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By adjusting the doping levels and gradients as well as the geometry of a semiconductor crystal, we can modify the behavior of the device. This page lists a wide range of diodes whose properties have been deliberately controlled to produce specific capabilities.

Each of these specialized diodes has its own schematic symbol, shown to the right of its description below. The symbols are all specific variations on the basic diode symbol, so that the nature and function of the device is clear on a schematic diagram.

Light Emitting Diode (LED) 

One of the questions semiconductor manufacturers asked themselves was, "What happens if we increase the doping levels in the silicon crystal?" Trying this gave rise, among other things, to the tunnel diode. Then they took the process even further, to the point where they skipped the silicon completely, and produced what is called a "III-V" device, named after the fact that P-type dopants are from column III of the Periodic Table (aluminum, gallium, indium) and N-type dopants are from column V (phosphorus, arsenic).

The resulting Gallium Arsenide (GaAs) crystal had the interesting property of radiating significant amounts of infrared radiation from the junction. By adding Phosphorus to the equation, they shortened the wavelength of the emitted radiation until it became visible red light. Further refinements have given us yellow and green LEDs. More recently, blue LEDs have been produced. This makes full-color flat-screen LED displays possible.

The mechanism of emitting light is interesting. The current through the junction provides enough energy to free a significant number of electrons from their parent atoms and allow them to become current carriers. Other electrons are given just enough energy to lift them to a higher orbit around their parent nucleus. Such electrons are in a quasi-stable state; they cannot remain at the higher energy state indefinitely, and drop back to their normal orbits. In doing so, each electron releases one quantum of energy according to the difference in energy levels between the two electron orbits. In the case of a visible LED, this quantum of energy has the same energy level as a particular color of visible light, and is therefore released as a visible photon.

If the current level through the diode is high enough, enough electrons are raised to higher orbits that the photon emitted by one electron actually triggers the release of an adjacent photon. When this happens, the new photon takes on the same direction and phase relationship held by the original, triggering photon. These two aligned photons then proceed to trigger the release of others, in an ongoing chain reaction. The light output thus becomes a coherent beam. LEDs manufactured to perform this way are known as laser diodes, and the chain reaction behavior is the basic description of laser behavior.






P-I-N Diode 

The p-i-n diode doesn't actually have a junction at all. Rather, the middle part of the silicon crystal is left undoped. Hence the name for this device: p-intrinsic-n, or p-i-n. Because this device has an intrinsic middle section, it has a wide forbidden zone when unbiased. However, when a forward bias is applied, current carriers from the p- and n-type ends become available and conduct current even through the intrisic center region. The end regions are heavily doped to provide more current carriers.

The p-i-n diode is highly useful as a switch for very high frequencies. They are commonly used as microwave switches and limiters.






Tunnel Diode 

As we mentioned in our discussion of semiconductor physics, the addition of either P-type or N-type impurities causes the Fermi level in the silicon crystal to shift towards the valence band (P-type impurities) or the conduction band (N-type impurities). The higher the doping level, the greater the shift. In the tunnel diode, the doping levels are so high that the Fermi levels in both halves of the crystal have been pushed completely out of the forbidden zone and into the valence and conduction bands.

As a result, at very low forward voltages, electrons don't have to gain energy to get over the Fermi level or into the conduction band; they can simply "tunnel through" the junction and appear at the other side. Furthermore, as the forward bias increases, the applied voltage shifts the levels apart, and gradually back to the more usual diode energy pattern. Over this applied forward voltage range, diode current actually decreases as applied voltage increases. Thus, over part of its operating range, the tunnel diode exhibits a negative resistance effect. This makes it useful in very high frequency oscillators and related circuitry.






Varactor Diode 

One characteristic of any PN junction is an inherent capacitance. When the junction is reverse biased, increasing the applied voltage will cause the depletion region to widen, thus increasing the effective distance between the two "plates" of the capacitor and decreasing the effective capacitance.

By adjusting the doping gradient and junction width, we can control the capacitance range and the way capacitance changes with applied reverse voltage. A four-to-one capacitance range is no problem; a typical varactor diode (sometimes called a "varicap diode") might vary from 60 picofarads (pf) at zero bias down to 15 pf at 20 volts. Very careful manufacturing can get a capacitance range of up to ten-to-one, although this seems at present to be a practical limit.

Varactor diodes are used in electronic tuning systems, to eliminate the use of and need for moving parts.






Zener Diode 

When the reverse voltage applied to a diode exceeds the capability of the diode to withstand it, one of two things will happen, yielding essentially the same result in either case. If the junction is wide, a process called avalanche breakdown occurs, whereby the current through the diode increases as much as the external circuit will permit. A narrow junction will experience Zener breakdown, which is a different mechanism but has the same effect.

The useful feature here is that the voltage across the diode remains nearly constant even with large changes in current through the diode. In addition, manufacturing techniques allow diodes to be accurately manufactured with breakdown voltages ranging from a few volts up to several hundred volts. Such diodes find wide use in electronic circuits as voltage regulators.

Experimentation is always in progress, and new applications are invented regularly. As new diode types come to my attention, I will add them to the list above. If you should hear of a diode type not yet in the list, please contact the webmaster and let me know there. I will research the device and add it as quickly as possible. Thanks.

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