A diode is an electronic component that only allows the electric current to pass in one direction, through its two terminals (anode and cathode).
Ideally we can consider that it has zero resistance in the first case and infinite resistance in the second case.
On several occasions we have discussed the different types of diodes and their applications, but we have not defined in detail how these electronic components work or what other types exist in addition to LEDs.
The semiconductor diode
Although there are several types of diodes, this term usually refers to the semiconductor diode, the most widespread. Another type would be thermionic diodes, which base their operation on vacuum valves and the generation of electrons in a single terminal by means of the joule effect, but we will not go into them in more detail here.
We are going to focus on the study of solid-state (semiconductor) diodes, since they are the technological foundation behind LED lighting. Even so, we will not detail in depth the scientific fundamentals of this technology in order to make the reading lighter.
This type of diode is mostly a junction of two semiconductor materials with special characteristics (p-n junction). One of the materials (n-type) is a negatively charged region (electrons). The other (p-type) is positively charged (holes).
The electrical symbol of the diode is an equilateral triangle with a line passing through one of its vertex at the same length and parallel to the opposite side. Its shape is similar to an arrow.
Interestingly, the direction in which the symbol points is the opposite direction in which the current is moving.
The junction of these two regions is what defines the behavior of the diode. Furthermore, it is in each of these regions that each terminal of the device is connected.
How a diode works
As we have just seen, the way in which this device is constructed is what defines its operation. Generally speaking, we can say that a diode behaves in two ways:
- Like a short circuit, allowing electricity to pass through (direct polarization).
- As an open circuit, preventing the passage of current (reverse polarization).
The different types of diodes can exhibit unique behavior depending on the polarization applied, which allows their use in a wide variety of applications as we will see below.
Types of diodes
There are a multitude of different diodes that have special characteristics depending on their physical appearance, materials, impurities, etc. These diodes are used in very specific applications.
Some of the most common and which we will discuss in more detail are: the laser diode, the Gunn, the varicap and, of course, the LED.
They are used in high-frequency electronics and are characterized by a negative resistance under certain conditions. Gunn diodes are used in the construction of microwave oscillators.
Depending on the materials used, oscillation frequencies above 10GHz can be achieved, and up to 3THz is possible with gallium nitride diodes.
Varicap or varactor diode
They are constructed in such a way that, polarized in reverse, they behave like a variable capacitor with a capacity that depends on the applied voltage.
They are used in tuned circuits where capacity changes are essential.
They are characterized because the recombinations of charge carriers generate photons, the elementary particles that form light.
As we have already mentioned on other occasions, there are different types of LEDs that form the basis of modern lighting systems.
Similar to LEDs, this type of diode is the most common way to generate a laser. They are applied in multiple fields such as: optical fiber communications, pointers, printers or bar code readers, to name a few.
A Zener diode is designed to work with reverse polarization. Instead of cutting off the current flow like a conventional diode, it starts conducting in the reverse direction when a certain voltage is reached. The point at which this voltage stabilizes is often called the Zener voltage.
This type of semiconductor diode is a fundamental part of constant voltage regulators.
The Schottky diode is named after the German physicist Walter H. Schottky. Its main feature is that it can switch from the cutoff state to the conduction state very quickly.
It is used in integrated circuits where high switching speeds are required.
The Shockley diode differs from the previous ones in that it contains four alternating layers of P and N semiconductors instead of two. They are used in digital applications such as counters and timer circuits.
It even has a statue in Mountain View (California) in front of the building where Shockley Semiconductor Laboratories used to be located.
Applications and uses of diodes
As we have seen, depending on the type of diode, the applications are very varied, being used in lighting, radio demodulators, surge protectors or logic gates (essential electronic device in the construction of microchips), but one of the most common is in power supplies where they act as wave rectifiers.
Wave bridge rectifier
Among the most common bridge rectifiers are the half-wave and full-wave bridge rectifiers. A half-wave rectifier eliminates the negative part of the alternating current.
Likewise, a full-wave rectifier converts the negative part of the current into positive.
A very common configuration is one that uses four diodes and is known as a Graetz bridge or dual-wave bridge rectifier. Diode bridges of this type are very common, for example, in DC power supplies.
As we can see, diodes are devices with endless applications and are more present than we think in all kinds of everyday devices, from light bulbs to Blu-ray players.