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CBSE Class 12-science Answered

what are semiconductors?why do they not conductors and insulator?how do they differ?  
Asked by baranikerthi5 | 27 May, 2018, 07:44: PM
answered-by-expert Expert Answer
In an isolated atom, electron energy level is decided by its orbit. when crytals are formed, each electron of the atom will have different energy level. These different energy level with continuous variation form what are called energy bands. Energy band which includes the energy level of valance electrons is called Valance band. The energy band above valance band is called conduction band. With no extra energy, all valance electron will reside in valance band. Normally Conduction band is empty. But when energy levels of Conduction band overlaps with energy levels of Valance band, electrons move freely. This is happening in metallic conductors.
If there is a gap between Conduction Band and Valance band, then all electrons in valance band remains bound and no free electrons are available in Conduction band. This makes the material as insulator. The gap between the top of valance band and bottom of conduction band is called the energy band gap (Eg).
Case I.   In metals, either  Conduction Band is partially filled and valance band is partially empty or Conduction band and valance band overlaps. This is shown in fig.1(a) and (b). When there is overlap, electrons from valance band can easily move into conduction band. Hence large number of elecrons are available for electrical conduction. When the valance band is partially empty, electron from its lower level can move to higher level making electrical conduction possible. Hence resistance is very low in metals.
 
Case II. This is the case of insulators. A large band gap, Eg > 3 eV exists between valance and conduction band. This is shown in fig.2. Conduction band is void of electrons. Hence electrical conduction is not possible for insulators. Gap energy Eg is so large, electron can not be excited from Valance band to conduction band by thermal excitation.
 
Case III. Energy gap is finite but small (Eg < 3 eV ). At room temperature electron from valance band can acquire enough energy to cross the energy gap and enter into conduction band. These electron can move in Conduction band(though small numbers). Hence resistance is not high as that of insulators
 
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