Semicinductors

 1. ENERGY BANDS IN SOLIDS  

(i) Overlapped energy levels are termed as energy bands

(ii) The energy band formed by the overlapping of valency electrons is known as valency energy

             band.

(iii) The energy band formed by the overlapping of conduction electrons is known as conduction energy band.

(iv) Electrical conduction in solid can take place only when electron remains present in its conduction energy band.

(v) The minimum energy required for exciting an electron  from valency energy band to conduction energy band is known as forbidden energy gap (Eg)

Eg = CEBmin – VEBmax

2.TYPES OF SOLID MATERIALS ON THE BASIS OF FORBIDDEN ENERGY GAP   

2.1 Conductors 

Those solid substances in which forbidden energy gap is zero are known as conductors

2.2 Insulators

These are solids in which the energy band formation occurs in such a manner, that valence band is completely filled while the conduction band is completely empty. Furthermore, the valence band and the conduction band are separated by a largely forbidden energy gap 

 Eg  6eV.

The energy band in diamond is shown is Fig. There occurs a forbidden band of width 6 eV between conduction and valence band. No electron can have an energy corresponding to the forbidden band. Thus an electron needs at least 6 eV to reach the empty conduction band. Such an energy can not be supplied by heat or electric fields that are generally used in laboratories. Therefore diamond is an insulator

2.3 Semiconductors

These are solids in which the forbidden energy gap between the valence band the conduction band is small, of the order of 1eV. At 0 kelvin temperature, the valence band is completely filled and the conduction band is completely empty. At  0 K, it behaves lie an insulator (electron can not absorb infinitesimal energy because there is a forbidden gap just above the top of the valence band). At a finitude temperature, (room temperature), some electrons gain energy due to thermal motion and jump from the tops of the valence band to the conduction band. These electrons contribute to the conductions to the conduction of electricity in a semiconductor.

 

The forbidden gap in semiconductor is small ~ 1eV . At finite temperature, some balance electron goes to conduction band. Then the formlessly is in the middle of the gap

The energy gap is some semiconductors is as follows :

Eg (Silicon) = 1.12 eV

Eg ( Germanium ) = 0.7 eV

Eg (Indium antimonide ) 0.17 eV

Eg (Gallium arsenide) = 1.43 eV

Eg ( Tellurium ) = 0.33 eV

The energy gap decreases slightly with increases in temperature. 

 4. COMMENT : BAND STRUCTURE AND OPTICAL PROPERTIES  

The optical properties of a solid are closely related with their energy band structure. The photons of visible light have energies between about 1eV and 3 eV, as sin the case of insulators like mica, diamond, then visible light from valence band can not go to the conduction band. Such solids are transparent to visible light.

In case of semiconductors, since band gap is 

~ 1 eV, the visible light is readily absorbed and these are usually opaque, to visible light,. Infrared photons have energies less than 1eV and therefore infrared light is not absorbed by Si or Ge.

The metals are usually opaque because electrons in the partially filled band can readily absorb visible light photon without leaving the valence band.

The ultraviolet photons energies are large and if they are more than the Eg of insulators, then those insulators will absorb UV radiation. Thus some special glasses are although transparent for visible light are opaque for UV light.