(a) As we know that n-type semi-conductor has more concentration of electrons than that of a hole and $p$-type semi-conductor has more concentration of holes than an electron. Due to the difference in concentration of charge carriers in the two regions of $p-n$ junction, the holes diffuse from $p$-side to $/ /$-side and electrons diffuse from $n$-side to $p$-side.When an electron diffuses from $n$ to $p$, it leaves behind an ionized donor on $n$-side. The ionized donor (+ve charge) is immobile as it is bounded by the surrounding atoms. Therefore, a layer of positive charge is developed on the $n$-side of the junction. Similarly, a layer of negative charge is developed on the $p$-side. Hence, a space-charge region is formed on both side of the junction, which has immobile ions and is devoid of any charge carrier, called as depletion layer or depletion region. (b) Using the circuit arrangements shown in fig (a) and fig (b), we study the variation of current with applied voltage to obtain the V-I characteristics shown below. (c) From the V-I characteristics of a junction diode, it is clear that it allows the current to pass only when it is forward biased. So when an alternating voltage is applied across the diode, current flows only during that part of the cycle when it is forward biased.