What Diodes are and How Does A diode Work? (Examples Explanation)

In the article today, we will introduce you all about diodes, what is this component and what are its characteristics, where to use it, etc.

Understand what diodes are and what they do in this video


I. What is a Diode?

II. Diode Characteristics

III. Diode Application

IV. Diode Types

V. Diode Conductive Property

VI. Diode Parameters

VII. Diode Testing


I. What is a Diode?

In electronic components, a diode is a device with two terminals. The most common function of diodes is to allow primarily the current to pass in one direction (called forward bias) and reverse blocking (known as reverse bias), which called asymmetric conductance. This characteristic of the current directionality of most diodes is commonly referred to as the rectifying function. The most common diodes made today are semiconductor materials such as silicon or germanium. The package of transistors has glass, plastic, and metal commonly.

forward bias and reverse bias of diode

In the early stage, the vacuum electron diode is an electronic device that can transmit current as unidirectional conduction. There is a PN junction and two lead terminals inside the semiconductor diode and it has unidirectional current conductivity according to the direction of the applied voltage. But the crystal diode is a p-n junction interface formed by the sintering of p-type semiconductor and n-type semiconductor, and a space charge layer is formed on both sides of the interface to form a built-in field. When the applied voltage is zero, the diffusion current caused by the concentration difference between the carriers on both sides of the p-n junction is equal to the drift current caused by the self-built electric field. This is a common characteristic of diodes in normal conditions.

II. Diode Characteristics

1. Forward direction

When the forward voltage is applied, it is small in the starting part of the forward characteristic, which is not enough to overcome the blocking effect of the electric field in the PN junction, and the forward current is almost zero, which is referred to as the headband. This forward voltage, which cannot lead the diode, is referred to as a deadband voltage. When the forward voltage is greater than the deadband voltage, the electric field blocking in the PN junction is overcome, and the diode is in conduction, and the current rises rapidly with the increase of the voltage. In the normal current range, the terminal voltage of the diode is almost unchanged at the time of conduction, which is referred to as the forward voltage of the diode. When the forward voltage across the diode exceeds a certain value, the internal electric field is rapidly weakened, in the case of this situation, the current increases rapidly and the diode leads forward, which called threshold voltage, and silicon tube is about 0.5V, germanium tube is about 0.1V, in addition, the forward on-voltage drop of silicon diode and germanium diode is about 0.6~0.8V and 0.2~0.3V respectively.

2. Reverse direction

When the applied reverse voltage does not exceed a certain range, the current passing through the diode is the reverse current formed by the minority current carrier drift motion. Because the reverse current is very small, the diode is in a cut-off state. This is also called reverse saturation current or leakage current, and it is greatly affected by temperature. In general, the reverse current of silicon tubes is much smaller than that of germanium tubes. The reverse saturation current of low power silicon tube and low-power silicon tube is in the order of nA and the low-power germanium tube is in the order of μA. When the temperature increases, the number of current carriers increases, and the reverse saturation current increases when the temperature is rising.

3. Breakdown

When the applied reverse voltage exceeds a certain value, the reverse current increases suddenly, which is called electric breakdown. The critical voltage that causes the breakdown is called the reverse breakdown voltage of diodes. When the electric breakdown occurs, the diode loses its unidirectional conductivity. If the diode is not overheated by electric breakdown, the unidirectional conductivity will not necessarily be permanently destroyed. After removing the applied voltage, its performance can still be restored. If not, the diode is damaged. Therefore, the reverse voltage should be avoided too high when using diodes.

Reverse: the reverse breakdown of the PN junction is divided into Zener breakdown and avalanche breakdown:

a. Zener breakdown 

The reverse breakdown is divided into Zener breakdown and avalanche breakdown according to the mechanism. In the case of high doping concentration, when the width of the barrier region is very small and the reverse voltage is large, the covalent bond structure in the barrier region is destroyed, the shared electron is separated from the covalent bond binding, and the covalent electron-hole is produced, which results in the sharp increase of the current. This phenomenon is called Zener breakdown. If the doping concentration is low, and the width of the barrier is wider, which will not result in Zener breakdown easily.

b. Avalanche breakdown 

Another breakdown is avalanche breakdown. When the reverse voltage is increased to a larger value, the external electric field accelerates the electron drift speed in the transition region, thus the valence electron in the covalent bond is collided out of the covalent bond by electric field electrodes, which produces a mobile or free electron-hole pair. The newly generated electron-hole is accelerated by the electric field and then bumped out of other valence electrons again. The carrier increases like an electron avalanche, resulting in a sharp increase in the current, which is called avalanche breakdown. Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials.

Regardless of the breakdown, if the current is not limited, it may cause permanent damage to the PN junction.

4. Voltage drop

Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance. Diode voltage drop: forward voltage drop silicon diode (no light-emitting type) is 0.7V, the forward voltage drop of germanium tube is 0.3V. In addition, the forward tube voltage drop of LED will vary with different light-emitting colors. But there are three main colors, the specific reference values are as follows: 

red LED is 2.0~2.2V

yellow LED is 1.8~2.0V

green LED is 3.0~3.2V

The rated current of normal luminescence is about 20mA.

The relation of voltage and current of the diode is not linear, therefore, it is necessary to select resistors properly in parallel with different diodes.

5. Characteristic curve

As above mentioned, the diode has a unidirectional conductivity. Applying a forward voltage to the diode, when the voltage value is small, the current is very small, and when the voltage exceeds 0. 6V, the current starts to increase exponentially, which is generally referred to as the opening voltage of the diode. When the voltage reaches about 0.7V, the diode is in a fully conductive state, and this voltage is generally referred to as the conduction voltage of the diode and is indicated by the symbol UD.

For germanium diodes, the turn-on voltage is 0.2V and the on-voltage UD is about 0.3V. A reverse voltage is added to the diode: when the voltage is small, the current is small, and this referred to as the reverse saturation current IS. When the reverse voltage exceeds a certain value, the current begins to increase sharply, called reverse breakdown, and meanwhile, the voltage of this phenomenon occurred is called the reverse breakdown voltage of the diode, which is represented by the symbol UBR. The UBR values of different types of diodes vary greatly, ranging from dozens of volts to kilowatts.

III. Diode Application

1. General Principle

The main function of a diode is to allow an electric current to pass in one direction (forward direction) and blocks it in the opposite direction (the reverse direction). Based on this function, the diode can be viewed as an electronic check valve. This unidirectional action is called rectification, which is used to transform alternating current (AC) to direct current (DC).

What's more, diodes also have other complicated behaviors than this simple on-off action(their nonlinear current-voltage characteristics as above mentioned). Diodes can conduct electricity if a certain threshold voltage or cut-in voltage is added in the forward direction (forward-biased). And the voltage drop across a forward-biased diode varies slightly with the current, which is affected by temperature; this effect can be used as a temperature sensor or as a voltage reference. In addition, diodes' high resistance to current flowing in the reverse direction drops to a low resistance sharply when the reverse voltage across the diode reaches a value called the breakdown voltage. 

The current-voltage characteristic of semiconductor diodes can be fixed by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture. And these technical indexes are used to create special-purpose diodes that perform many different functions. For example, diodes are used to regulate voltage, to protect circuits from high voltage surges, to electronically tune radio and TV receivers, to generate radio-frequency oscillations, and to produce light.

2. Operational Principle

In this video, we will explore the inner workings and applications of the diode in great detail. Apart from the basic working of the diode, this video also explains V-I characteristics and applications of diode (rectification using Bridge rectifier) with help of animation.

The crystal diode is a PN junction formed by p-type semiconductors and n-type semiconductors, and a space charge layer is formed on both sides of the interface with a self-built electric field. When there is a positive voltage bias, the mutual suppression of the external electric field and the self-built electric field leads to the increase of the carrier diffusion current and then the positive current caused by this interaction between them. When there is a reverse voltage bias, the external electric field and the self-built electric field are further strengthened, forming in a certain reverse voltage range independent of the reverse bias voltage, and the reverse saturation current be generated. When the applied reverse voltage excesses a certain value, the electric field intensity in the space charge layer of PN junction reaches the critical value to cause the multiplying process of carriers, resulting in a large number of electron-hole pairs and a very large reverse breakdown current, which is called a breakdown phenomenon of diodes.

3. Specific Explanations

There are many types of diodes, and according to electronic fabrication, the following diodes are often used: Zener diodes for voltage regulators, switching diodes for digital circuits, various for resonance, and so on. The most common diode is the light-emitting diode. Light-emitting diodes (LEDs) are widely used in various electronic products, light sources for optical fiber communication, indicators, and lighting for various instruments. LEDs have many characteristics which can not be compared with ordinary light-emitting devices. These characteristics include safety, high efficiency, environmental protection, long life, fast response speed, small size, and solid structure. And the following are some of their main applications:

(1) Application summary

(1) In electronic equipment

LEDs are generally used in electronic devices as backlight or display, lighting applications. Displays ranging from large LCD televisions, computer displays, and media players such as MP3, MP4, and mobile phones.

(2) In the automobile and large machinery

The light-emitting diode is widely used in automobiles and large machinery. Light-emitting diodes are used in the direction lights, in-vehicle lighting, mechanical equipment instrument lighting, large-light, turn-light, brake light, tail lights, and so on. It is mainly because the response of the light-emitting diode is fast and the service life is long (the service life of the general light-emitting diode is longer than that of the service life of automobiles and large machinery).

(3) In coal mine

Owing to the advantages of high efficiency, low energy consumption, long life, strong luminosity and so on, LEDs are used in miner lighting devices. Although not fully popularized, it will be widely used in the near future, and LEDs will replace common light-emitting devices in coal mine applications.

(4) In decoration lights of the city

Neon is an important symbol of modern urban prosperity, but there are many shortcomings, such as a short life span. Therefore, there are many advantages in replacing neon with LED. Compared with neon, LEDs not only have a longer life, but also save energy, be easily driven and controlled, and do not need maintenance. It is the inevitable result of LED equipment to replace the neon lamps with LED.

(2) Selection of several common diodes

(1) detector diode 

Generally, the detector diode is usually a point-contact type germanium diode. The detector with high working frequency, low reverse current, and large forward current should be selected according to the specific requirements of the circuit.

(2) rectifier diode

Rectifier diodes are generally planar silicon diodes, used in various power rectifier circuits. When selecting rectifier diode, the parameters such as maximum rectified current, maximum reverse working current, cutoff frequency, and reverse recovery time should be considered. The rectifier diode used in the common series regulated power supply circuit is not strict with the reverse recovery time of cutoff frequency, so long as the maximum rectified current and the maximum reverse working current are selected according to the requirements of the circuit which can meet the requirements.

(3) Zener diode

The Zener diode is generally used as a reference voltage source in a regulated power supply or as a protection diode in an overvoltage protection circuit. The selected Zener diode shall meet the main parameters based on the applying requirement. The stable voltage value of the Zener diode shall be the same as the reference voltage value of the application circuit, and the maximum stable current of the Zener diode shall be higher than the maximum load current of the application circuit by about 50%.

(4) switching diode 

Switching diodes are mainly used in video recorders, TV sets, DVDs, and other household appliances and electronic equipment, such as switching circuits, detection circuits, high-frequency pulse rectifier circuits, and so on.

Medium-speed switching circuit and detection circuit, it is suitable to choose the 2AK series of ordinary switching diodes. High-speed switch circuits can choose RLS series, 1SS series, 1N series, and 2CK series high-speed switch diode.

According to the main parameters of the application circuit (such as forward current, maximum reverse voltage, reverse recovery time, etc.) to select the specific type of switch diode.

(5) variode

When selecting variodes, the parameters such as working frequency, maximum reverse working voltage, maximum forward current, and zero-bias junction capacitance should be considered. A variode with a small reverse leakage current and various junction capacitance should be selected.

IV. Diode Types

There are many kinds of diodes. According to its semiconductor materials, it can be divided into germanium diodes (Ge-diodes) and silicon diodes (Si-diodes). According to its different applications, it can be divided into detector diode, rectifier diode, Zener diode, switching diode, isolation diode, Schottky diode, LED, silicon power-switch diode, rotary diode, and so on. 

Semiconductor diodes work mainly on PN junctions. The point-contact type and Schottky type, which are the most common type based on the PN junction, and they also included in the range of general diodes. According to the characteristics of the PN structure(core structure), it can be divided into the point-contact diode, surface-contact diode, and planar diode.

(1) point-contact type

The point-contact diode is pressed on the surface of a clean semiconductor wafer with a thin metal wire, passing through a pulse current, so that one end of the contact wire is firmly sintered with the wafer to form a PN junction. Due to its point-contact characteristic, only a small current can flow through, thus it is suitable for high frequency and small current circuits, such as radio detection. However, compared with the surface junction type, the point-contact diode has poor forward and reverse characteristics, so it can not be used in high current and rectifier. Because the structure is simple, the price is cheap.

(2) surface-contact type

The PN junction of surface contact is made by alloy method or diffusion method. As for the surface-contact diode, its area of PN junction is larger, allowing a larger current to across through, it is suitable for the conversion of AC to DC circuit, that is rectifying function of diodes, but it is not suitable for the high-frequency circuit.

(3) bond types 

A bond diode is formed by melting gold or silver filaments on a single crystal sheet of silicon or germanium. and the characteristics are between the point-contact type diode and the alloy type diode. Compared with the point-contact type, although the PN junction capacitance of the bond diode is slightly increased, and its forward characteristic is particularly excellent. It is used as a switch and sometimes applied to the detection and power supply rectification (not greater than 50mA). In a bond diode, a diode of a fused gold wire is sometimes referred to as a gold bond type, and a diode of a fused silver wire is sometimes referred to as a silver bond type.

(4) alloy type

PN junctions were fabricated on N-type germanium or silicon single crystal wafers by adding indium, aluminum, and other metals. Small forward voltage drop, suitable for the large current rectifier. The PN junction is not suitable for high-frequency detection and high-frequency rectifier because of its large electrostatic capacity.

(5) diffusion type

In the high-temperature P-type impurity gas, the single crystal wafer heated with N-type germanium or silicon makes one part of the surface of the single crystal become P-type. Due to the small forward voltage drop of the PN junction, it is suitable for a high current rectifier. In addition, the use of high-current rectifiers has changed from silicon alloy to silicon diffusion.

(6) mesa type

Although its fabrication method of PN junction is the same as that of diffusion type, only the PN junction and its necessary parts are retained, and the unnecessary part is corroded by chemical. The rest of it takes on a mesa shape, hence its name. The initial production of this type is made of semiconductor materials by diffusion method. Therefore, this type is also called diffusion mesa. It usually used for small current switches.

(7) planar type

It is named after the surface of the semiconductor is made flat. In a semiconductor single crystal chip (mainly an N-type silicon single crystal chip), a P-type impurity is diffused, and a PN junction formed by selectively diffusing a part of the N-type silicon single crystal chip by a shielding effect of a silicon wafer surface oxide film. Therefore, it is not necessary to use chemicals. In addition, the surface of the PN junction is recognized as a type that having good stability and long service life due to the coating of the oxidized film. Initially, the semiconductor material used is formed by chemical extension, and the planar type is also referred to as an epitaxial planar type. 

The planar diode is a kind of special silicon diode, it not only can pass through a large current, but also has stable and reliable performance, and it is widely used in switching, pulse, and high-frequency circuits.

(8) alloy diffusion type

It is a kind of alloy type. Alloy materials are easily diffused materials, which can be over diffused with the alloy by skillfully mixing impurities so that the proper concentration distribution of impurities can be obtained in the formed PN junctions. This method is suitable for the manufacture of high-sensitivity varactor diodes.

(9) epitaxial type

A diode formed by the manufacture of a PN junction by using an epitaxial surface length process. Manufacturing requires great skill. Because of its ability to control the distribution of impurities at random, it is suitable for the manufacture of high-sensitivity capacitive diodes.

(10) Schottky

Schottky rectification diode

The basic principle is that: the formed substrate is used to block the reverse voltage on the contact-surfaces of metals (such as lead) and semiconductors (N-type silicon wafers). Schottky and PN junction have fundamental difference in the principle of rectifying function. Its voltage resistance is only about 40V. Its advantages are: switch speed is very fast: reverse recovery time is particularly short. Therefore, switching diodes and low-voltage high current rectifiers can be made based on this method.

According to application, diodes can be divided as :

1. detector diode 

Gumanium detector diode

The main function of the detector is to detect the low-frequency signal in the high-frequency signal. It belongs to the point-contact type, so its junction capacitance is smaller and its working frequency is higher, and it is generally made of germanium. In principle, when the modulation signal is extracted from the input signal, usually, and the output current less than the 100mA( the rectifier current 100mA is used as the boundary) is called the demodulation. Its advantages include: the working frequency can reach 400MHz, the forward voltage drop is small, the junction capacitance is small, the detection efficiency is high, and the frequency characteristic is good. In addition to being used for detection, it can also be used for limiting, clipping, modulating, mixing, switching, and other circuits. Furthermore, there are also two diode assemblies dedicated to FM demodulation.

2. rectifier diode

rectifier diode

In principle, the output from the input AC DC is rectified. The rectified current size (100mA) is usually used as the boundary of the output current greater than the 100mA called a rectifier. Surface junction type, so junction capacitance is larger, generally below 3kHZ. Maximum reverse voltage from 25 volts to 3000 volts a total of 22 volts. Classified as follows: 1 silicon semiconductor rectifier diode 2CZ type, 2 silicon bridge rectifier QL type, 3 for television high voltage silicon stack working frequency near 100KHz 2CLG type.

3. clipper diode 

The forward voltage drop of the diode is substantially unchanged after the diode is in conduction (the silicon tube is 0.7V, and the silicon tube is 0.3V). With this characteristic, the amplitude of the signal can be limited to a certain range with this limiting element in the circuit.

Most of the diodes can be used as a clipping component, but there is also a dedicated clipping diode like a protective instrument and a high-frequency Zanner diode. To have a particularly strong effect on limiting the sharp amplitude, a diode typically made of a silicon material. There is also a component set: a number of necessary rectifying diodes are connected in series to form a whole, depending on the need for limiting the voltage.

4. modulation diode

It usually refers to the ring modulation dedicated diode. It is a combination of four diodes with good forward characteristics and consistency. Even though other varactor diodes have modulation applications, they are usually used directly as FM.

5. mixer diode 

In the frequency range of 500~10000Hz, Schottky type and point-contact type diodes are usually used when diode mixing mode is used.

6. amplifier diode

The amplification of a negative resistance device, such as a tunnel diode and a bulk diode, is generally performed with a diode, and also the parametric amplification of the variode. Thus, the amplification diodes generally refer to a tunnel diode, a bulk diode, and a variode.

7. switching diode

The resistance of the diode is very small under the forward voltage, which is equivalent to that of an on-on switch; under the action of reverse voltage, the resistance is very large, and in the cut-off state, that is turn off state. All kinds of logic circuits can be formed by using the switching characteristics of diodes.

A logic operation with a small current and a magnetic core excitation switching diode for use in milliamperes. The small current switching diode is usually a point-contact type and a bond diode, and also has a silicon diffusion type, a mesa type, and a planar type diode which can work at high temperature. The advantage of the switching diode is that the switching speed is fast, and the switching time of the Schottky diode is very short, thus it is the ideal switching diode. The 2AK point-contact is used for medium-speed switch circuits; the 2CK-type plane is used for high-speed switching circuits, usually for switches, clipping, clamp bits, or detection circuits, and the Schottky-barrier diode has the advantages of small positive voltage drop, high speed, and high efficiency.

8. variode

Low-power diode for automatic frequency control (AFC) and tuning. Other manufacturers also have many other terms. When applying reverse voltage, the electrostatic capacity of the PN junction will change. Therefore, it is used for automatic frequency control, scanning oscillation, frequency modulation, and tuning. Generally, although silicon diffusion diodes are used, special diodes such as alloy diffusion type, epitaxial bonding type, and dual diffusion type can be used, because the electrostatic capacity of these diodes has a very large change rate for voltage. Junction capacitance changes with reverse voltage and replaces variable capacitance, used in tuning circuit, oscillating circuit, phase-locked loop circuit. For example, it is often used in TV high-frequency channel conversion and tuning circuits and mostly made of silicon material.

9. frequency multiplication diode

For the frequency multiplication of diodes, the frequency doubling depends on the frequency doubling of the variode and the frequency multiplication of the snap-off diode. The variode used for frequency multiplication is called a variable reactor. Although the variable reactor works the same principle as the variode used in automatic frequency control, the construction of the reactor can withstand high power. Snap-off diode, also called step recovery diode, has a short reverse recovery time when switch on to switch off. If sine waves are applied to snap-off diodes because the on-off time is short, so the output waveform is quickly cut off, it can produce a lot of high-frequency harmonics.

10. Zener diode

Zener diode

This type is based on the reverse breakdown characteristic to be made. The voltage at both ends of the circuit remains basically unchanged, which plays the role of stabilizing the voltage. It is made into a diffusion or alloy type of silicon. Its reverse breakdown characteristic curve changes sharply. Made as a control voltage and a standard voltage component. Diode terminal voltage (also known as Zener voltage) from about 3V to 150V, which can be divided into many grades. In terms of power, there is 200mW to 100W or more. Working in the reverse breakdown state, the dynamic resistance RZ is very small. The two complementary diodes are connected in reverse series to reduce the temperature coefficient, which is turned into a 2DW type.

The p-n junction of Zener diodes is highly doped. And normal diodes will also break down with a reverse voltage but the voltage and sharpness of it may not as well as defined. Also, normal diodes are not designed to operate in the breakdown region, but Zener diodes can reliably operate in this case.

Zener diodes are widely used in electronic devices(almost all kinds) and are one of the basic parts of electronic circuits. It is used to generate low-power stabilized supply rails from a higher voltage and to provide reference voltages for circuits, particularly stabilized power supplies. It is also used to protect circuits from overvoltage, especially electrostatic discharge.

11. PIN diode

This is a crystal diode constructed by a layer of intrinsic semiconductors (or low concentration impurity semiconductors) between the P and N regions. When the operating frequency exceeds 100MHz, the diode becomes an impedance element due to the memory effect of minority carriers and the transit time effect in the "intrinsic" layer, it becomes an impedance element because of losing rectifying function, and its impedance value varies with the bias voltage. The impedance of the "intrinsic" region is very high when the bias is zero or the DC reverse bias, and the "intrinsic" region is low impedance due to the carrier injection into the "intrinsic" region when the DC is positive bias. Therefore The PIN diode can be used as a variable impedance element. It is often used in high-frequency switches (microwave switches), phase shift, modulation, amplitude limiting, and other circuits.

12. avalanche diode

It is a transistor that can produce high-frequency oscillation under the behavior of applied voltage. The working principle of producing high-frequency oscillation is that the carrier is injected into the crystal by avalanche breakdown. Because the carrier transit chip takes a certain time, the current lags behind the voltage, and the delay time occurs. If the transit time is controlled properly, there will be a dynatron effect in the relationship between current and voltage, which will produce high-frequency oscillation. So it is often used in oscillating circuits in the microwave field.

13. tunnel diode

It is a crystal diode based on tunneling effect current as of the main current component. The substrate materials are gallium arsenide and germanium, and the N-type region of the P region is highly doped. 

A tunnel diode is a dual terminal active device, and it can be used in low-noise and high-frequency amplifiers and high-frequency oscillators (whose operating frequency can be up to millimeter-wave level) or in high-speed switching circuits.

(Note: Tunneling is the quantum mechanical phenomenon where a subatomic particle passes through a potential barrier that it cannot surmount under the provision of classical mechanics.

Tunneling plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the Sun. It has important applications in the tunnel diode, quantum computing, and scanning tunneling microscope. The effect was predicted in the early 20th century, and its acceptance as a general physical phenomenon came mid-century.

Fundamental quantum mechanical concepts are central to this phenomenon, which makes quantum tunneling one of the novel implications of quantum mechanics. Quantum tunneling is projected to create physical limits to the size of the transistors used in microprocessors, due to electrons being able to tunnel past them if the transistors are too small.)

14. step recovery diode

It is also a diode with a PN junction. Its structural characteristics are that there is a steep impurity distribution area at the boundary of the PN junction, thus forming a "self-help electric field". The reverse current of the PN junction can be reduced to the minimum value (reverse saturation current) after a "storage time" because of the charge storage effect in the vicinity of the PN junction due to the conduction of a few carriers at the forward bias voltage. The self-help electric field of the step recovery diode shortens the storage time, makes the reverse current cut off quickly, and produces abundant harmonic components. The comb spectrum generation circuit can be designed by using these harmonic components. Fast turn-off (step recovery) diodes are used in pulse and high-order harmonic circuits.

15. Schottky barrier diode

It is a metal-semiconductor junction diode with Schottky characteristics. The forward starting voltage is lower. In addition to materials, gold, molybdenum, nickel, titanium and other materials can be used in the metal layer. Its semiconductor materials are silicon or gallium arsenide, mostly N-type semiconductors. This device is conductive by most carriers, so its reverse saturation current is much larger than that of PN junction with minority carrier conduction. Because the memory effect of minority carriers in Schottky diodes is very small, the frequency response of it is limited only by the RC time constant, so it is an ideal device for high frequency and fast switching. Its working frequency can reach 100GHz. And, MIS (metal-insulator-semiconductor) Schottky diodes can be used as solar cells or light-emitting diodes.

It also can be used as a continuation diode in the switching power supply inductance and plays a role in the continuation of the current in the relay and another inductive load.

16. damping diode 

Damping diodes are widely used in high-frequency voltage circuits, with high reverse working voltage and peak current, but their forward voltage drop is small. It is a kind of high frequency and high voltage rectifier diodes, and often used in TV line scanning circuits for damping and boost rectifying. The commonly used damping diodes are 2CN1, 2CN2, BSBS44, and so on.

17. transient voltage suppressor(TVS)

TVS is used to protect the circuits when having a fast overvoltage. They are divided into two types: bipolar and unipolar, classified by the values of peak power (500W-5000W) and voltage (8.2V~200V).

18. double-base diode (unijunction diode)

A three-terminal negative resistive device with two base electrodes and emitter used in an oscillating circuit, has the advantages of easy frequency adjustment and good temperature stability.

19. LED

It is made of gallium phosphide and gallium arsenide. Low working voltage, small operating current, uniform luminescence, long life, emitting red, yellow, green, blue monochromatic light. With the development of technology, white light and highlight diode to forming the new industry of LED lighting. It is also used in VCD, DVD, calculators, and other displays.

20. silicon power switching diode

The silicon power switching diode has the capability of high-speed conduction and cut-off. It is mainly used for high-power switch or voltage-stabilizing circuit, DC converter, high-speed motor speed-regulating, and high-frequency rectification and free-wheeling, and has the advantages of soft recovery property and strong overload capacity. And it is widely applied to the computer, radar power supply, stepper motor speed-regulation, and so on.

According to characteristic, diodes can be divided as:

Point-contact diodes, classified by forward and reverse characteristic, are as follows:

1. Common point contact diode

This kind of diode, is usually used in demodulation and rectifier circuits and is an intermediate product with forward and reverse characteristics, such as SD34, SD46, 1N34A, and so on.

2. High reverse voltage resistance point contact diode

A kind of component with maximum peak reverse voltage and the maximum DC reverse voltage, which used in the detection and rectification of high voltage circuits, but this type of diode generally has poor or moderate forward characteristics. In point-contact type germanium diode, there are SD38, 1N38A, OA81, and so on. 

3. High reverse resistance point-contact diode

Forward voltage characteristics are the same as general diodes. Although its reverse voltage is also particularly high, the reverse current is small. Used in circuits with high input resistance and high resistance load. For example, SD54 and 1N54A belong to high reverse resistance diodes made of germanium material.

4. High conduction point-contact diode

It is the opposite of the high reverse resistance type. Its reverse characteristics are poor, but the forward resistance is small. For high conduction point-contact diodes, there are SD56,1N56A and so on. For high conduction bond diodes, it has better properties when operating. When the load resistance is especially low, its rectifier efficiency is good. 

V. Diode Conductive Property

The most important characteristic of diodes is unidirectional conductivity. In the circuit, the current can only flow from the positive, flow out from the negative.

(1) forward characteristic 

In electronic circuits, if the positive electrode of the diode is connected to the high potential terminal and the negative electrode to the low potential terminal, the diode will be switched on. This connection is called forward bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode cannot be switched on, and the forward current flowing through the diode is very weak. 

(2) reverse characteristic 

In the electronic circuit, the positive electrode of the diode is connected to the low potential terminal, and the negative electrode is connected to the high potential terminal. In this case, there is almost no current flowing through the diode, and the diode is in the cut-off state. This connection mode is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called leakage current. When the reverse voltage at both ends of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose the unidirectional conductivity, this state is called the breakdown of the diode as forward mentioned.

VI. Diode Parameters

The parameters of the diode are used for indicating the performance of the diode and the technical index of the applications. Different types of diodes have different characteristic parameters, and for beginners, the following main parameters must be understood:

(1) rated forward working current

It refers to the maximum positive current allowed by the diode during long-term continuous operation.

(2) maximum surge current 

It is an excess forward current that is allowed to flow. It is a transient current, and it is usually about 20 times the rated forward current.

(3) maximum reverse operating voltage

When the reverse working voltage at both ends of the diode reaches a certain value, the tube will break down and lose its unidirectional conductivity. In order to keep safe, a maximum reverse working power value is specified. For example, the reverse voltage of an lN4001 diode with a reverse voltage of 50V, and is 1000V for IN4007.

(4) reverse current

Reverse current is a kind of current that the diode flows through the diode at a specified temperature and maximum reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. The reverse current is closely related to the temperature, the reverse current increases twice when the temperature rises 10℃ at one time. In addition, silicon diode has better stability than germanium diode at high temperature.

(5) reverse recovery time

When the forward voltage converts into the reverse voltage, the current can not stop at a short time, because it has a delay time, which is called reverse recovery time. It directly affects the switching speed of the diode.

(6) maximum power

The maximum power is the voltage applied at both ends of the diode multiplied by the current. 

(7) dynamic resistance

The ratio of the voltage variation near the static operating point to the variation of the corresponding current in the diode characteristic curve.

(8) frequency characteristic

Due to the existence of junction capacitance, when the frequency is up to a certain degree, the capacitance reactance is small enough to make the PN junction short-circuit, resulting in the diode loses unidirectional conductivity and cannot work. The larger the PN junction area is, the larger the junction capacitance is, so it can’t work at high frequency.

VII. Diode Testing

General diodes (including detection diodes, rectifier diodes, damped diodes, switching diodes, continuous diodes) have unidirectional conductivity. It is suitable to use a multimeter to detect the positive and reverse resistance, the electrode of the diode can be identified and the damage of the diode can be estimated.

1. The multimeter is placed in the R×100 barrier or R×1k barrier for polarity discrimination. The two-meter pens are connected with two electrodes of the diode respectively. After one result is measured, the two-meter pens are adjusted to obtain another result. In the two measurements, the large resistance value measured is reverse resistance, and the smaller resistance value measured is forward resistance. In addition, the black meter pen is connected with the positive pole of the diode, and the red meter pen is connected with the negative pole of the diode during a small resistance measurement.

2. In general, the positive resistance of GE diode is about 1kΩ and the reverse resistance is about 300. The resistance of the silicon diode is about 5kΩ and the reverse resistance is infinity. The smaller the forward resistance, the better the reverse resistance. The greater the difference between the positive and reverse resistance values, the better the unidirectional conductivity of the diode. If the positive and reverse resistance values of the diodes are all close to zero or the resistance values are small, the internal breakdown short circuit or leakage damage of the diode is indicated. If the positive and reverse resistance values of the diode are infinite, then the dipole is proved. The pipe is open and damaged.

3. Detection of reverse breakdown voltage(withstand voltage) of the diode can be measured by a transistor DC parameter meter. The method is: when measuring the diode, the "NPN/PNP" selection key of the testing meter should be set to the NPN state, and the negative pole should be inserted into the "e" jack of the testing meter and the positive pole of the diode should insert to the "c" jack, then press the V (BR) key, finally the reverse breakdown voltage of the diode can be detected. Another way is that a megohmmeter and a multimeter are used to measure the reverse breakdown voltage of the diode. When measured, the negative electrode of the diode is connected to the positive pole of the megohmmeter, and the positive electrode of the diode is connected to the negative pole of the megohmmeter, and meanwhile, the voltage across the diode is monitored by a multimeter (placed in the appropriate DC voltage level). 

Several Detection methods of Common Diode

1. Detection of a low power crystal diode

A. Distinguishing positive and negative electrode

(a) Observe the symbol mark on the shell. Usually, the symbol of the diode is marked on the shell with the one end with the triangular arrow being the positive and the other end is the negative pole.

(b) Observe the color dots on the shell. On the shell of a point-contact diode, it is usually marked with a polar color dot (white or red). One end with a general colored point is a positive pole. There are also diodes marked with color bands, and one end with the color bands is a negative pole. For example, a diode shell with a silver band is the negative pole.

(c) the one end of the black meter pen is the positive pole, and one end of the red meter pen is the negative pole, whichever takes the smaller value.

B. Detecting the maximum reverse breakdown voltage. 

For alternating current, the maximum reverse operating voltage is the AC peak voltage the diode receives because it is constantly changing.

2. Detection bidirectional trigger diode

Put the multimeter in the corresponding DC voltage block. When testing, shake the mega-meter to measure the VBR value. Finally, comparing VBO with VBR, the smaller the difference between the absolute values, the better the symmetry of the measured bidirectional trigger diode.

3. Detection of transient voltage suppression diode (TVS)

The multimeter is used to measure the quality of the tube. According to the method of measuring the common diode, the positive and reverse resistance can be measured for the single-pole TVS. The general forward resistance is about 4kΩ, and the reverse resistance is infinity.

For the bidirectional polar TVS, the resistance between the two pins measured by two-meter pens should be infinite, otherwise, the diode performance is poor or damaged.

4. Detection of high-frequency variable-resistance diodes

The difference between the positive and negative of high-frequency resistive diodes and ordinary diodes in appearance is that the color code is different. Ordinary diodes are generally black, while high-frequency resistive diodes are always a light color. Its polarity of the band is similar to that of the ordinary diode, that is, one end with a green band represents a negative pole, and the other end without a green band is a positive pole.

5. Detection of variode

Inter-modulation by adjusting the red meter pen and the black meter pen of the multimeter, the resistance value between the two pins of the variode should be infinite. During the measurement, it is found that the multimeter pointer has a slight swing to the right or resistance of zero, indicating that the measured variode has a leakage fault or has broken down.

6. Detection of monochromatic light-emitting diodes

A 1.5V dry battery is attached to the outside of the multimeter, and the multimeter is placed in R×10 or R×100 block. This method is equivalent to giving the multimeter a voltage of 1.5V, which increases the detection voltage to 3V (the starting voltage of the LED is 2V). When detecting, rotate the two pins of the LED with the two-meter pens of the multimeter. If the diode performance is good, there must be a normal luminous, at this time, the black pen is connected to the positive pole and the red pen is connected to the negative pole.

7. Detection of Infrared light-emitting diode

A. Identify the positive and negative electrodes of infrared LEDs. An infrared LED has two pins, usually, the long pin is positive and the short pin is negative. Because the infrared LED is transparent, the electrode inside the tube and shell are clearly visible. The larger electrode is the negative electrode, and the narrower and smaller one is the positive electrode.

B.Measure the positive and reverse resistance of infrared LED firstly, usually, the forward resistance should be about 30k, reverse resistance should be more than 500k so that the device can be used normally.

8. Detection of IR receiver

A. identify pin polarity

(a) Detection in appearance. The common IR receiver appearance color is black. When recognizing pins, facing the light window, from left to right, it is positive and negative respectively. In addition, there is a small oblique plane at the top of the IR receiver, usually a negative pin at one end with the oblique plane and a positive electrode at the other end.

(b) First uses multimeter to judge the positive and negative electrodes of common diodes, that is, to exchange red and black meter pen to measure the resistance between the two pins of the diode twice. Under normal conditions, the obtained resistance values should be various. Taking the smaller resistance, the connected end by the red-meter pen is negative and the black-meter pin is positive.

B. detection performance

The forward and reverse resistance of the IR receiver is measured by a multimeter electric barrier. According to the value of forward and reverse resistance, the quality of the IR receiver can be primarily judged.

9. Laser diode detection

The pin arrangement order of the laser diode can be determined according to the method of detecting the forward and reverse resistance of ordinary diodes. However, it is important to note that since the forward voltage drop of the laser diode is larger than that of the ordinary diode, the multimeter pointer only slightly swings to the right when detecting the forward resistance.


1. What is diode and its symbol?

Diode, an electrical component taaaaaaaaaaaaaaaaaahat allows the flow of current in only one direction. In circuit diagrams, a diode is represented by a triangle with a line across one vertex.

2. What is special about a diode?

Some semiconductor junctions, composed of special chemical combinations, emit radiant energy within the spectrum of visible light as the electrons change energy levels. Simply put, these junctions glow when forward biased. A diode intentionally designed to glow like a lamp is called a light-emitting diode, or LED.

3. Are diodes AC or DC?

It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. Diodes are also known as rectifiers because they change alternating current (ac) into pulsating direct current (dc). Diodes are rated according to their type, voltage, and current capacity.

4. Why do we use 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.

5. What is unit of diode?

A diode is not a measurable quantity. Hence,it does not have a unit. Generally,for a diode,we measure characteristics like forward voltage drop,reverse voltage drop and reverse breakdown voltage which are usually measured in Volts.

6. Do diodes have resistance?

Just like a resistor or any other load in a circuit, a diode offers resistance in a circuit. Unlike resistors, though, diodes are not linear devices. This means that the resistance of diodes does not vary directly and proportional to the amount of voltage and current applied to them.

7. Does diode reduce current?

Ideally, diodes will block any and all current flowing the reverse direction, or just act like a short-circuit if current flow is forward. Unfortunately, actual diode behavior isn't quite ideal. Diodes do consume some amount of power when conducting forward current, and they won't block out all reverse current.

8. How are diodes classified?

Diodes are classified according to their characteristics and are offered in a number of different types, including rectifiers, switching diodes, Schottky barrier diodes, Zener (constant voltage) diodes, and diodes designed for high-frequency applications.

9. What is the most common diode?

The most commonly used signal diode is the 1N4148. This diode has a close brother called 1N914 that can be used in its place if you can't find a 1N4148. This diode has a forward-voltage drop of 0.7 and a peak inverse voltage of 100 V, and can carry a maximum of 200 mA of current.

10. What is the difference between a Zener diode and a Schottky diode?

As their switching speed is very high, Schottky diodes recover very fast when the current reverses, resulting in only a very small reverse current overshoot. ... A special type of diode, called the Zener diode, blocks the current through it up to a certain voltage when reverse biased.

11. What is difference between Schottky diode and normal diode?

In the normal rectifier grade PN junction diode, the junction is formed between P type semiconductor to N type semiconductor. Whereas in Schottky diode the junction is in between N type semiconductor to Metal plate. The schottky barrier diode has electrons as majority carriers on both sides of the junction.


12. Why it is called diode?

A diode is called a diode because it has two distinct electrodes (i.e. terminals), called the anode and the cathode. A diode is electrically asymmetric because current can flow freely from the anode to the cathode, but not in the other direction. In this way, it functions as a one-way valve for current.

13. Is a diode the same as a resistor?

Key Difference: A diode is a type of electrical device that allows the current to move through it in only one direction. ... A resistor is an electric component that is used to provide resistance to current in the circuit. They are mostly used to produce heat or light.

14. How much voltage can a diode take?

Silicon diodes have a forward voltage of approximately 0.7 volts. Germanium diodes have a forward voltage of approximately 0.3 volts. The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.

15. Can a resistor replace a diode?

Diodes only conduct in one direction whereas resistors conduct in both directions. Without analyzing the actual circuit the results would be unpredictable but, generally speaking, being that diodes & resistors are designed to do different things, substituting one for the other is something you wouldn't want to do.

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