Write briefly on $n$-type and $p$-type semiconductors.

(N/A) When we dope $Si$ or $Ge$ with a pentavalent element (e.g., $As, Sb, P$), the impurity atom occupies a position in the crystal lattice. Four of its valence electrons form covalent bonds with the four neighboring $Si$ or $Ge$ atoms, while the fifth electron remains very weakly bound to its parent atom. This is because the four electrons participating in bonding effectively screen the fifth electron from the nucleus.
As a result, the ionization energy required to free this electron is very small ($ \approx 0.01 \, eV$ for $Ge$ and $0.05 \, eV$ for $Si$), making it free to move in the lattice even at room temperature. Since the pentavalent dopant donates an extra electron for conduction, it is called a donor impurity. In $n$-type semiconductors, the number of free electrons $(n_e)$ is much greater than the number of holes $(n_h)$, i.e., $n_e \gg n_h$. The majority charge carriers are electrons (negative), hence the name $n$-type.
When $Si$ or $Ge$ is doped with a trivalent element (e.g., $Al, B, In$), the impurity atom replaces a $Si$ or $Ge$ atom. Three of its valence electrons form covalent bonds with neighbors, but the fourth bond has an electron deficiency, creating a 'hole'. This hole can attract an electron from a neighboring covalent bond, causing the hole to move through the crystal. Thus, the trivalent dopant is called an acceptor impurity. In $p$-type semiconductors, the majority charge carriers are holes (positive), hence the name $p$-type. Here, $n_h \gg n_e$.