CBSE Class 12 · 2020 · SemiconductorsShort Answer

(a) Circuit: an unregulated dc supply is connected to the zener (reverse biased) through a series resistance $R_S$, with the load $R_L$ in parallel with the zener.
Working: once the input exceeds $V_z$, the zener operates in breakdown where its voltage stays constant at $V_z$.
If the input rises, the extra voltage drops across $R_S$ (the zener current increases) so the load voltage stays at $V_z$; thus the output is regulated.
(b) Heavy doping of both sides makes the depletion region very thin, producing a strong electric field at a low reverse voltage, so breakdown occurs sharply at a low, well-defined $V_z$.
$\boxed{\text{Constant }{V}_z\text{ regulates output; heavy doping}\Rightarrow \text{thin layer, low }{V}_z.}$