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Updated: Aug 12, 2023

From our childhood days and even now, we are all listening to radios. They are used to hear songs and also an effective costless way of acquiring information in olden days. Have you ever wondered how the radio waves in air are detected and delivered to as a clear sound? This action is performed purely with the help of #diodes. The diodes are constructed with the help of semiconductors.

Starting from 1873 till the 1960s, vacuum tube diodes are mostly employed in sound systems, radios, and televisions. But due to the introduction of semiconductors in the electrical industry, vacuum tubes are replaced by semiconductor-based diodes. In this article, we are going to discuss in detail the diodes, its types, and its applications in various domains.

What is a diode?

A diode is defined as a two-terminal electrical component that only conducts current in one direction as long as it is operated at a specified voltage level. An ideal diode will have zero (low) resistance in one direction, and infinite (high) #resistance in the reverse direction. However, practical diodes do not show zero or infinite resistance. At least they have negligible resistance in one direction and high resistance in the opposite direction.

A thermionic diode or vacuum tube diode is a vacuum tube with a heated cathode and a plate as the two electrodes, in which electrons can flow only from cathode to plate. The heated cathode provides the necessary electrons. John Ambrose Fleming patented the first thermionic diode in 1904. Even though these diodes are replaced by semiconductor diodes, still they are employed in musical instruments, audiophiles, and other high power applications.

first diode
vacuum tube diode

Semiconductor diodes are developed in the early 1930s with the help of crystal detector #technology. These diodes have unique properties such as high forward resistance, lower #capacitance, and greater reverse leakage. The current-voltage characteristics of semiconductor diodes can be controlled by the selection of semiconductor materials and the number of impurities doped in the material. These diodes are operated in both forward-bias and reverse-bias.

silicon based
Semiconductor diode

Types of semiconductor diodes

PN junction diodes and Schottky diodes are two important types of semiconductor diodes. Apart from these two, there are also several diodes manufactured for their unique applications.

P-N junction diode:

The p-n junction can be defined as a boundary between n-type and p-type semiconductors inside a single crystal of semiconductor. This junction can be created by doping processes such as diffusion, epitaxy, and ion implementation.

The p-side contains an excess of holes, whereas the n-side of the junction contains an excess of electrons. This diode constructed using p-n junction is known as p-n junction diode and this configuration allows the electric current to pass through the junction in only one direction.

semiconductor diode
Schematic diagram of p-n junction diode

The p-n junction diode can be operated in two operating regions, and three "biasing" conditions. The region with no charge carriers is called as depletion region and the width of this region is varies with respect to the applied voltage. Zero bias, forward bias, and reverse bias are the three biasing conditions.

  • When a diode is connected in a zero bias condition, no external potential energy is applied to the PN junction. An equilibrium is reached in which a potential difference is formed across the junction. This state is called a dynamic equilibrium in which the majority carriers are equal and moving in the opposite direction which results in zero current flow in the circuit.

  • When a diode is connected in a forward bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type of the junction. If this applied external voltage overcomes the potential barrier of the semiconductor employed in the diode, the current will start to flow. The current flow is established because the negative voltage repels the electrons and they combine with the holes which are also repelled by the positive voltage.

  • When a diode is connected in a reverse bias condition, a negative voltage is applied to the P-type material and a positive voltage is applied to the N-type material of the junction. The positive voltage which is applied to the N-type material attracts electrons towards it. Similarily the P-type attracts the holes towards it. The net result is the depletion layer gets wider due to the absence of holes and electrons and becomes almost an insulator. This high potential barrier prevents the flow of current through the semiconductor.

Schottky diode

Schottky diodes are constructed using a metal electrode bonded to an n-type semiconductor. Unlike the p-n junction diode, this diode has no depletion layer and also possesses a low forward voltage drop. The most employed contact metal in this diode is silicide. This is used for its low ohmic resistance value which in turn helps in more current flow.

semiconductor diode
Schottky diode

The width of the metal-silicon junction is based on the type of metal and semiconductor used in the diode. When forward-biased voltage is applied to the diode, the electrons flow from n-type semiconductors to the metal electrode enabling the flow of current. Due to the absence of a p-type semiconductor, there are no minority charge carriers (holes). So when reverse-biased voltage is applied to the diode, conduction stops very quickly.

Other than these, there are also avalanche diodes, Zener diodes, Gunn diodes, crystal diodes, thermal diodes, and most commonly known Light Emitting Diodes (LEDs).

  • Avalanche diodes conduct the electric current in the reverse direction when the barrier voltage is less than the applied reverse-biased voltage.

  • Zener diodes are electrically similar to the avalanche diodes but possess different breakdown mechanisms (i.e) they have reduced breakdown voltage when compared to avalanche diodes.

  • Crystal diodes are kind of point-contact diodes which are employed in mixer and detector applications in microwave receivers and in radars.

  • Thermal diodes have the property of varying its forward voltage with respect to the temperature. These diodes also include Peltier heat pumps used for thermoelectric heating and cooling purposes.

  • Light Emitting Diodes are manufactured using the semiconductor such as gallium arsenide which emits photons when minority charge carriers cross the junction and combine with the majority carriers. All LEDs produce incoherent, narrow-spectrum light.

  • Laser diodes are formed by placing LED in a resonant cavity with a polished parallel end faces. laser diodes are used for high-speed optical communication.

  • Gunn diodes are similar to the tunnel diodes that are employed in building high-frequency microwave oscillators.

semiconductor diode
Light Emitting Diode

Applications of diodes

Diodes are primarily employed in demodulating the amplitude-modulated radio #signals. The diodes rectify the AM radio signals with only positive peaks. The audio is extracted from this signal and fed into a filter or amplifier which in turn delivers us sound waves. Schottky diodes are used in the microwave, radar wave detectors. They are also employed in the manufacturing of AND, OR logic gates.

Diodes protect the electric circuits from damage caused due to the flow of reverse voltage. The reverse voltage can be generated when the polarity of #power supply inputs are reversed. Rectifiers help us to convert the alternating current (AC) to direct current (DC). These rectifiers are constructed by keeping diodes as their basic unit. Diodes possess a negative temperature coefficient. They are mostly employed in temperature monitoring sensors.

They are also employed as clampers in many circuits (i.e) they can take an alternating current signal as input that has both negative and positive peaks, displace them with either only positive or negative peaks at a prescribed level. Diodes also play a major role in the functioning of #electronic musical keyboards and pinball machines.


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Anushka Gupta
Anushka Gupta

This blog on diodes by Learn Electronics India is nothing short of brilliant! I've seen many articles on this subject, but none have been as thorough and easy to follow as this one. The practical examples and real-world applications have really enriched my understanding of diodes. I can't thank the author enough for sharing their expertise and making electronics learning enjoyable. Keep up the excellent work.


Clear and concise. LearnElectronics India's expertise shines through.

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