What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.

(N/A) Semiconductors are substances having conductance in the intermediate range of $10^{-6}$ to $10^{4} \ ohm^{-1} \ m^{-1}$.
The two main types of semiconductors are:
$(i)$ $n$-type semiconductor
$(ii)$ $p$-type semiconductor
$n$-type semiconductor: The semiconductor whose increased conductivity is a result of negatively-charged electrons is called an $n$-type semiconductor. When the crystal of a group $14$ element such as $Si$ or $Ge$ is doped with a group $15$ element such as $P$ or $As$,an $n$-type semiconductor is generated.
$Si$ and $Ge$ have four valence electrons each. In their crystals,each atom forms four covalent bonds. On the other hand,$P$ and $As$ contain five valence electrons each. When $Si$ or $Ge$ is doped with $P$ or $As$,the latter occupies some of the lattice sites in the crystal. Four out of five electrons are used in the formation of four covalent bonds with four neighbouring $Si$ or $Ge$ atoms. The remaining fifth electron becomes delocalised and increases the conductivity of the doped $Si$ or $Ge$.
$p$-type semiconductor: The semiconductor whose increased conductivity is a result of electron holes is called a $p$-type semiconductor. When a crystal of group $14$ elements such as $Si$ or $Ge$ is doped with a group $13$ element such as $B$,$Al$,or $Ga$ (which contains only three valence electrons),a $p$-type semiconductor is generated.
When a crystal of $Si$ is doped with $B$,the three electrons of $B$ are used in the formation of three covalent bonds and an electron hole is created. An electron from the neighbouring atom can come and fill this electron hole,but in doing so,it would leave an electron hole at its original position. The process appears as if the electron hole has moved in the direction opposite to that of the electron that filled it. Therefore,when an electric field is applied,electrons will move toward the positively-charged plate through electron holes. However,it will appear as if the electron holes are positively-charged and are moving toward the negatively-charged plate.