Electronic – Rationale for operating the diode in the (reverse biased) breakdown region

biasdiodesvoltage-regulatorzener

For the circuits I've studied thus far involving diodes (which admittedly are not that many), they have been nominally used in the forward-bias mode. For example, the LED only lights up when it's operated in the forward-biased region, and is not designed for reverse bias, let alone the burn-out breakdown region.

However, I recently read about the Zener diode, and I found that this particular diode is predominantly used in the reverse-biased, breakdown region, with the following regulator circuit being a popular example:

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Although this circuit works, why can we not achieve the same voltage regulation functionality by operating the diode in the forward-biased mode, like this:

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This is I-V curve I am assuming for the diode:
enter image description here

Best Answer

Three reasons:

First, operating in the forward orientation only allows operation at a single voltage, nominally about 0.7 volts for a silicon diode. Diode construction can be tailored to produce a wide range of breakdown voltages, with a consequent choice of different regulator outputs.

Second, your V-I curve overstates the sharpness of a forward-biased junction. There is no relatively flat portion other than in the vicinity of zero, and that's not very useful.

Third, with an exponential V-I curve, the forward-biased junction cannot be operated at useful current levels with good regulation.