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The Function and Operating Principle of Diode

Author: Apogeeweb
Date: 3 Jan 2019
how diodes work

Warm hints: This article contains about 6000 words and the reading time is about 30 mins.

Ⅰ Introduction

A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate.


A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.[5] Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

In this Tutorial We Look at What is a Diode and Diode Function


Ⅰ Introduction

Ⅱ Basic Concepts of Diode

2.1 Definition of Diode

2.2 Operating Principle of Diode

2.3 Volt-ampere Characteristics of Diode

2.4  Forward Characteristics of Diode

2.5 Reverse Characteristics of Diode

2.6 Rectifier Circuit

Ⅲ Diode Function

Ⅳ Main Parameters of Diode

4.1 Maximum Rectifier Current (IF)

4.2 Highest Reverse Working Voltage (Udrm)

4.3 Reverse Current (Idrm)

4.4 Dynamic Resistance (Rd)

4.5 Highest Working Frequency (Fm)

4.6 Voltage Temperature Coefficient (αuz)

4.7 Parameter Symbols

Ⅴ Types of Diode

5.1 Rectifier Diode

5.2 Detection Diode

5.3 Switch Diode

5.4  Zener Diode

5.5 Fast Recovery Diode

5.6  Schottky Barrier Diode

5.7 Transient Voltage Suppression Diode

5.8 Light Emitting Diode

5.9 Avalanche Diode

5.10 DIAC

5.11 Varactor Diode

Ⅵ Identification and Detection of Diode

6.1 Diode Identification

6.2 Diode Detection

Ⅶ Frequently Asked Questions about LED Working Principle

Ⅱ Basic Concepts of Diode

2.1 Definition of Diode

The diode consists of a die, a package and two electrodes. The die is a PN junction. A lead is drawn at each end of the PN junction, and a plastic diode, glass, or metal material is used as the package to form a crystal diode, as shown in the following figure. The electrode drawn in the P region is referred to as a positive electrode or an anode, and the electrode extracted in the N region is referred to as a negative electrode or a cathode.


Figure 1. Diode Structure and Diode Symbol


2.2 Operating Principle of Diode 

The crystal diode is a pn junction formed by a p-type semiconductor and an n-type semiconductor, and 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 on both sides of the pn junction is equal to the drift current caused by the self-built electric field and is in an electrical equilibrium state. When there is a forward voltage bias externally, the external electric field and the self-built electric field inhibit each other, which increases the diffusion current of carriers and forms a forward current.


When there is a reverse voltage bias externally, the external electric field and the self-built electric field are further strengthened and reverse saturation current I0 independent of the reverse bias voltage value is formed within a certain reverse voltage range. When the applied reverse voltage is high to a certain extent, the electric field strength of the space charge layer of the pn junction reaches a critical value, a carrier multiplication process occurs, a large number of electron-hole pairs are generated, and a large reverse breakdown current is generated. This is called the breakdown phenomenon of the diode. The reverse breakdown characteristics of the pn junction are Zener breakdown and avalanche breakdown.


2.3 Volt-ampere Characteristics of Diode

The volt-ampere characteristic of a diode is the relationship between the voltage applied to the diode and the current flowing through the diode. The curve used to qualitatively describe the relationship between the two is called the volt-ampere characteristic curve. The volt-ampere characteristics of the silicon diode observed with a transistor plotter are shown in the figure below.

Volt-ampere Characteristics of Diode

Figure 2. V-A Characteristics Curve


2.4  Forward Characteristics of Diode

1) When the applied forward voltage is small, the diode shows a larger resistance, and the forward current is almost zero. The OA section of the curve is called the non-conductive area or dead zone. Generally, the dead zone voltage of a silicon tube is about 0.5 volts, and the dead zone voltage of a germanium tube is about 0.2 volts. This voltage value is also called the threshold voltage.


2) When the applied forward voltage exceeds the deadband voltage, the PN junction of the electric field is almost canceled, the resistance of the diode is small, and the forward current begins to increase, and enter the forward conduction area, but it is not proportional to the voltage and current, for example, the AB section. As the applied voltage increases, the forward current increases rapidly. For example, the characteristic curve of the BC section is steep, and the volt-ampere relationship is approximately linear, in a fully conductive state.


3) After the diode is turned on, the forward voltage across both ends is called the forward voltage drop (or tube voltage drop), which is almost constant. The pressure drop of the silicon tube is about 0.7V, and the pressure drop of the germanium tube is about 0.3V.


2.5 Reverse Characteristics of Diode

1) When the diode is subjected to a reverse voltage, the internal electric field of the PN junction is strengthened, and the diode exhibits a large resistance with only a small reverse current. In practical applications, the smaller the reverse current, the greater the reverse resistance of the diode, and the better the reverse cutoff performance. Under normal circumstances, the reverse saturation current of silicon diodes is less than tens of microamperes, the reverse saturation current of germanium diodes is hundreds of microamperes, and high-power diodes are slightly larger.


2) When the reverse voltage increases to a certain value, the reverse current sharply increases and enters the reverse breakdown region. The corresponding voltage is called the reverse breakdown voltage. If the current is too large after the diode breaks down, it will damage the tube. Therefore, except for the Zener diode, the reverse voltage of the diode must not exceed the breakdown voltage.


2.6 Rectifier Circuit

2.6.1 One-way Half-wave Rectifier Circuit

The diode is like an automatic switch. When U2 is a positive half cycle, the power supply is automatically connected to the load. When U2 is a negative half cycle, the power supply and load are automatically cut off. Therefore, as shown in the figure below, the pulsating DC voltage U0 with the same direction and magnitude change on the load is shown in the figure below. Since this circuit only outputs in the positive half cycle of U2, it is called a half-wave rectification circuit. If the polarity of the rectifier diode is reversed, a negative DC ripple voltage can be obtained.

One-way Half-wave Rectifier Circuit

Figure 3. One-way Half-wave Rectifier Circuit

2.6.2 Full Wave Rectifier Circuit 

Full Wave Rectifier Circuit

Figure 4. Full Wave Rectifier Circuit

  • Rectification Principle

Set the voltage on the secondary side of the transformer to:


1) When U2 is a positive half cycle, the potential at point A is the highest, the potential at point V is the lowest, diodes V1 and V3 are turned on, V2 and V4 are turned off, and the current path is A→V1→RL→V3→B.


2) When U2 is a negative half cycle, the potential at point B is the highest, the potential at point A is the lowest, diodes V2 and V4 are on, V1 and V3 are off, and the current path is B→V2→RL→V4→A.


It can be seen that in a period of change of u2, the current from top to bottom always flows through the load RL, and the waveform of the voltage and current is a full-wave pulsating DC voltage and current, as shown in the following figure.

Rectification principle

Figure 6. Full-wave Pulsating DC Voltage and Current 

Ⅲ Diode Function

Diodes are one of the most commonly used electronic components. Its biggest characteristic is unidirectional conduction, that is, current can only flow through one direction of the diode. The function of the diode includes a rectifier circuit, a detection circuit, a voltage regulator circuit, and various modulation circuits. It is mainly composed of diodes. The principle is simple. Because of the invention of diodes and other components, we have the birth of our colorful world of electronic information. Since the role of diodes is so great, how should we detect them? This component is actually very simple. Just tap the resistance file with a multimeter and measure the reverse resistance. If it is small, it means that the diode is broken. If the reverse resistance is large, the diode is good. For such basic components, we should firmly grasp our own operating principles and basic circuits to lay a good foundation for future electronic technology learning.


Ⅳ Main Parameters of Diode

A specification used to express the performance and range of a diode is called a diode parameter. Different types of diodes have different characteristic parameters. For beginners, you must understand the following main parameters:

4.1 Maximum Rectifier Current (IF)

It refers to the maximum forward average current value allowed by the diode in long-term continuous operation. This value is related to the PN junction area and external heat dissipation conditions. When current passes through the tube, the die heats up and the temperature rises. When the temperature exceeds the allowable limit (about 141 for silicon tubes and about 90 for tubes), the mold overheats and is damaged. Therefore, under the specified heat dissipation conditions, the diode should not exceed the maximum rectified current value of the diode. For example, the commonly used IN4001-4007 type germanium diode has a rated forward operating current of 1A.


4.2 Highest Reverse Working Voltage (Udrm)

When the reverse voltage applied across the diode is high enough, the tube will break down and lose its unidirectional conductivity. In order to ensure safe use, the highest reverse operating voltage value is specified. For example, the IN4001 diode has a reverse withstand voltage of 50V and the IN4007 has a reverse withstand voltage of 1000V.


4.3 Reverse Current (Idrm)

Reverse current refers to the reverse current and the highest reverse voltage flowing through the diode at normal temperature (25°C). The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current has a close relationship with temperature and the reverse current doubles for every 10°C increase in temperature. For example, a 2AP1 germanium diode has a reverse current of 250uA at 25°C, and a reverse current of 500uA when the temperature rises to 35°C, and so on. At 75°C, its reverse current reaches 8mA. Not only will it lose its unidirectional conductivity, but it will also overheat and damage the tube. Another example is the 2CP10 silicon diode, which has a reverse current of only 5uA at 25°C, and a reverse current of 160uA when the temperature rises to 75°C. Therefore, silicon diodes have better high-temperature stability than germanium diodes.


4.4 Dynamic Resistance (Rd)

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


4.5 Highest Working Frequency (Fm)

Fm is the upper limit frequency of diode operation. Since the diode is the same as the PN junction, its junction capacitance is composed of blocking capacitance. Therefore, the size of the frequency modulation mainly depends on the size of the PN junction capacitance. If it exceeds this value, it will affect unidirectional conductivity.


4.6 Voltage Temperature Coefficient (αuz)

Αuz refers to the relative change in the steady voltage for every one degree Celsius increase in temperature. The temperature stability of the Zener diode with a uz of about 6v is better.


4.7 Parameter Symbols 

CT---barrier capacitance

Cj---junction (interelectrode) capacitance; indicates the total capacitance of the germanium detection diode under the specified bias voltage across the diode

Cjv---bias junction capacitance

Co---zero bias capacitor

Cjo---zero-bias junction capacitance

Cjo/Cjn---junction capacitance change

Cs---shell capacitor or package capacitor

Ct---total capacitance

CTV---voltage temperature coefficient. The ratio of relative change in steady voltage to absolute change in ambient temperature at test current

CTC---capacitor temperature coefficient

Cvn---nominal capacitance

IF---forward DC (forward test current). Germanium detector diode passes the current between the poles under the specified forward voltage VF; the maximum operating current (average value) that the silicon rectifier and the silicon stack are allowed to pass continuously in the sine half-wave under the specified use conditions, the silicon switch The maximum forward DC that the diode is allowed to pass at rated power; the current given when measuring the forward voltage of the Zener diode

IF(AV)---forward average current

IFM(IM)---forward peak current (forward maximum current). The maximum forward pulse current allowed through the diode at rated power. LED limit current.

IH---constant current, holding current.

Ii---; LED flashing current

IFRM---forward repeat peak current

IFSM---positive peak current (surge current)

Io---rectifying current. Operating current through specified frequency and specified voltage conditions in a particular line

IF(ov)---forward overload current

IL---Photocurrent or steady current diode limiting current

ID---dark current

IB2---single-junction transistor

IEM---emitter peak current

IEB10---Reverse current between the emitter and the first base in a double-base single-junction transistor

IEB20---Emitter current in double base single-junction transistor

ICM---Maximum output average current

IFMP---positive pulse current

IP---peak current

IV---valley current

IGT---thyristor gate trigger current

IGD---thyristor control pole does not trigger current

IGFM---control positive peak current

IR(AV)---reverse average current

IR (In)---reverse DC (reverse leakage current). When measuring the reverse characteristic, the given reverse current; the silicon stack is in the sinusoidal half-wave resistive load circuit, the current passed when the reverse voltage is specified; the reverse polarity of the silicon switching diode plus the reverse operating voltage VR The current passed through; the leakage current generated by the Zener diode under reverse voltage; the leakage current of the rectifier at the highest reverse operating voltage of the sine half-wave.

IRM---reverse peak current

IRR---Thyristor Reverse Repeated Average Current

IDR---thyristor off-state average repeat current

IRRM---reverse repeat peak current

IRSM---reverse peak current (reverse surge current)

Irp---reverse recovery current

Iz---stabilize voltage and current (reverse test current). Given reverse current when testing reverse electrical parameters

Izk---Stabilized tube knee current

IOM---maximum forward (rectifier) current. The maximum forward instantaneous current that can withstand under specified conditions; the maximum operating current that allows continuous conduction through the detection diode in a sinusoidal half-wave rectification circuit with resistive load

IZSM---Zener diode surge current

IZM---Maximum regulated current. Current that the Zener diode allows passing at maximum dissipated power

iF---forward total instantaneous current

iR---reverse total instantaneous current

Ir---reverse recovery current

Iop---working current

Is--- steady current diode steady current


N---capacitance change index; capacitance ratio

Q---good value (quality factor)

Δvz---voltage regulator voltage drift

Di/dt---on-state current critical rise rate

Dv/dt---on-state voltage critical rise rate

PB---withstand pulse burnout power

PFT (AV)--- forward conduction average power dissipation

PFTM---positive peak power dissipation

PFT---positive conduction total instantaneous power dissipation

Pd---dissipated power

PG---gate average power

PGM---gate peak power

PC---control pole average power or collector dissipation power

Pi---input power

PK---maximum switching power

PM---rated power. The maximum power that a silicon diode can withstand no more than 150 degrees

PMP---maximum leakage pulse power

PMS---maximum pulse power

Po---output power

PR---reverse surge power

Ptot---total dissipated power

Pomax---maximum output power

Psc---continuous output power

PSM---Do not repeat surge power

PZM---Maximum dissipated power. The maximum power that the Zener diode is allowed to withstand for a given service condition

RF(r)---forward differential resistance. In the forward conduction, the current exhibits significant nonlinear characteristics as the voltage index increases. Under a certain forward voltage, the voltage increases by a small amount ΔV, and the forward current increases by ΔI, then ΔV/△I is called differential resistance.

RBB---base resistance between double base transistors

RE---RF resistance

RL---load resistor

Rs(rs)----Series resistance

Rth----thermal resistance

R(th)ja----thermal resistance from junction to environment

Rz(ru)---dynamic resistance

R(th)jc---junction-to-shell thermal resistance

r; δ---attenuation resistance

r(th)---transient resistance

Ta---ambient temperature

Tc---shell temperature

Td---delay time

Tf---fall time

Tfr---forward recovery time

Tg---circuit commutation shutdown time

Tgt---gate gate opening time

Tj---junction temperature

Tjm---highest junction temperature

Ton---opening time

Toff---off time

Tr---rise time

Trr---reverse recovery time

Ts---storage time

Tstg---temperature storage diode storage temperature

a---temperature coefficient

Λp---luminescence peak wavelength

---spectral half-width

η---single-junction transistor divider ratio or efficiency

VB---reverse peak breakdown voltage

Vc---rectified input voltage

VB2B1---base voltage

VBE10---emitter and first base reverse voltage

VEB---saturated pressure drop

VFM---maximum forward voltage drop (forward peak voltage)

VF---forward voltage drop (forward DC voltage)

△VF---positive pressure drop difference

VDRM---off state repeated peak voltage

VGT---gate trigger voltage

VGD---gate does not trigger voltage

VGFM---gate positive peak voltage

VGRM---gate reverse peak voltage

VF (AV)--- positive average voltage

Vo---AC input voltage

VOM---Maximum output average voltage

Vop---working voltage

Vn---center voltage

Vp---peak point voltage

VR---reverse working voltage (reverse DC voltage)

VRM---reverse peak voltage (highest test voltage)

V(BR)---breakdown voltage

Vth---valve voltage (threshold voltage, dead band voltage)

VRRM---reverse repeat peak voltage (reverse surge voltage)

VRWM---reverse working peak voltage

Vv---valley voltage

Vz---stable voltage

△Vz---voltage range voltage increment

Vs---to voltage (signal voltage) or steady current tube to stabilize current and voltage

Av---voltage temperature coefficient

Vk---Knee point voltage (steady current diode)

VL--- limit voltage

If you have more interest in diodes, you can check the Diode Physical Maps and Symbols.

diode symbol

Diode Symbol

Ⅴ Types of Diode

There are many types of diodes: according to materials, there are germanium diodes, silicon diodes, gallium arsenide diodes, etc.; according to the manufacturing process can be divided into surface contact diodes and point contact diodes; according to different uses can be divided into rectifier diodes, detection diodes, Zener diodes, varactors, photodiodes, light-emitting diodes, switching diodes, fast recovery diodes, etc.; according to the type of connection can be divided into semiconductor junction diodes, metal-semiconductor contact diodes, etc.; Conventional packaged diodes, specially packaged diodes, etc. The following uses the application as an example to introduce the characteristics of different types of diodes.

5.1 Rectifier Diode

The function of the rectifier diode is to use the unidirectional conduction characteristic of the diode to rectify the AC power source into a pulsating DC. Due to the large forward working current of the rectifier diode, a surface contact structure is often used in this process. The diode junction capacitance of this structure is relatively large, so the operating frequency of the rectifier diode is generally less than 3khz.


Rectifier diodes are mainly available in hermetic metal package and plastic package. Normally, the rectifier diode with rated forward T current LF above l A is packaged in a metal case to facilitate heat dissipation; the rated forward operating current is below 1A in an all-plastic package. In addition, due to the continuous improvement of T-technical technology, many high-power rectifier diodes are packaged in plastic, which should be distinguished in use.


Since the rectifier circuit is usually a bridge rectifier circuit, some manufacturers package four rectifier diodes together. Such redundant components are usually called rectifier bridges or rectifier full-bridge (referred to as a full-bridge).


When using a rectifier diode, the parameters such as large rectification current, large reverse current, cutoff frequency and reverse recovery time should be considered.


The rectifier diode used in the common series stabilized power supply circuit does not require a reverse recovery time for the cutoff frequency. As long as the rectifier current is selected according to the requirements of the circuit, the rectifier diode (for example, the N series, 2CZ series, RLR series, etc.).


The rectifier circuit used in the switching regulator power supply and the rectifier diode used in the pulse rectifier circuit should use a rectifier diode or a fast recovery diode with a higher operating frequency and a shorter reverse recovery time.

Rectifier Diode


5.2 Detection Diode

The detector diode is a device that detects a low-frequency signal superimposed on a high-frequency carrier and has high detection efficiency and good frequency characteristics.


The detection diode requires a small forward voltage drop, high detection efficiency, small junction capacitance, and good frequency characteristics, and its shape is generally EA glass package structure. The general detection diode adopts a point contact type structure of tantalum material.


When selecting a detector diode, the detector diode with high operating frequency, small reverse current, and sufficient forward current should be selected according to the specific requirements of the circuit.

Detection Diode


5.3 Switch Diode

Since the semiconductor diode has a positive bias, the on-resistance is small. When the reverse bias is applied, the cutoff is applied. The off-resistance is large, and the unidirectional conduction characteristic of the semiconductor diode in the storage switching circuit can turn on and off the current, so the semiconductor diode used for this purpose is called a switching diode.


Switch diodes are mainly used in household appliances and electronic equipment such as tape recorders, televisions, and DVD players, such as switching circuits, detection circuits, and high-frequency pulse rectifier circuits.


The 2AK series of common switch diodes can be selected for the medium-speed switching and detection circuits. High-speed switching circuits can be selected from RLS series, 1sS series, 1N series, 2CK series high-speed switching diodes. The specific model of the switching diode should be selected according to the main parameters of the application circuit (such as forward current, high reverse voltage, reverse recovery time, etc.).

Switch Diode


5.4  Zener Diode

The Zener diode is characterized by the fact that the voltage of the PN junction reversely changes without changing with the current to achieve the purpose of voltage regulation. Because it can regulate the voltage in the circuit, it is called a Zener diode (referred to as a voltage regulator). The Zener diode is classified according to the breakdown voltage, and its voltage regulation value is the breakdown voltage value. The Zener diode is mainly used as a voltage regulator or voltage reference component. Zener diodes can be connected in series to obtain a higher voltage regulation value.


The selected Zener diode should meet the requirements of the main parameters in the application circuit. The stable voltage value of the Zener diode should be the same as the reference voltage value of the application circuit. The large current of the Zener diode should be about 50% higher than the large load current of the application circuit.

Zener Diode


5.5 Fast Recovery Diode

The Fast Recovery Diode is a new type of semiconductor diode. This diode has good switching characteristics and short reverse recovery time and is commonly used as a rectifier diode in high-frequency switching power supplies.


The fast recovery diode has the characteristics of short recovery time and is suitable for high frequency (such as TV line frequency) rectification. The fast recovery diode has an important parameter that determines its performance-reverse recovery time. The reverse recovery time is defined as the diode falling from the output pulse to the zero lines, and transitioning from the forward conducting state to the off state. The time required for the reverse power supply to recover to 10% of the large reverse current is represented by a symbol.


Ultra-fast recovery diodes (SRDs) are developed based on fast recovery diodes, the main difference is that the reverse recovery time is smaller. The reverse recovery time of a normal fast recovery diode is several hundred nanoseconds, and the reverse recovery time of an ultra-fast recovery diode (SRD) is typically several tens of nanoseconds. The smaller the value, the higher the operating frequency of the fast recovery diode.


When the operating frequency is in the range of tens to hundreds of k Hz, the change of the forward and reverse voltage of the ordinary rectifier diode is slower than the recovery time, and the ordinary rectifier diode cannot normally conduct unidirectional conduction and rectification. At this time, it is necessary to use a fast recovery rectifier diode to be competent. Therefore, the rectifier diodes powered by a switching power supply (such as a color TV) are usually fast recovery diodes and cannot be replaced by ordinary rectifier diodes. Otherwise, the device may not work properly.

 Fast Recovery Diode


5.6  Schottky Barrier Diode 

The Schottky diode is an abbreviation for Schottky Barrier Diode (SBD). It is a low-power, high-current, ultra-high-speed semiconductor device produced in recent years. The reverse recovery time is extremely short (can be as small as a few nanoseconds), the forward voltage drop is only about 0.4 V, and the rectified current can reach several thousand amperes. These excellent characteristics are unmatched by fast recovery diodes.


A Schottky diode is a metal-semiconductor device in which a noble metal (gold, silver, aluminum, platinum, or the like) is used as a positive electrode, and an N-type semiconductor is used as a negative electrode, and a barrier formed on the contact surface thereof has a rectifying property.


Schottky diodes are commonly used in high frequency, high current, low voltage rectifier circuits.

Schottky Barrier Diode


5.7 Transient Voltage Suppression Diode

The transient voltage suppression diode is abbreviated as TVP tube (transient-voltage-suppressor). It is a semiconductor device developed on the basis of the process of the Zener diode and is mainly used in a fast overvoltage protection circuit for voltage. It can be widely used in computers, electronic instruments, communication equipment, household appliances, and onboard or marine and automotive electronic equipment for field operations, and can be used as a protection element for over-voltage shock caused by human operation or an electric shock to equipment.


Transient voltage suppression diodes can be classified into four categories according to their peak pulse power: 50 () w, 1000 W, 1500 W, 5000 w. Each class is divided into several types according to its nominal voltage.


When the voltage at both ends is higher than the rated value, the transient voltage suppression diode will turn on instantaneously, and the resistance at both ends will change from high resistance to low resistance at a very high speed, thereby absorbing a very large current and the voltage across the tube. Clamp at a predetermined value.

Transient Voltage Suppression Diode


5.8 Light Emitting Diode

The English abbreviation for the light-emitting diode is LED, which is a device made of semiconductor materials such as gallium phosphide or phosphorus gallium arsenide, which can directly convert electrical energy into light energy. In addition to the unidirectional conduction characteristics of conventional diodes, LEDs can convert electrical energy into light energy. When a forward voltage is applied to the LED, it is also in a conductive state. When the forward current flows through the mold, the LED emits light, which converts electrical energy into light energy.


The color of the light-emitting diode is mainly determined by the material of the tube and the type of impurities. At present, the common LED light-emitting colors are mainly blue, green, yellow, red, orange, white, and so on. Among them, white LED is a new type of product, mainly used in mobile phone backlights, LCD backlights, lighting and other fields.


The operating current of the LED is usually 2 to 25 mA. The operating voltage (ie, forward voltage drop) varies from material to material: normal green, yellow, red, and orange light-emitting diodes operate at approximately 2V; white LEDs typically operate at voltages greater than 2.4V; blue LEDs usually work at a voltage higher than 3.3V. The working current of the LED should not exceed the rated value too high, otherwise, there is a danger of burning. Therefore, a resistor R is usually connected in series in the LED circuit as a current-limiting resistor.

Light Emitting Diode


5.9 Avalanche Diode

The avalanche diode is a microwave power device developed based on the Zener process technology, which can generate high-frequency oscillation under the action of an applied voltage.


The avalanche diode uses avalanche breakdown to inject carriers into the crystal. Since it takes a certain time for the carrier to pass through the semiconductor wafer, its current lags behind the voltage, resulting in a delay time. If the passage time is properly controlled, the current is a negative resistance effect that occurs in the voltage relationship, leading to high-frequency oscillations. It is commonly used in microwave communications, radar, tactical missiles, remote control, telemetry, instrumentation and other equipment.

Avalanche Diode


5.10 DIAC

The bidirectional trigger diode is also called a two-terminal AC device (DIAC). It is a silicon bidirectional voltage-triggered switching device. When the voltage applied across the bidirectional trigger diode exceeds its breakdown voltage, both ends are turned on, and conduction will continue until the current is interrupted or dropped to a smallholding current of the device. Turn it off again. Bidirectional trigger diodes are commonly used in overvoltage protection circuits, phase shifting circuits, thyristor trigger circuits, and timing circuits.



5.11 Varactor Diode

The varactor diode (English name variable-Capacitance Diode, abbreviated as VCD) is a special semiconductor device that uses reverse bias to change the PN junction capacitance. A varactor diode is equivalent to a variable-capacity capacitor whose PN junction capacitance varies between the two electrodes and changes with the magnitude of the reverse voltage applied to the varactor diode. When the reverse voltage applied to the varactor increases, the capacity of the varactor decreases. Because varactors have this characteristic, they are mainly used in electrical tuning circuits (such as high-frequency heads of color TVs), as an automatic trimming capacitor that can be controlled by voltage.


When selecting a varactor, it should be considered whether its operating frequency, high reverse operating voltage, large forward current and zero-bias junction capacitance meet the requirements of the application circuit. The junction capacitance should be changed greatly, high Q value, reverse A varactor diode with a small leakage current.

Varactor Diode


Ⅵ Identification and Detection of Diode

6.1 Diode Identification

The crystal diode is commonly used in the circuit as a VD plus a digital representation, such as VD5 represents a diode numbered 5.


The identification of the diode is simple: the negative pole of the low-power diode is usually marked with a color ring on the surface; some diodes also use the "P" and "N" symbols to determine the polarity of the diode, "P" for the positive pole and "N" for the negative pole. Metal-encapsulated diodes usually have a diode symbol printed on the surface with the same polarity; LEDs usually use the length of the pins to identify the positive and negative poles, the long legs are positive, and the short legs are negative.


The surface of the rectifier bridge is usually marked with the internal circuit structure or the name of the AC input and the DC output. The AC input is usually indicated by “AC” or “~”; the DC output is usually indicated by the “+” and “~” symbols.


Due to the variety of shapes of the chip diodes, the polarity is also marked by a variety of methods: in the leaded chip diode, the end of the tube with a white color ring is the negative electrode; in the chip diode with lead and colorless ring, the longer end of the lead wire is the positive electrode; in the leadless chip diode, the end of the ribbon or the notched end is the negative electrode.


6.2 Diode Detection

When using an analog multimeter to test the diode, one end of the black pen with a smaller value is the positive pole, and the end connected to the red test pen is the negative pole. If the forward resistance and reverse resistance are infinite, the diode is open; if the forward resistance and reverse resistance are both 0, the diode is short-circuited. Under normal conditions, the forward resistance of a germanium diode is about 1.6kΩ.


When measuring a diode with a digital multimeter, connect the red pen to the anode of the diode and the black test lead to the cathode of the diode. The measured resistance is the forward conduction resistance of the diode, just like the pointer of a pointer multimeter.


It is more convenient to use the diode block detection diode of the digital multimeter: put the digital multimeter in the diode block, connect the negative pole of the diode to the black multimeter of the digital multimeter, and connect the positive pole of the digital multimeter to the red test lead. The forward voltage drop value of the diode. Diodes of different materials have different forward voltage drop values: 0.55-0.7V for silicon diodes and 0.15-0.3V for germanium diodes. If the display shows “0000”, the tube is short-circuited; if “0L” or “overload” is displayed, it means that the diode is open or in reverse state. At this time, the meter can be re-tested.


Ⅶ Frequently Asked Questions about LED Working Principle

1. What is a diode?

A diode is reverse biased when it acts as an insulator and is forward biased when it allows current to flow. A diode has two terminals, the anode and the cathode.


2. 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.


3. What is diode in basic electronics?

A diode is a semiconductor device that essentially acts as a one-way switch for current. It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction.


4. What are the three main types of diodes?

Types of diode
Backward diode: This type of diode is sometimes also called the back diode.
BARITT diode: This form of diode gains its name from the words Barrier Injection Transit Time diode.
Gunn Diode: Although not a diode in the form of a PN junction, this type of diode is a semiconductor device that has two terminals.


5. What is diode equation?

I0 is the dark saturation current, q is the charge on the electron, V is the voltage applied across the diode, η is the (exponential) ideality factor.


6. What is the symbol of diode?

The basic schematic symbol for a diode looks like an arrow head that points in the direction of conventional current flow from its Anode (A) terminal to its Cathode (K) terminal. The schematic symbol of a diode also shows that if forward-biased, current will flow through the direction of the arrow.


7. What is diode characteristics?

Basic static characteristics of diodes are the forward voltage VF and forward current IF, and the reverse voltage and current VR and IR. The area surrounded by the orange dashed line in the diagram on the right indicates the usable area of rectifying diodes.


8. What is ideal diode model?

An ideal diode is one kind of an electrical component that performs like an ideal conductor when voltage is applied in forward bias and like an ideal insulator when the voltage is applied in reverse bias. So when +ve voltage is applied across the anode toward the cathode, the diode performs forward current immediately.


9. What are diode parameters?

Peak Inverse Voltage, PIV: This diode characteristics is the maximum voltage a diode can withstand in the reverse direction. This voltage must not be exceeded otherwise the device may fail. ... The diode can withstand a reverse voltage up to a certain point, and then it will breakdown.


10. How do you read a diode value?

Diode Mode Testing Procedure
Connect the red probe to the anode and black probe to the cathode. This means diode is forward-biased. Observe the reading on meter display. If the displayed voltage value is in between 0.6 to 0.7 (since it is silicon diode) then the diode is healthy and perfect.

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