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A diode is a crucial electrical component having two terminals which allows current to flow in only one direction. The resistance to electric current has a very high value at one of the two ends and a low value at the other end. They are electrical components that protect the electrical circuit by limiting the electrical voltage. Very high voltage can lead to a fire in the circuit. Diodes can be classified into several types: LED (diode that emits light), laser diode, solar cell, Zener diode, photodiode, and p-n junction diode. They are made of semiconductors like germanium or silicon (group 14 elements).
Light Emitting Diode
LED or Light Emitting Diode emits light when the electric current is made to pass through it. It is one of the most standard sources of light. It emits light that, however, may not always be visible to the human eye. Light has a range of wavelengths, and the only light in the visible wavelength range (400-750nm) can be seen.
Sometimes, one LED can emit as many as three coloured lights at once. LEDs have several applications, including billboards, mobile phones, and television sets.
Like any other diode, LED allows current to pass only in one direction and blocks the current trying to flow in the reverse direction. Its working principle is very simple. Upon the application of voltage, minority electrons travel from the p-side of the diode to the n-side, and minority holes travel from the n-side to the p-side. The two combine at the junction and the energy is released in the form of photons instead of heat.
Since LED allows current to pass only in forward bias, the I-V curve of the diode that emits light looks as follows:
The conduction of current is limited until the forward voltage is reached. The voltage for red is lower, while the voltage for blue is the highest in the colour spectrum. IR LEDs have an even lower voltage.
Photodiode
The working of a photodiode is completely reversed to that of an LED. While an LED converts electrical energy into light (emits light), a photodiode converts light to electrical energy. It is used to measure the intensity of light with high accuracy.
Photodiodes are reverse biassed. When a photon (light) of enough energy strikes the p-n junction photodiode, an electron is knocked off, and an electron-hole pair is created. These electrons and holes move in opposite directions. Electrons move towards the cathode (which has a positive potential value). Holes move toward the anode (which has a negative potential value).
The I-V curve for a photodiode looks as follows:
Solar Cell
A photovoltaic solar cell converts solar energy into an electric current. It is used in solar panels and is greatly in demand these days for solar energy.
When the sunlight falls on the semiconductor material of the solar cell, electrons from the semiconductor are released. It is also reverse biassed. The I-V characteristics of the solar cell are as follows:
In an open-circuit solar cell, the voltage has the highest possible value, whereas the magnitude of electrical current is zero. That voltage is known as open-circuit voltage. In a short-circuit solar cell, the voltage is zero (minimum value), but the value of current is the highest possible. At this point, the current is called short-circuit current.
The point at which the maximum power is obtained is called Maximum Power Point (MPP). It is where the solar cell should ideally work. The voltage and current at MPP are denoted by Vmp and Imp, respectively.
The value of both current and voltage depends on temperature. Hence, the power of the solar cell actually varies with changes in temperature.
Zener Diode
Zener diode is a voltage regulator and one of the most widely used semiconducting diodes. They are popular because they are able to provide a stable voltage. The I-V characteristics of the Zener diode look as follows:
Two kinds of breakdowns occur in a Zener diode—Avalanche and Zener. When the magnitude of reverse voltage is high, there is a rapid collision between electrons, and there is a huge spike in the conductance value. Any other diode would have malfunctioned at such a rapid increase in current, but the Zener diode is engineered in a manner that it can sustain this.
Zener breakdown, on the other hand, occurs at a lower voltage value than Avalanche breakdown.
The Zener effect is defined as the flow of current in the reverse direction as the Zener voltage (also known as knee voltage) is reached. It is a quantum phenomenon.
Conclusion
Different types of diodes exhibit different I-V characteristics depending upon their working principle. LED emits light and operates in the forward bias mode. Photodiodes and solar cells operate in reverse bias. Zener diode is a special voltage regulator diode that is capable of operating in both forward and reverse bias. It is used as a shunt voltage regulator across small loads.
