What are the different types of semiconductor diodes used in circuits

Diodes are an important semiconductor device used in every electrical circuit to pass current in one direction. Although a diode is a very basic device of all semiconductor materials, it performs several important functions, including controlling the flow of an electrical current. In simple words, a diode is like a switch used to pass current, without which a circuit is incomplete.

Diodes pass current through its built-in electric field and are mostly made of semiconductor materials, mainly doped silicon and germanium. Widely used in modern devices, diodes are highly useful elements and available in various types.

Different types of semiconductor diodes:

• Zener diode:

This is a kind of diode that allows current to flow in both forward and reverse direction. The diode passes the current when the reverse voltage hits the breakdown voltage, also known as Zener voltage. The doping concentration in a Zener diode is heavier than a standard P-N junction diode. As a result, it has a very narrow depletion region.

In forward bias, the Zener diode functions as a standard P-N junction diode. In reverse bias mode, the diode blocks till the reverse voltage reaches breakdown and then enables the flow of current with a constant voltage drop. Avalanche breakdown and electron quantum tunnelling are two causes of Zener's reverse breakdown.

Zener diodes are commonly utilized in reverse bias configurations as it offers a constant voltage to safeguard the circuits from overvoltage.

• PN junction diode:

These diodes are two-electrode or two-terminal semiconductor devices that pass current in only one direction while blocking the current in reverse or opposite. The electric current usually flows through a forward biassed diode and is blocked in a reversed biased diode. PN junction diodes are also known as PN junction semiconductor devices.

In n-type semiconductors, the majority of charge carriers are free electrons whereas minority charge carriers in p-type diodes are holes. PN junction semiconductors are created by connecting n-type semiconductors to a p-type semiconductor.

These diodes are produced using semiconductor materials like gallium arsenide, silicon and germanium. However, silicon is the most preferred material over germanium for constructing diodes. In comparison to germanium PN junction diodes, silicon PN junction semiconductor can operate at a higher temperature.

• Schottky diode:

This diode does not feature a p-n type intersection but has a small junction between a metal and an n-type semiconductor. One of the key advantages of schottky diodes is that it has fast switching and low forward voltage drop than a regular p-n junction diode. The voltage drop can be between 0.15-0.4 volts at low currents in schottky diodes whereas 0.6 volts in a p-n junction diode. These diodes are constructed uniquely to achieve this efficiency.

Schottky diodes switching speed is fast since they don't have the p-n junction (capacitive junction). These diodes also have a limitation because they have high reverse leakage current and lower reverse breakdown voltage. These diodes are widely used in rectifier and clamping diode applications, as well as in RF applications.

• Varactor diode:

Known as varicap diodes, varactor diodes are voltage controlled capacitors, which comprises a p-n junction that has a variable junction capacitance. These diodes work in reverse bias modes. The depletion layer of the diode between p-type and n-type is differentiated by making changes to the reverse voltage.

These diodes have been carefully engineered and improved. Junction capacitance of all diodes varies with reverse voltage, but the varactor diode can benefit from this condition with a higher range of capacitance.

These diodes are used in applications where variable capacitance is selected and can be achieved by controlling voltage. Varactor diodes are used in phase-lock loops as voltage regulated oscillators as well as in frequency multipliers and RF tuning filters.

• Photodiode:

This is a PIN semiconductor or PN junction device, which generates electric current by consuming light energy. They are also known as photo-detector, photo-sensor or light detector, photodiodes are nearly identical to those of a standard PN junction diode in terms of design and operation. It is mostly made with a PIN (p-type, intrinsic, n-type) structure because the PIN framework has a faster reaction time than the PN (p-type and n-type). PIN photodiodes are utilised in high-speed applications.

These diodes are particularly designed to work in reverse bias conditions. In reverse bias condition, the p-side of the photodiode is attached to the battery's negative terminal, while the n-side is joined to the positive terminal. Photodiodes are light-sensitive, they quickly convert light into electric current when it comes in contact with light.

Since solar cells transform light or solar energy into electric energy, they are also called wide-area photodiodes. Furthermore, PN junction diodes use voltage to produce electric current, while photodiodes use voltage and light to release electric current.

• Gunn diode:

Known as the TED (Transferred Electron Device), gunn diodes basically have a negative resistance diode. The gunn diode does not have any p-n junction and it is made entirely of n-type material. Due to the non-existence of p-type material, these diodes do not rectify AC or work as a regular diode. This is one of the reasons why it is called a Transferred Electron Device rather than a diode.

They have three n-type layers; the two near the terminal side feature high doping concentration and the one near the middle thin layer has low doping concentration. When voltage is passed to this diode, its current increases initially as the voltage increases. As voltage increases, the middle layers resistance also begins to increase, which results in the drop of flow of current. This state is known as the negative resistance field, where the gunn diode works. These diodes are utilized in an oscillator to generate high-frequency microwaves.

• Pin diode:

These diodes have three layers, p-layer, l-layer and n-layer, where the intrinsic semiconductor layer (I) is placed between p-type and n-type semiconductors. The electrons from n-type holes from p-type flow into the intrinsic region. The pin diode begins conduction once the intrinsic fills with electron-holes.

The intrinsic layer can stop and bear high reverse voltages in reverse bias. PIN diodes work like a linear resistor at high frequency whereas functions as a rectifier diode at low frequency. At high frequency, these diodes have bad reverse recovery time and get ample time.

These diodes are utilized in high-voltage rectification and RF application as a switching and attenuator feature.

• Crystal diode:

This diode has a small point junction between the n-type semiconductor crystal and a metal wire. The springy thin wire is made of tungsten or Phosphorus bronze called a cat whisker, which joins at the n-type semiconductor, making a point contact junction.

Crystal diodes are also called point contact diodes and their capacitance is very poor because of the small junction. As a result, the charge storage space is very limited, making it a rapid switching system.

When a significant amount of current is passed through the wire, a tiny p-region is formed on the n-type semiconductor. This small junction forms a P-N junction. They are used in microwave mixers and detectors for low voltage signals.

• Vacuum diode:

It is the most basic diode, produced from two electrodes (cathode and anode) and a vacuum tube. Both anode and cathode are present in the vacuum tube.

When heated, the free electrons on the cathode are released into the vacuum in forward bias. The electrons are gathered by the anode continuing the current flow. The electric current doesn't flow in reverse bias condition as an anode is attached to the negative end and the free electrons present in the vacuum are repelled by the anode.

• Light emitting diode:

Known as LEDs, light emitting diodes are optical diodes used widely among other types of semiconductors. They release visible or invisible infrared light in forward biased conditions on application of voltage. In simpler words, LEDs are light-emitting diodes that transform electrical energy into light energy. The invisible infrared light releasing LEDs are mostly used in remote controls.

When the LED is forward biased, the free electrons present in the conduction-band reunite with the holes present in the valence-band, releasing light energy. The process of releasing light energy against a strong current flow or strong electric field is called electroluminescence.

Light emitting diode is made by connecting n-type material to the negative point and the p-type to the positive point of the battery; meaning n-type materials must be negatively charged, while p-type materials must be charged positively.

The LED diodes structure is identical to a standard p-n junction diode. Instead of silicon or germanium, LEDs are made from phosphorus, arsenic and gallium products because silicon and germanium emit energy as heat and not light.

• Laser diode:

The main function of these diodes is to produce coherent light of high-intensity by converting electrical energy to light energy. They are optoelectronic devices, in which the semiconductors PN junction acts as an active medium or laser medium.

These diodes are inexpensive, compact and the tiniest of all the lasers that have been discovered. Laser diodes are known by the names semiconductor lasers, junction lasers, junction diode lasers, and injection lasers too. The laser diode works similarly to a light emitting diode, but there is one difference among both diodes. LEDs produce incoherent light whereas laser diodes produce coherent light.

These diodes are made from two layers of doped gallium arsenide. One layer of doped gallium arsenide produces n-type semiconductors while another layer of doped gallium arsenide makes p-type semiconductors. Silicon, selenium, and magnesium are used as doping agents in laser diodes.

• Organic light emitting diode:

These diodes are solid-state monolithic devices made up of a sequence of organic thin films that are placed between two conductive thin-film electrodes. After supplying electricity to an OLED (organic light emitting diode), an electrical field is created. This electric field allows the holes and electrons to flow from the electrodes into the organic films till they recombine and form excitons. The excitons further relax to a lower energy level by emitting light and/or unwanted heat.

The cell structure of basic organic light emitting diodes features a pile of thin organic layers placed between anode and cathode.

These are a few most common types of diodes used to design and operate electronic circuits. Every diode is different from the other and has various applications. A few main applications of diodes are radio demodulation, power conservations, reverse-voltage protection, over-voltage protection, logic gates, temperature measurements and current steering. Semiconductor diodes are relatively cheap and easily available from various suppliers on Moglix.