An intrinsic conductor has $1018/m^3$ free electrons and is doped with pentavalent impurity of $1024/m^3$. Then the free electron density will increase by:
A) 4
B) 3
C) 5
D) 6

Hint: The pentavalent material has excess electrons in it. When they are doped with another intrinsic semiconductor having some electron density then the total order of the electron density increases is the difference of their individual electron density. These free electrons cause a flow of electric current.

Complete step by step answer:
It is given that free electron density in intrinsic semiconductor is $1018/m^3$ and the electron density of pentavalent impurity is $1024/m^3$. So the free electrons are donated and thus the density order of electrons will be $$(24 - 18) = 6$$. There are two types of semiconductors namely intrinsic and extrinsic. Intrinsic semiconductors are the pure semiconductors which do not have impurities in it whereas the extrinsic are those semiconductors which contain some amount of impurities to make it more conductive. Also there are two types of dopants namely trivalent and pentavalent.

Trivalent are the electron deficient substances whereas the pentavalent dopants have excess electrons. So when pentavalents are doped in intrinsic semiconductors then the order of free electrons is the difference of their individual electron density. These free electron density gives the amount of current it can pass through it. The dopant value increases the conductivity in an intrinsic semiconductor. A small amount of impurity can increase a large amount of free electrons.

Note: According to the question we have to calculate the increase in free electron density. So the total increase in free electrons is the difference in their individual free electron density. If we have to calculate the total free electron density then it would be an order of 30.