Zener Diodes: How are different voltage drops achieved
From your comments I see that you have to make a distinction between the forward voltage of a PN junction (diode) and the reverse breakdown voltage.
The forward voltage is dependent on the material. Example: a silicon diode has a forward voltage of around 0.7 V while a Schottky diode (which uses a metal-silicon junction) has a lower forward voltage of around 0.2 V.
The reverse voltage relates more directly to zener diodes. Remember that a zener diode is used &quot;in reverse&quot;. If you don't use a zener diode in reverse, it will have a forward voltage similar to that of a &quot;normal&quot; silicon diode: about 0.7 V.
The doping levels of the P-type and N-type silicon do influence the maximum reverse voltage that a diode can withstand before it shows breakdown (starts to conduct).
This is because the thickness of the depletion regions is directly related to doping levels. Low doping levels result in a thick depletion regions and a higher breakdown voltage.