Ans. OR (a) Formation of depletion region: When $p$ type and $n$ type semiconductors are brought in contact, electrons diffuse from $n$-side to $p$-side and holes diffuse from $p$-side to $n$-side. This is due to the higher concentration of holes in $p$-side and higher electron concentration in $n$-side. The electrons and holes cross the junction and recombine with each other. Thus a certain region of both side of the actual junction becomes depleted from free charge carriers. There remain only immobile positive ions in $n$-side and immobile negative ions in $p$-side which restrict further electrons and holes to cross the junction. Thus a depletion region is formed. (b) Full wave rectifier: A full wave rectifier is constructed using two $\text{p-n}$ junction diodes, a centre-trapped transformer and a load resistor. Anodes of the diodes are connected at the two end terminals of the secondary of the transformer. Cathodes of the diodes are joined together and connected to the centre tap of the transformer through the load resistor. $\text{AC}$ is fed to the input of the transformer. For the positive half cycle of $\text{AC}, \text{A}$ is $\text{+v e}, \text{C}$ is 0 and $\text{B}$ is $\text{-v e}$. So, $D_1$ is forward biased and $D_2$ is reverse biased. Current flows only through $\text{D}_1$ in the direction as shown. For the negative half cycle of $\text{AC}, \text{A}$ is $\text{-v e}, \text{C}$ is 0 and $\text{B}$ is $\text{+v e}$. So, $D_2$ is forward biased and $D_1$ is reverse biased. Current flows only through $\text{D}_2$ in the direction as shown. So, in both the cases, the direction of current through load resistor is in the same direction. So, a direct potential difference i.e., DC voltage is developed across the resistor . Input Output waveform: (c) To obtain steady DC output from the pulsating voltage, filter is to be used. Capacitor in parallel is an effective filter.