$(4$ marks$)$

$3.($a$)$ Explain the physical meaning of the following equation to understand why we can simply use the equation to calculate the hole concentration in p$-$type semiconductor at room temperature.

$p=N_v$exp$(\frac{E_v-E_F}{kT})$

$(4$ marks$)$

$($b$)$ Write down the equation of charge neutrality condition in part $($a$).$

$(3$ marks$)$