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What is the Function of a Diode?

Author: Apogeeweb
Date: 12 Mar 2020
diode functions


Ⅰ. Working Principle

1.1 Terminology

1.2 PN Junction

1.2.1 Principle

1.2.2 Feature: unidirectional conductivity

1.2.3 Supplementary note

Ⅱ. Diode Applications

2.1 Main Functions

2.2 Typical Diode Applications

Ⅲ. One Question Related Diode Functions and Going Further

3.1 Question

3.2 Answer

Ⅳ. Diode Manufacturers Recommendations

Ⅰ. Working Principle

1.1 Terminology

A diode is a two-terminal electronic device with unidirectional conductivity. It is divided into an electron diode and a crystalline diode. Because of the heat loss of the filament, the efficiency of the electron diode is lower than that of the crystalline diode. So the latter is commonly used in electronics.

The main principle of a diode is to use the unidirectional conductivity of the PN junction. And adding leads and packages to the PN junction becomes a diode.

The crystalline diode is a PN junction formed by a P-type semiconductor and an N-type semiconductor. A space charge layer is formed on both sides of the interface, and a self-built electric field is built. When there is no applied voltage, the diffusion current caused by the difference in carrier concentration between the two sides of the PN junction and the drift current caused by the self-built electric field are equal, that is, they are in an electric equilibrium state.

When there is a forward voltage bias from the outside, the mutual suppression of the external electric field and the self-built electric field causes the carrier's diffusion current to increase, causing a forward current. When there is a reverse voltage bias outside, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current that is independent of the reverse bias voltage value within a certain reverse voltage range.

When the applied reverse voltage reaches a certain level, the electric field strength in the space charge layer of the PN junction reaches a critical value, resulting in a multiplication process of carriers to generate a large number of electron-hole pairs, and finally generating a large reverse breakdown current, which called the breakdown phenomenon of the diode. To say a few more words, the reverse breakdown of PN junction is divided into Zener breakdown and avalanche breakdown.

P-type Semiconductor and N-type Semiconductor  

Figure 1. P-type Semiconductor and N-type Semiconductor 

1.2 PN Junction

A PN junction refers to a special charged thin layer formed on the interface between the p-type semiconductor and the N-type semiconductor by using a special process. In addition, P-type semiconductors and N-type semiconductors are commonly referred to as P-regions and N-regions. The formation of the PN junction is due to the existence of a large number of positive holes in the P region and a large number of free electrons in the N region. Therefore, a carrier concentration difference cause a diffusion motion. The positive holes in the P region diffuse into the N region, and the free electrons in the N region diffuse into the P region. In addition, the positive holes and the free electrons move in opposite directions.

A lead is respectively connected from the two sides of the PN junction. The wiring on the P-region  is called the positive pole (anode) and connects with the positive pole of the power supply; the lead on the N-region is called the negative pole (cathode) and connects with the negative pole of the power source.

1.2.1 Principle

P-type semiconductors are doped with intrinsic semiconductors (a completely pure, structurally intact semiconductor crystal) with a small amount of trivalent element impurities, such as boron. Because of tri-boron atom, when it forms a covalent bond with the surrounding silicon atom, due to the lack of an electron, a vacancy is created in the crystal. Once the electrons on adjacent covalent bonds gain energy, it is possible to fill this to make the boron atom an immovable negative anion, while the covalent bond of the original silicon atom forms holes due to the lack of an electron, but the entire semiconductor is still neutral. In this P-type semiconductor, holes are mainly conductive, holes are majority carriers and free electrons.

The principle of N-type semiconductor formation is similar to that of P-type. When pentavalent atoms such as phosphorus are doped in intrinsic semiconductors, it forms covalent bonds with silicon atoms, creating free electrons. In an N-type semiconductors, electrons are majority carriers and holes are minority carriers.

PN junction 

Figure 2. PN Junction

Therefore, the trivalent and pentavalent impurity elements doped into two different regions of the intrinsic semiconductor forms a P-type region and an N-type region. According to the characteristics of the N-type semiconductor and the P-type semiconductor, it can be seen that the difference in the concentration of electrons and holes occurs at interface between them. There is a difference in the concentration of electrons and holes. In addition, both electrons and holes diffuse from regions with high concentrations to the low concentrations, which destroys the electrical neutrality at the original junction.

1.2.2 Feature: unidirectional conductivity

A forward voltage is applied to the PN junction. Under the action of this applied electric field, the equilibrium state of the PN junction is broken. The holes in the P region and the electrons in the N region both move the PN junction, and negative ions neutralize electrons and positive ions in the N region of the PN junction, which narrows the PN junction space. With the increase of the applied electric field, the diffusion motion is further enhanced and the drift motion is weakened. When the applied voltage exceeds the threshold voltage, the PN junction is equivalent to a small resistance, that is, the PN junction is turned on.

1.2.3 Supplementary note

When a forward voltage is applied to the PN junction, the internal current direction is the same as  the power supply, and the current can easily form a current loop through the PN junction. At this time, the PN junction is in a low-impedance state (the impedance in the forward-biased state is small), and the circuit is in a conducting state.

When a reverse voltage is applied to the PN junction, the direction of the current inside it is opposite to the power supply, and it is not easy for the current to form a loop through the PN junction. At this time, the PN is set to a high-impedance state, and the circuit is in an off -state.


Ⅱ. Diode Applications

Diode is one of the earliest semiconductor devices, based on its unidirectional conductivity, semiconductor diodes are used in almost all electronic circuits. It plays a major role in many electronic circuits. The diode is a fundamental component you have to understand if you want to grow your electronics prowess.

Function of a Diode in a Circuit Uses

2.1 Main Functions

Diodes are used in virtually all electronic circuits. The use of semiconductor diodes in the circuit can protect the circuit and extend the life of the circuit. The development of semiconductor diodes has made integrated circuits better and played an active role in various fields. The following briefly describes the role of the diode in the following four circuits.

(1)In switching circuit (current steering)

In digital and integrated circuits, the unidirectional conductivity of a diode is used to turn on or off a circuit, and this technology has been widely used. For example, switching diodes can protect the circuit well, prevent the circuit from being burned out due to short circuits, etc., and also keep the function of traditional switches. Another feature of the switch diode is the fast speed of the switch. This is not comparable to traditional switches.

(2)In limiter circuit (signal control)

In electronic circuits, limiter circuits are commonly used to process various signals. It is used to selectively transmit a part of the signal within a preset level range. Most diodes can be used as limiters, but sometimes special limit diodes, such as protection instruments, are needed.

(3)In regulator circuit (over-voltage protections)

Zener diodes are usually used in voltage stabilizing circuits. It is a surface-junction silicon semiconductor diode manufactured using a special process. This special diode has a high impurity concentration, a high charge density in the charge space, and is easy to form an electric field. When the reverse voltage across the Zener diode is increased to a certain value, the reverse current increases sharply, causing reverse breakdown.

(4)In varactor circuit (signals demodulation)

Varactor diodes are commonly used in varactor circuits to realize automatic frequency control, tuning, frequency modulation, and scanning oscillation of circuits. They are widely used in microwave circuits such as parametric amplifiers, electronic tuners, and frequency doublers.


2.2 Typical Diode Applications

  • Light-emitting diode (LED)

Light-emitting diode.jpg

Figure 3. Light-emitting Diode

With the development of technology, light-emitting diodes are widely used in indicators of various electronic products, light sources for optical fiber communications, indicators of various instruments, and lighting. Many characteristics of light-emitting diodes are unmatched by ordinary light-emitting devices, such as safety, high efficiency, environmental protection, long life, fast response, small size, solid structure and so on. Therefore, the light emitting diode is a pretty light source that meets the requirements of green lighting.

Here are some of their main applications:

Light-emitting diodes are commonly used as screen backlights or display and lighting source in electronic appliances. For example, from large LCD TVs, computer displays to media players MP3, MP4, and mobile displays, LEDs are used as screen backlights.

Light-emitting diodes are widely used in automobiles and large machinery. The direction lights, interior lighting, machinery and instrument lighting, headlights, turn signals, brake lights, and tail lights in automobiles and large mechanical equipment are all LEDs. In addition, the life of LEDs is generally longer than that of automobiles and large machinery).

Because light-emitting diodes have the characteristics of higher efficiency, lower energy consumption, longer life, and strong luminosity than ordinary lighting devices, they have been used in such equipment as miner lamps and underground lighting. They will replace ordinary lighting devices in mining applications.

In modern society, neon lights are an important symbol of urban cities, but neon lights have  short service life. Fortunately, the use of light-emitting diodes to replace neon lights has many advantages, because light-emitting diodes have the advantages of long life, energy saving, easy driving and control, and no long-time maintenance. Therefore, the replacement of neon by light emitting diodes will be an inevitable result of the development of lighting equipment.

  • Zener diode

Zener diode, its current varies within a wide range but the voltage remains basically unchanged. They are classified by breakdown voltage. If you want to obtain a higher voltage, you can use it in series, which can get a higher output constant voltage. For example, 1N4620 regulated 3.3V, 1N4625 regulated 5.1V, etc, and the power ranges from 200mW to 100W.

 Zener Diode Circuit

Figure 4. Zener Diode Circuit

  • Rectifier diode

The rectifier diode can use one-way electric conduction to rectify the AC into a DC output. This unidirectional behavior is called rectification. That is, the rectifier diode can form a rectifier circuit based on its characteristics. Generally, it is widely used in circuits with low processing frequencies, such as rectifier circuits, embedded circuits, and protection circuits. Also, the main consideration in the use of rectifier diodes is that the maximum rectified current and the maximum reverse working voltage should be greater than the values in actual work.

Full Wave Rectifier Circuit 

Figure 5. Full Wave Rectifier Circuit

For the reverse recovery time of the cut-off frequency of the rectifier diode in the ordinary regulated power supply, the working requirement is not very strict. Generally, the maximum rectifier current and the maximum reverse working voltage are the key points. Such rectifier diodes include 1NXX series and 2CZ series. For high-frequency switching power supplies, rectifier diodes with higher operating frequency and shorter reverse recovery time are used, such diodes include RU series, V series, and 1SR series.

  • Detector diode

Detector diodes have high detection efficiency and good frequency characteristics, and are usually used in small signal circuits such as semiconductor radios and televisions. As for the detection, it is to take the modulation signal from the input signal, and the size of the rectified current is generally 100mA as the dividing point, and the output current is usually less than 100mA.

Detector Diode Circuit 

 Figure 6. Detector Diode Circuit

  • Schottky diode

A Schottky diode is a low-power, ultra-high-speed diode. Its main characteristics are short reverse recovery time and small switching loss. In operation, its forward voltage drop is only about 0.4V. Such a low-power, ultra-high-speed semiconductor device is widely used in switching power supplies, inverters, drivers, etc. They are commonly used as high-frequency, low-voltage, high-current rectifier diodes, free-wheeling diodes, protection diodes, or in microwave communication and other circuits as rectifier diodes, small signal detection diodes.

  • Switching diode

A switching diode is a type of semiconductor diode. When there is a forward current, the current flows and the device is turned on. When negative current flows, the diode is not conducting. It plays the role of switching and isolation in the circuit.

Switching Diode Circuit 

Figure 7. Switching Diode Circuit

VD1 is a switching diode in the circuit, and its function is equivalent to a switch, which is used to switch on and off the capacitor C2.

  • Fast recovery diode

Fast recovery diode (referred to as FRD) is a semiconductor diode with good switching characteristics and short reverse recovery time. It is mainly used in switching power supplies, PWM pulse width modulators, inverters and other electronic circuits, as a high-frequency rectifier diode, freewheeling diode or damping diode. At present, fast-recovery diodes are mainly used as rectifiers in inverter power supplies, limiting and clamping high-frequency signals.

  • Transient voltage suppressor

Transient voltage suppressor TVS is a solid-state diode specifically designed for EsD protection.

 Diode Symbol

Figure 8. Diode Symbol


3.1 Question

Why we are using diode in a circuit?

3.2 Answer

The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking it in the opposite (the reverse). As such, the diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current (AC) to direct current (DC). Forms of rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers.


Ⅳ. Diode Distributors Recommendation

Now that your current is flowing in the right direction, it's time to put your new knowledge to good use. Whether you're looking for a starting point or just stocking up, the following are some good diode distributors as our recommendation for you:


Frequently Asked Questions about Diode Function

1. What is a diode used for?
Main functions. The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking it in the opposite direction (the reverse direction). As such, the diode can be viewed as an electronic version of a check valve.


2. What is the main function of PN diode?
A p-n junction diode is a basic semiconductor device that controls the flow of electric current in a circuit. It has a positive (p) side and a negative (n) side created by adding impurities to each side of a silicon semiconductor.


3. What is the function of rectifier diode?
Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode.


4. Are diodes AC or DC?
A single diode or four diodes transform 110V household power to DC by forming a halfway (single diode) or a full-wave (four diodes) rectifier. The diode allows only half of the AC waveform to travel through it.


5. What is the function of zener diode?
Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits. The load voltage equals breakdown voltage VZ of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.


6. What is the difference between diode and rectifier?
A diode is an electronic component that allows the current to flow in only one direction. It is a two-terminal semiconductor device. The rectifier is a device that is used to convert an AC voltage to a DC voltage. A diode is used as a switch and a rectifier is used for the conversion of an AC voltage to a DC voltage.

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