Explanation of heavily doping of both $p$ and $n$ sides of Zener diode Circuit diagram of Zener diode as a $d c$ voltage regulator Explanation of the use of Zener diode as a $d c$ voltage regulator. By heavily doping both $p$ and $n$ sides of the junction, depletion region formed is very thin, i.e. < $10^{-6}\,\mathrm{m}$ . Hence, electric field, across the junction is very high (even for a small reverse bias. This can lead to a 'breakdown' during reverse biasing. If the input voltage increases/decreases, current through resister $R_s$, and Zener diode, also increases/decreases. This increases/decreases the voltage drop across $R_s$ without any change in voltage across the Zener diode. This is because, in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes.