91Ó°ÊÓ

At room temp, due to thermal excitation semiconductor bonds are broken and electrons from these bonds which come out in the lattice are the reason for conductivity. Doping with other elements increases conductivity either by creating holes or extra electrons in the lattice which function as charge carriers

Here As is dopant, which is a member of group 5, it means it will have 5 valence electrons.

So, when Ge is doped with As, As will replace some of the Ge atoms in the lattice and occupy their positions. 4 out of 5 valence electrons will form the bond with neighboring atoms while 1 atom will remain free.

So,

$\text{No. Of holes}=\text{holes due to breakdown of semiconductor}-\text{semiconductor bonds}$

Here, electrons are more in no. than holes, so electrons are the main charge carriers I.e., involved in charge transfer throughout the semiconductor so they formed a negative type semiconductor or n-type semiconductor.

Hence, Ge on doping with As would form an n-type semiconductor because of more no. Of negatively charged (electrons) carriers.

Here B is dopant which is a member of group 3, which means it will have 3 valence electrons

So, when Si is doped with B, B will replace some of the Si atoms in the lattice and occupy their positions. 3 bonds will be formed by 3 valence electrons while one space will remain empty creating a hole. So, the electron-hole balance will be changed.

So,

$$\begin{gathered} \text{number of free electrons}=\text{free electron due to breakdown of semiconductor} \\ -\text{semiconductor bonds} \\ \text{No. Of holes}=\text{holes due to breakdown of semiconductor}-\text{semiconductor bonds} \\ +\text{hole created due to dopant}B\text{as it forms only 3 bonds} \end{gathered}$$

Here holes are more in no. than electrons, the current is mainly carried by holes.

Holes accept free electrons here so considered to be positively charged in nature. so, they formed a positive type semiconductor or p-type semiconductor.

Therefore, Si on doping with B would form a p-type semiconductor because of holes as majority carriers.