A p$-$n junction with horizontal distance x set to be zero at the metallurgical junction, has the edge of the depletion region on the p$-$type side at $$ lp and in the n$-$type side at $+$ln$.$ The acceptor concentration in p$-$type is NA and the donor concentration in n$-$type is ND$.($i$)$ Using the depletion approximation, derive expressions for the electric field variation across the junction in both the p$-$type and n$-$type sides. $($ii$)$ Hence, or otherwise, show that NAlp $=$ NDln$.$