitive resistances, for ex- thermisiter.

If the doping level is 1 part per million part of intrinsic semiconductor, than it is extrinsic semiconductor.

How free charge increases.

initial- 1 free e for every 10^(12) atoms.

doping level rate- 1 atom in 10^(7) atoms.

so impurity ratio is -  10^(12) / 10^(7) = 10^(5) atoms of impurity, so these many atoms will contribute as holes or e to inc conductivity.