1Q: The conduction band of a solid is partially filled at 0 K.Will it be a conductor, a semiconductor or an insulator ?

Answer :

An insulator has a large energy band gap between the conduction band and valence band. There will be no carriers in the conduction band at 0 K as well as at 300 K.

A semiconductor has a moderate energy band gap and there will be no carriers in the conduction band at 0 K. There will be carriers in conduction band at room temperature.

A conductor has overlapped conduction and valence band (energy band gap do not exist). The conduction band is partially filled with carriers at 0 K and it will be fully filled at room temperature.

 Therefore, the material is a conductor in which the conduction band is partially filled at 0 K.





2Q:  In semiconductors, thermal collisions are responsible for taking a valence electron to the conduction band.
 Why does the number of conduction electrons not go on increasing with time as thermal collisions continuously take place ?

Answer :

Thermal collisions continue to generate some number of electron-hole pairs due to the jumping of electrons from valence band to conduction band while other electron-hole pairs disappear due to the recombination process.

Recombination is the process where an electron moves from the conduction band to the valence band so that a mobile electron-hole pair disappears.

The electron in the conduction band will lose its energy when it collides with atoms and hence it comes back to the valence band filling an empty place, thus recombination takes place.

Therefore, recombination is the main reason for the conduction electrons not go on increasing with time as thermal collisions continuously generates electron-hole pair.





3Q: When an electron goes from the valence band to the conduction band in silicon, its energy is increased by
1.1 eV. The average energy exchanged in a thermal collision is of the order of kT which is only 0.026 eV at room temperature. How is a thermal collision able to
take some of the electrons from the valence band to the conduction band ?

Answer :

Thermal collision means that the electrons have already acquired sufficient energy due to the room temperature (300 K) and it moves freely thus colliding with lattice atoms and transferring its energy to other electrons in the atom.


At room temperature, some of the electrons occupying the highest energy level in the valence band will acquire enough energy (greater than 1.1 eV) and hence they jump to the conduction band from the valence band before any collisions take place. 

Also some excited electrons (energy less than 1.1 eV) occupying lower energy levels in the valence band collides with other excited electrons occupying the highest energy level in the valence band.

 Due to this collision, there is the exchange of 0.026 eV which makes these electrons to jump from higher energy level of valence band to the conduction band. The thermal collision as well as already applied room temperature are the reason for this movement of electrons.





4Q: We have valence electrons and conduction electrons in a semiconductor. Do we also have ‘valence holes’ and ‘conduction holes’ ?

Answer :

We have valence electrons in the outer orbit of a semiconductor. We have conduction electrons in the conduction band of semiconductor at 300 K. 

When an electron jumps from the valence band to the conduction band, a hole is created in the valence band at the place from where that electron jumped.

 There is no such concept of ‘valence holes’ and ‘conduction holes’. A place void of electron is a hole.