# Zener Diodes: How are different voltage drops achieved

How do semiconductor makers make zener diodes with a fairly wide range of voltage drops? In particular I'm curious how they get small increments - 3.3V, 3.6V 3.9V, 4.2V, 4.8V etc as I thought the semiconductor material basically always resulted in a fix voltage drop which could not be adjusted.

• It depends on doping concentrations. And I'm not even slightly qualified to explain much beyond that, so I'll leave the answer field for someone else to explain it better than I could! Apr 4 '21 at 20:07
• I can promise you it's very dependent on dopant concentration. You don't see as strong of an effect on forward voltage (it is there though! 0.6/0.7 volts is an approximation!), but zener/avalanche voltage is strongly dependent on dopant concentration. Apr 4 '21 at 20:10
• "I thought the semiconductor material basically always resulted in a fix voltage drop" No. Definitely not. You know how Schottky diodes are silicon against metal? Doesn't that make you wonder how you connect wires to any semiconductor without it always turning into a diode wherever a wire enters? Because metal on silicon isn't all there is to it. It's all in how the doping affects the bandgap Apr 4 '21 at 20:15
• try Wikipedia Zener diode
– RoyC
Apr 4 '21 at 20:17
• Not only is the zener voltage dependent on the dopant concentration (which is much higher than for most components) but it’s quite difficult to control. I don’t make zeners but as far as I know they are sorted by voltage after they are made, hence the large range of voltages available.
– Frog
Apr 4 '21 at 20:36

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 "in reverse". If you don't use a zener diode in reverse, it will have a forward voltage similar to that of a "normal" 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.

• The forward voltage also does depend on dopant levels, but to a much lesser extent than the reverse voltage. Apr 4 '21 at 21:10

According to this document, the breakdown voltage is inversely proportional to the doping density N of the pn junction of the diode.

See equations:. 4.5.1 and 4.5.2