Forward and Reverse Bias Diodes

I've been reading Getting Started in Electronics (by Forrest M. Mims III) - just for fun, to learn for myself - and I got really stuck understanding a diagram in the 'Diode Gates' section.

Realizing that I was probably stuck because I missed something basic, I decided to go back and learn a little better.

The Diode's "why"

The purpose of a diode is to allow current (holes) to flow in only one direction and to be blocked in the other direction.

You could put a diode in line with the batteries of a device to prevent damage in the case that someone inserts the batteries in backwards by mistake (or accidentally plugs in one of those terrible reverse-polarity wall warts...)

You could can also use diodes to convert AC to DC (by blocking half of the AC wave), or to prevent the sort of electrical backwash where some sort of inductor or some such kicks voltage in the wrong direction under certain conditions.

The Ideal Diode

In a perfect world, in a vaccum, with no friction, etc, an ideal (meaning hypothetical) diode would have no resistence, no minimum voltage, no voltage drop, no heat, and would convey any amount of current in when used its "normal" (forward bias) mode of operation.

Also, it would be perfectly insulated from "backwards" (reverse bias) operation - no current, no voltage signal, infinite breakdown voltage, etc.

Essentially, it would be the same as a copper or gold wire in one direction, and the same as ceramic or air in the other direction.

Forward Bias vs Reverse Bias

The thing that confused me the most is just the way the terminology is used.

It's not so much that a diode is "forward biased" or "reverse biased" - because it's both - depending on whether you're either putting it in the circuit normally, or backwards...

and why.

Forward Bias

"Forward Bias" means that you place the diode in the circuit such that P goes to positive (the anode lead) and N goes to negative, or ground (the cathode lead).

The diode introduces a small voltage drop, generates a moderate amount of heat - acting somewhat like as a low-ohm, high-current resistor.

However, if the voltage applied to the diode is too low, 0 current will flow. Not "a little bit", but literally inperceptible with a common multimeter. This is due to the nature of the energy levels of electrons and such (see the explainer video below).

Reverse Bias

"Reverse Bias" means that you place the diode such that P goes to Negative, and N goes to positive.

Ironically, in this configuration current will flow - but only a tiny bit.

In this case the diode acts kinda like a high-ohm, low-current resistor. There will be a huge voltage drop (probably down to below 1v), and very little current will flow.

If enough voltage is applied it will breakdown, and break (of course).

However, there is a very narrow window of voltage in which it could potentially be useful without overheating and bursting into flames.

Forward AND Reverse Bias

And so, the diode allows you to let either 0, or a sizable amount of current in one direction, depending on whether a nominal voltage is applied (perhaps 1v+), simultaneously blocking all but a very small amount of reverse current (AC, or "backwash" of sorts) up to the breakdown voltage - which is probably significantly higher than what you intended to exist in your system.

Variations, Zener, & Schottky Diodes

Since we live in the real world (and not the ideal world), we have to use real diodes rather than ideal diodes.

And so, depending on whether you need something that is faster or higher current or what have you, you get a diode that's designed and rated differently for voltage, current, speed, heat, etc for your use case.

For example:

1N4004 is great for rectifying AC power
- High Current (1a)
- High Voltage (400v forward, 280v reverse)
- Low speed (30µs recovery time)
1N4148 (previously 1N914) is great for high-speed (ish) signals
- high-speed 100 MHz, 4ns recovery
- low current (200ma)
1N4733A is common zener diode, which can be used as a voltage regulator

Zener diodes are intended to be used in reverse - they can have relatively low breakdown voltages (3.6v, 5.1v) and, rather than breaking down (fire) at the breakdown voltage, they can conduct a useful amount of current (in the milliampere range).

They still have a small forward voltage drop, but when put in a circuit in reverse they can be used as a voltage regulator (get the same 5.1v out whether the input is 6v or 7v). They're not very efficient regulators (excess power becomes heat), but they can be particularly useful for providing reference voltage.

Schottky diodes are useful in high current (>= 3A).