Explain the electrical conduction in semiconductors.

(N/A) $1$. In semiconductors,the energy gap between the valence band and the conduction band is small. Therefore,some electrons can jump to the conduction band,showing some conductivity.
$2$. The electrical conductivity of semiconductors increases with an increase in temperature because more electrons can jump into the conduction band.
$3$. $Si$ and $Ge$ exhibit this behavior and are called intrinsic semiconductors.
$4$. The conductivity of semiconductors can be increased by adding an appropriate amount of suitable impurity. This process is called 'doping'.
$5$. Doping can be done using electron-rich or electron-deficient impurities,which creates electronic defects.
$6$. There are two types of electronic defects:
$7$. $(i)$ Electron-rich impurity: Since $Si$ and $Ge$ belong to group $14$ of the periodic table,they have four valence electrons. In their crystal lattice,each atom forms four covalent bonds with neighbors.
$8$. When doped with group $15$ elements like $P$ or $As$,which have five valence electrons,some lattice sites of $Si$ or $Ge$ are occupied by these atoms. Four of the five electrons are used in forming covalent bonds with four neighboring atoms.
$9$. The fifth electron is extra and becomes delocalized. This delocalized electron increases the conductivity of the doped $Si$ or $Ge$.