In an $n-$type semiconductor,which of the following statements is true?

In a semiconductor,the intrinsic carrier concentration of electrons and holes is $6 \times 10^8 \ m^{-3}$. After doping with some impurity,the electron concentration increases to $9 \times 10^{12} \ m^{-3}$. Find the new hole concentration.

If ${n_e}$ and ${n_h}$ are the number of electrons and holes in a semiconductor heavily doped with phosphorus,then

Why is it required to add impurity to a pure semiconductor? Mention its condition and explain what impure semiconductors are.

The impurity atom added to germanium to make it $N-$type semiconductor is

In pure silicon,the electron-hole concentration at $T = 300 \ K$ is $7 \times 10^{15} \ m^{-3}$. Antimony is added as an impurity to silicon at a rate of $1$ atom per $10^7 \ Si$ atoms. Assume that half of the impurity atoms contribute their electrons to the conduction band. Calculate the factor by which the number of charge carriers increases. Given: the number density of silicon atoms is $5 \times 10^{28} \ m^{-3}$.