CBSE Class 12 · 2008 · All India · SemiconductorsShort Answer

Principle: a p-n diode conducts appreciably only when forward biased, so it passes current in one direction only; applied to alternating input it rectifies it.
Full-wave rectifier: the ac is applied to a centre-tapped transformer; each end of the secondary feeds one diode, and the load $R_L$ is between the centre tap and the common cathode point.
In the first half-cycle $D_1$ is forward biased and conducts while $D_2$ is reverse biased; in the next half-cycle $D_2$ conducts while $D_1$ is off.
In both half-cycles the current through $R_L$ is in the same direction, so the output is unidirectional for the full input cycle; the output waveform consists of successive positive humps.
$\boxed{\text{Both half-cycles rectified}\Rightarrow \text{full-wave output.}}$