CBSE Class 12 · 2009 · SemiconductorsLong Answer

Fabrication: a zener diode is made by heavily doping both the p- and n-sides, so the depletion layer is very thin and a high field appears at a low reverse voltage, giving a sharp breakdown at a well-defined $V_z$.
I-V characteristic: an ordinary forward branch, and in reverse bias a tiny current until $V_z$, beyond which the current increases steeply at almost constant voltage.
Significance of breakdown voltage: at $V_z$ the voltage across the diode stays constant over a wide current range, which is what makes it usable as a voltage regulator.
Half-wave rectifier: with the diode in series with a load $R_L$, it conducts only during the half-cycle that forward biases it, giving output for that half only; the other half is blocked, so the output is unidirectional pulsating dc.
$\boxed{\text{Heavy doping}\Rightarrow \text{sharp }{V}_z;\text{diode conducts one half-cycle.}}$