What is the valency of donor impurity? Name these impurities.

In $n$-type semiconductor,electrons are majority charge carriers,but it does not show any negative charge. The reason is:

In an $n$-type semiconductor:

$A$ $Ge$ specimen is doped with $Al$. The concentration of acceptor atoms is $10^{21} \text{ atoms}/m^3$. Given that the intrinsic concentration of electron-hole pairs is $10^{19} /m^3$, the concentration of electrons in the specimen is:

An intrinsic semiconductor is electrically neutral. An extrinsic semiconductor having a large number of current carriers would be:

In a pure silicon, the number of electrons and holes per unit volume is $1.6 \times 10^{16} \,m^{-3}$. If silicon is doped with Boron in a way that the hole density increases to $4 \times 10^{22} \,m^{-3}$, then the electron density in the doped semiconductor will be: