We might tend to take the word “diode” for granted if we’re thinking of a “diode” as just a two-lead or two-terminal device that gets used in this or that place for this or that purpose. It can become a bit humbling to contemplate just how many kinds of diodes we actually have at our disposal and what they’re used for.
Let’s take a brief, if super-simplistic, look. The schematic symbols shown for each case are not the only applicable symbols I’ve ever seen. In some cases, there are symbol variations in use, but these few shown here will just have to suffice for now.
1. Rectifier Diode (power, signal)
This is a device that simply carries an electrical current in one direction and blocks current flow in the other direction. It can be a small and familiar device like the 1N4148 or something pretty big like a 1N4045 275A 100-V rated diode for a bridge rectifier for wind turbine generator service, or bigger still. It can also be a piece of pencil lead touching a rusty razor blade, a stiff wire (a cat’s whisker) making a point contact on a block of galena, or a low-power, point-contact germanium diode like the 1N34A.
2. Schottky Diode (hot carrier)
This kind of diode is made by forming a junction between a metal (many different types of metal can be used) and some semiconductor material. It has the advantage of a lower forward voltage drop than a semiconductor-to-semiconductor diode and very little storage charge, resulting in a really fast turn-off time.
3. Step Recovery Diode
This device is a semiconductor-to-semiconductor diode with a useful amount of stored charge that allows a brief conduction time in the reverse direction. Time things right and you can cause the reverse conduction to halt at the 270° point of an input RF sinusoid when the storage charge very abruptly runs out. Extremely abrupt current halts make this device a really nice harmonic generator in frequency multiplier applications.
4. PIN Diode (P-type semiconductor, intrinsic semiconductor, N-type semiconductor)
This device is a semiconductor-to-semiconductor diode with a useful amount of stored charge that allows intentional conduction time in the reverse direction. For high enough frequencies, typically 1 GHz and up, this diode’s dynamic impedance can be varied by controlling the DC bias current. That variable impedance is useful for making programmable signal attenuators.
5. Photo Diode
A photo diode will generate an electrical output in response to stimulation by light. Some devices can even be used to detect ultraviolet and/or X-rays.
6. Light Emitting Diode (LED)
A light-emitting diode will generate light in response to stimulation by an electrical current. Some diode devices can generate visible light, as red, yellow, amber, green, blue, or white, while others can generate infrared or ultraviolet. My dentist uses a hand-held ultraviolet LED light to speed up the setting process of dental cement. I questioned him about that. He used to expose dental cement to an ultraviolet lamp.
7. Laser Diode
A laser diode uses a PIN diode structure to pump the intrinsic region in the center of that diode into laser action inside an optical cavity. One of these things is hiding inside that laser pointer of yours, and another one is in your CD player.
8. Zener Diode
A Zener diode is an ordinary diode, but one whose reverse voltage characteristic has a deliberately low breakdown threshold. There is very little current flow through the Zener diode in response to the application of a reverse bias voltage until that reverse bias voltage gets high enough to cross the breakdown threshold and induce a substantial current flow. Voltage regulation is a practical application of this effect.
9. Transient Absorbing Diode
A transient-absorbing diode is very much like a Zener diode, but with the ability to withstand brief intervals of high power during breakdown. Protection of electronic circuitry from otherwise damaging voltage transients is the practical purpose of these devices.
10. Back Diode
A back diode is a diode whose reverse breakdown threshold is very low, even lower than the forward voltage drop of other diodes and even lower than the forward voltage drop of the back diode itself. Low-level RF detection is the practical application for these devices.
11. Varactor Diode
A varactor diode is a diode that is normally operated with reverse voltage applied. The capacitance across the reverse-biased device varies inversely with the applied reverse bias voltage. RF tuning, especially the tuning of voltage-controlled oscillators, is the most common practical purpose of these devices.
12. Tunnel Diode (Esaki)
Tunnel diodes are diodes whose voltage versus current characteristic is discontinuous. They have “voltage-controlled negative resistance” properties. As I personally recall, they were invented in 1957 and were once thought to herald a new age in semiconductor technology. Heathkit even made a tunnel diode DIP oscillator, superseding its earlier grid dip oscillator product. Today, tunnel diodes are still available, although not too commonly used.
13. Gunn Diode
Gunn diodes are single-material semiconductors with no PN junction; nevertheless, they exhibit a negative resistance property that can be exploited to make a microwave oscillator. The lack of a PN junction makes some folks object to the word “diode” as a descriptor for these devices, but the term has become a well-known colloquialism, so who am I to try to change things?
14. Current Limiting Diode
This device might not be called a “diode” either, but as with the Gunn diode, there is a commonly used colloquialism. This device is really a junction field effect transistor (JFET) with the gate tied to the source. The voltage versus current characteristic curve is that of a JFET with Vgs of zero, which, when the device is pulled out of JFET saturation by a sufficiently high voltage, behaves as a constant current driver.
15. Vacuum Diode (Yes, we’re looking at tubes too.)
We may have come full circle at this point. This device is thermionic and, just like its solid-state counterparts, it will conduct current only in one direction. Think 5Y3GT and 35Z5GT. If those part numbers don’t look familiar, go ahead and look them up.
16. Mercury Vapor Diode
A close cousin to the vacuum diode, these devices have an internal atmosphere of heated mercury. In fact, you have to allow enough time (60 seconds if I recall correctly) for the filament to make the mercury hot enough to become a vapor before you try to press the diode into actual service. Also, the device must be operated only in the vertical position with the base pins at the bottom and the plate cap on top. When this tube is doing its thing, the ionized mercury glows blue. Think of the 866A, and again, if that part number doesn’t look familiar, go ahead and look it up.
17. Xenon Gas Diode
Another vapor-dependent diode, but this time the atmosphere is xenon. There is no need to heat the xenon before use, as it is already a gas. When this tube is doing its thing, the ionized xenon glows a somewhat yellowish-white color. Think of the 3B28 and again, if that part number doesn’t look familiar, go ahead and look it up.
18. Magnetron
A magnetron is essentially an educated vacuum tube diode used for generating microwave signals. (Please see “Magnetron.”)
19. Cold Cathode Gas Voltage Regulator
This device isn’t normally referred to as a “diode”, but it meets my idea of being one. It is filled with an ionizable gas, which, when it does get ionized, the plate-to-cold-cathode voltage tends to be stable. It’s a lot like a zener diode in that sense, but it has one troublesome trait of which to be aware. The “striking voltage” for which ionization begins is quite a bit higher than the steady state voltage under steady state gas ionization. That yields a negative resistance property, which, if you put capacitance in parallel with this device, yields relaxation oscillation. When this tube is doing its thing, its gas has a violet glow. Think 0A2 (That first character is a numeral zero, not a letter “oh”.) and yet again, if that part number doesn’t look familiar, go ahead and look it up.
I just happen to have one of those on hand:
20. Mogen Diode
An imaginary device dreamed up by the late Bob Pease. No further discussion necessary.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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