How does the diode work?

How the diode works ？

Diode is a kind of double element semiconductor device , Including anode and cathode , And provide one-way conduction . Diodes allow current to flow in one direction , But it is not allowed to flow in the other direction . Many types are used for rectifiers 、 Geophone 、 Clipper 、 Mixers 、 Modulator 、 amplifier 、 Oscillators and test instruments .

In circuit symbols , The cathode is displayed as a bar , The anode is shown as a triangle . On some circuit diagrams , The anode of the diode can also use letters “a” Express , The cathode can be written in letters “k” Express .

To understand how diodes work , We should know what semiconductors are .

A material that allows electrons to flow is called a conductor . Materials that block the flow of electrons are called insulators . A material whose conductivity is between a conductor and an insulator is called a semiconductor . Semiconductors are “ Part-time job ” conductor , Its conductivity can be controlled . Semiconductors are very useful in the field of electronics .

How the diode works

A diode is an electronic component that directs current in one direction . They are called “ Active components ”, With transistors and IC identical . It is a basic component made of semiconductors . It can adjust the current , Keep the voltage constant , And detect waves .

First , Let's review the “ semiconductor ” The nature of .“ Can this material conduct electricity ？” According to this question , It's divided into “ conductor ”,“ semiconductor ” and “ insulator ”.“ semiconductor ” It is a kind of material , Its properties lie between a good conductor and a non-conductive insulator .

Generally speaking , Metals conduct electricity very well , Because when metal elements combine with each other , The electrons of every atom become free electrons . When voltage is applied , Free electrons in metal crystals move around and carry electric charges , This is how current flows .

Semiconductors can behave as conductors or insulators , It depends on the state of the current flowing through them . Semiconductors do not have as many free electrons as metals . When voltage is applied , Electrons move in turn to fill the missing holes , Or they carry less free electrons than metal bonds .

Semiconductors are divided into P Type a semiconductors and N Type semiconductor ;P Type a semiconductors are those in which the electrons of the former move in sequence to fill the missing holes . Tetravalent elements such as silicon are mixed with trivalent additives such as boron or boron , Become P Type semiconductor . Because it lacks an electron , It is therefore considered positively charged .

N Type a semiconductors are those that carry electricity , It has fewer free electrons than the metal bonds of the latter . Silicon and other tetravalent elements are mixed with phosphorus and other monovalent additives , Become N Type semiconductor . Because it has an extra electron , So it is considered to be negatively charged .

stay PN In the diode , Connect to P The electrode of a type a semiconductor is called an anode （A）, Connect to N The electrode of type a semiconductor is called a cathode （K）.（ chart 1）

When “-” Connect to PN Anode side of diode ,“+” Connect to PN On the cathode side of the diode , Electricity in the semiconductor is attracted to the electrode side , And in PN A blank area is generated at the junction . result , No current flows .（ chart 2）

contrary , If “+” Connect to anode side ,“-” Connected to cathode side , In semiconductors “+” and “-” Electricity will be in P and N The knots stick together and cancel each other out , But the next electricity will be sent from the electrode , So the current will flow .（ chart 3）

In this way , Diodes only have the property of conducting in a fixed direction . Light emitting diodes we often see in our daily life （LED） It is designed to allow current to flow through PN The knot glows . Diodes are also used where we can't see them , To support our daily life .

The role of the diode

Diodes have the following four main roles .

• （1） Rectification

  Due to the AC current in the general power supply , The direction of the current is always changing . The diode has the characteristic of allowing current to flow only in a specific direction , Therefore, only the forward current can be extracted from the AC current . This is called the rectifying effect of the diode .

• （2） Radio wave detection

  Diodes play a role in extracting audio signals from radio waves . This is called wave detection . Radio waves are generated by combining high-frequency signals for communication with low-frequency signals such as voice .

• （3） Voltage control

  Generally speaking , Diodes carry current only in one direction , But when the voltage in the opposite direction exceeds a certain value , The voltage begins to flow . however , When the reverse voltage exceeds a certain value , The voltage begins to flow , Even if the current increases , The voltage does not change . This is called a breakdown phenomenon , The voltage at which breakdown occurs is called “ Breakdown voltage ” or “ Zener voltage ”. The yield phenomenon is used for diode voltage control , Diodes used in this way are called zener diodes .

• (4) Current conversion

  When light hits PN At the end of the day , Near knot N The electrons on the side will move . therefore , As long as the light shines , Electricity will continue to flow . This is the composition of solar cells . When no voltage is applied from the outside , It acts as a battery , But when voltage is applied , It acts as a diode . Some diodes respond to visible light , And those diodes that respond to invisible light are used in applications such as the light receiving part of the infrared remote controller .

Type of diode

There are various types of diodes . Here is a list of some of the most common types .

• Silicon diodes

  The most common PN Diode type . It usually refers to rectifier diode .

• Germanium diode

  Like silicon diodes , These are combinations PN The diode . They are usually used to detect waves , Because their forward drop voltage is very low , Especially when the flowing current is as small as 0.1mA Region . However , Due to the high cost of germanium , Schottky barrier diodes are now widely used .

• Schottky diode

  This is a kind of diode made by bonding metal and semiconductor . Compared with silicon diode , These diodes have excellent switching characteristics , Therefore, it is used in high-speed circuits .

• Switching diodes

  A diode , Used to turn on and off the power circuit , Like a switch . When the voltage is applied in the direction of power flow , It's on , When voltage is applied in the direction where power does not flow , It closes .

• Kawasaki diode

  Using Nobel Prize Winners Leona Esaki The tunneling effect of diodes was found . Tunneling effect is characterized by high impurity concentration PN A characteristic of junction diode , Even if it should not flow due to quantum mechanical effects , It also allows current to flow . Due to its extremely fast response time , They are used to generate microwaves .

• Light-emitting diode

  A diode , When current flows through PN At the end of the day , Nodes will glow . When current flows through a semiconductor ,P The holes and electrons in type a semiconductors are combined , Energy is emitted in the form of light . It is sometimes used both as a power lamp , Also used as a rectifier .

• Zener diode

  A diode , It is used to apply voltage in the direction opposite to the normal flow direction of current . It is used to obtain a constant voltage , And protect the circuit from overvoltage .

Typical diode characteristics

Before the diode turns on , The forward threshold voltage must be exceeded . The forward threshold voltage must be high enough , To completely remove the depletion layer and force the charge carriers to move on the node . For silicon diodes , The forward threshold voltage is approximately 0.6 V to 0.7 V. For Germanium Diodes , The forward threshold voltage is approximately 0.2 V to 0.3 V.

The figure below shows typical characteristics of small germanium and silicon diodes . It is worth noting that , Diodes are limited by the forward current and reverse voltage they can withstand . This limitation is based on the physical size and structure of the diode .

In case of reverse bias diode ,P Type a material relative to N Type a material is negatively biased . under these circumstances , Imposed on P The negative potential of A-type material attracts positive charge carriers , Pull them away from the knot . Again , Imposed on N The negative charge carriers are attracted away from the junction by the positive potential of A-type material . This depletes the junction region ; There is actually no charge carrier . therefore , The junction region becomes an insulator , The current flow is inhibited . The reverse bias potential can be increased to the rated reverse breakdown voltage of a particular diode . Same as for the maximum forward current rating , The reverse breakdown voltage is specified by the manufacturer . The reverse breakdown voltage is usually much higher than the forward threshold voltage . A typical General-purpose diode can be specified with a forward threshold voltage of 0.6 V, The reverse breakdown voltage is 200 V. If it exceeds the latter , The diode may be irreversibly damaged .