Definition of Diode

A diode, also called a rectifier, is an electronic component that conducts current primarily in one direction. It comprises a semiconductor material with two regions, creating a P-N junction. Diodes act as one-way switches for current, allowing current to flow smoothly in one direction but severely restricting the flow of current in the opposite direction. They also convert alternating current (AC) into pulsating direct current (DC) and are rated based on their type, voltage, and current capacity.

Construction of Diode

The anode, marked with an indicator on the diode's casing, serves as the positive terminal, while the cathode, usually indicated opposite to the anode, functions as the negative terminal; the diode's core is the P-N junction, where a region of p-type semiconductor material meets a region of n-type semiconductor material, creating a barrier to electric current flow in one direction (forward-biased) while permitting it in the other (reverse-biased); typically made from materials like silicon, germanium, or gallium arsenide, diodes feature metal contacts that connect to the semiconductor, forming the anode and cathode terminals; encased in a package or casing, diodes are protected and can be connected to external circuits, with the size and shape varying depending on the type and intended application.

Working of Diode

Semiconductors like silicon and germanium are poor conductors due to the equal number of electrons and holes in the material. To address this, trivalent elements like boron and pentavalent elements like phosphorus or arsenic are introduced. Boron introduces holes, while phosphorus or arsenic introduces free electrons, creating the P-region and N-region.

The intersection between the P-region and the N-region creates a thin depletion region, where positive and negative charges interact to create an internal electric field. This field blocks diffusion movement, impeding charged particles through the space charge region, and causing minority carrier drift.

Once diffusion and drift reach dynamic equilibrium, the P-N junction is formed, laying the foundation for diode operation. By adding appropriate electrode leads and packaging, a diode can be constructed.

Types of Diodes

There are many different types of diodes. The most common ones are:

● Point contact diode

● Photodiode

● Schottky Diode

● Voltage Regulator Diode

● PIN Diode

● Light-Emitting Diodes (LED)

● Zener diode

Point Contact Diode

Point-contact diodes are formed by pressing a metal needle onto a single crystal of germanium or silicon and then applying a current. Therefore, their PN junction has a small static capacitance, making them suitable for high-frequency circuits. However, compared to junction diodes, point-contact diodes have poor forward and reverse characteristics, so they cannot be used for large currents and rectification. Because of their simple construction, they are inexpensive. They are widely used for general purposes such as small-signal detection, rectification, modulation, mixing, and limiting.

Photodiode

The photodiode produces current when a certain amount of light energy falls over them, They are special diodes that can detect any light that falls on them. They work in reverse bias conditions and are used in solar cells and photometers.

Schottky Diode

The basic principle is that a reverse voltage is blocked by a Schottky that has been formed on the contact surface of a metal (e.g. lead) and a semiconductor (N-type silicon wafer). The Schottky differs fundamentally from the rectification principle of action of the PN junction. Its degree of voltage resistance is only about 40V. Its specialty is the very fast switching speed: the reverse recovery time is particularly short. Therefore, it is possible to make switching diodes and low-voltage high-current rectifier diodes.

Voltage Regulator Diode

A voltage regulator diode is a replacement for a voltage regulator electronic diode. It is made as a diffusion type or alloy type of silicon. It is a diode whose reverse breakdown characteristic curve changes sharply. It is made to be used as control voltage and standard voltage. The terminal voltage (also known as Zener voltage) of a diode when it is in operation ranges from about 3V to 150V, and it can be classified into many grades at 10% intervals. In terms of power, there are also products ranging from 200mW to more than 100W. Work in the reverse breakdown state, made of silicon material, the dynamic resistance RZ is very small, generally 2CW type; two complementary diodes in reverse series to reduce the temperature coefficient is 2DW type.

PIN Diode

The PIN diode is a crystal diode constructed by sandwiching a layer of intrinsic semiconductor (or a semiconductor with a low concentration of impurities) between the P region and the N region, and the I in PIN is an abbreviation of the meaning of "intrinsic". When its operating frequency exceeds 100MHz, due to the storage effect of the minority carriers and the "intrinsic" layer of the transition time effect, the diode loses the rectification effect and becomes an impedance element, and, its impedance value with the bias voltage and change. In the zero bias or DC reverse bias, the "intrinsic" region of the impedance is very high; in the DC forward bias, due to carrier injection "intrinsic" region, and make the "intrinsic" region presents a low impedance state. Therefore, the PIN diode can be used as a variable impedance element. It is often used in high-frequency switching (i.e., microwave switching), phase shifting, modulation, limiting and other circuits.

Light Emitting Diode (LED）

Light Emitting Diode is a semiconductor solid-state display device that converts electrical energy directly into light energy, referred to as LED (Light Emitting Diode). Similar to ordinary diodes, light-emitting diodes are made of a PN structure. Light-emitting diode PN junction encapsulated in a transparent plastic shell, the shape of the square, rectangular round and so on. Light-emitting diode drive voltage is low, small operating current, has a strong resistance to vibration and impact capacity, small size, high reliability, power consumption and long life, etc., widely used in signal instructions and other circuits.

Zener Diode

Zener diodes are semiconductor devices that stabilize voltage levels in electronic circuits. They operate in the reverse breakdown region, maintaining a constant voltage when the applied voltage exceeds a specific threshold. Widely used in voltage regulators, they provide a precise voltage reference for components like integrated circuits.

Symbol of a Diode

The standard diode symbol is shown in the above figure. In the above figure, we can see that there are two terminals called anode and cathode. The anode pointed by the arrow represents the regular direction of current flow in the forward biased state. The other terminal is the cathode.

Characteristics of Diode

The characteristics of the diode can easily be understood under the following four headings.

● Forward Characteristic

● Reverse Characteristic

● Voltammetric Characteristic

● Breakdown Characteristic

The technical specifications used to indicate the performance of a diode and its range of applications are called diode parameters. Different types of diodes have different characteristic parameters.

Forward Characteristic

When a diode is forward biased, it conducts current (IF) in the forward direction. IF depends directly on the forward voltage magnitude, known as the diode's IV characteristic. Initially, with a small forward voltage, the current is almost zero due to the electric field in the PN junction, creating a dead band. This voltage, called the deadband voltage, doesn't allow conduction. Once the forward voltage exceeds the deadband voltage, the diode conducts, and the current increases rapidly. In normal operation, the terminal voltage remains nearly constant, known as the forward voltage.

Reverse Characteristic

When reverse-biased, a diode conducts a small reverse current, typically in microamperes. This current remains nearly constant over a wide range of reverse voltage, with only a slight change as voltage increases. At the breakdown voltage (VBR), the current increases rapidly, while the voltage across the diode remains stable. This constant-voltage characteristic is useful in various applications. The conduction processes in reverse-biased diodes are known as Avalanche breakdown and Zener breakdown.

Voltammetric Characteristic

A diode is a device that allows current to flow in only one direction. When a forward voltage is applied, the current begins to increase exponentially after a threshold of around 0.6V and becomes fully conductive at about 0.7V. For germanium diodes, the turn-on voltage is 0.2V, and the conduction voltage (UD) is about 0.3V.

When a reverse voltage is applied, the current remains negligible at low voltage values and is equal to the reverse saturation current (IS). When the reverse voltage exceeds a certain value, the current increases sharply, leading to a reverse breakdown. This voltage is known as the diode's reverse breakdown voltage (UBR), which varies between different types of diodes.

Breakdown Characteristic

When the reverse voltage exceeds a critical value, the reverse current suddenly increases, causing electrical breakdown. This voltage is called the reverse breakdown voltage. At this point, the diode loses its unidirectional conductivity. If not overheated, conductivity may recover when the voltage is removed; otherwise, it's damaged. Excessive reverse voltage should be avoided.

Reverse breakdown includes Zener and avalanche breakdown. In heavily doped situations with small barrier widths and high reverse voltage, Zener breakdown occurs, generating electron-hole pairs and a sharp current increase. With lower doping concentrations and wider barrier widths, Zener breakdown is less likely.

Avalanche breakdown happens as reverse voltage increases, accelerating electron drift and causing collisions with valence electrons, generating new pairs. These pairs, accelerated by the field, collide with other electrons, causing an avalanche increase in carriers and a rapid current rise. Regardless of the type of breakdown, without limiting its current, it may cause permanent damage to the PN junction.

Reverse Current

Reverse current is the reverse current that flows through a diode at room temperature (25°C) and the highest reverse voltage. 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, approximately every 10°C rise in temperature doubles the reverse current.

Detection Methods of Diode

Before taking a look at various detection methods of diodes, let us quickly take a peek at a small list of common detection testing of diodes.

● Low Power Crystal Diodes

● High Frequency Varistor Diode

● Varactor Diode

● Monochrome Light Emitting Diode

● Infrared Light Emitting Diode

● Laser Diode

Low Power Crystal Diodes

1. Identify positive and negative electrodes

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

(2) Observe the color dots on the shell. In the point of contact diode shell, usually marked with a polarity color point (white or red). Generally marked with a color point of the end that is the positive pole. There are diodes marked with a color ring, with a color ring on the end of the negative pole.

(3) Take the smaller resistance value of a measurement shall prevail, the black pen connected to the end of the positive pole, the red pen connected to the end of the negative pole.

(4) Observe the diode shell, with a silver band at one end for the negative pole.

2. Detect the highest reverse breakdown voltage.

For alternating current, since it is constantly changing, the maximum reverse operating voltage is also the peak AC voltage to which the diode is subjected.

High Frequency Varistor Diodes

Identify the positive and negative poles of high-frequency varistor diodes and ordinary diodes in the appearance of the difference is the color of its color scale is different, ordinary diodes are generally black, while the color of the color scale of high-frequency varistor diodes is light-colored. Its polarity law is similar to that of ordinary diodes, i.e., the end with a green ring is the negative pole, and the end without a green ring is the positive pole.

Varactor Diode

The multimeter red and black pen how to measure, varactor diode resistance between the two pins should be infinity. If in the measurement, found that the multimeter pointer to the right has a slight swing or resistance value of zero, the measured varactor diode leakage fault or has been broken.

Monochrome Light Emitting Diode

In the multimeter attached to the external energy-saving 1.5V dry battery, the multimeter set R × 10 or R × 100 block. This connection is equivalent to giving the multimeter series connection on the 1.5V voltage so that the detection voltage increases to 3V (light-emitting diode turn-on voltage of 2V). Detection, with the multimeter two pens rotating contact light-emitting diode two pins. If the tube performance is good, there must be normal light-emitting, at this time, the black pen is connected to the positive pole red pen is connected to the negative pole.

Infrared Light Emitting Diode

1. Identification of infrared light-emitting diode positive and negative electrodes. An infrared light-emitting diode has two pins, usually the long pin for the positive pole, and the short pin for the negative pole. Because the infrared light-emitting diode is transparent, the electrode inside the tube shell is visible, the internal electrode is wider and larger for the negative pole, and narrower and smaller for the positive pole.

2. First measure the red light-emitting diode's positive and negative resistance, usually the forward resistance should be about 30k, and the reverse resistance should be more than 500k so that the tube can be used normally.

Laser Diode

By the detection of ordinary diodes, and forward and reverse resistance methods, the laser diode pin arrangement order can be determined. However, it should be noted that, because the forward voltage drop of the laser diode is larger than that of the ordinary diode, the multimeter pointer is slightly deflected to the right when detecting the forward resistance.

Applications of Diodes

Electronic Circuit Applications

Semiconductor diodes are an essential component of almost all electronic circuits. They are used to protect circuits, extend their lifespan, and optimize integrated circuits. Diodes play a crucial role in maintaining the normal operation of integrated circuits. In the following sections, we will provide a brief description of how diodes function in four different circuits.

(1) Varactor circuits

Varactor diodes are commonly used in varactor circuits for automatic frequency control, tuning, frequency modulation, and sweeping oscillation.

(2) Voltage Regulator Circuit

The voltage regulator circuit usually needs to use the Zener diode, which is a use of the special process for the manufacture of surface junction type silicon to the semiconductor diode, this special diode impurity concentration is relatively high, and the charge density in the space charge region is large, easy to form a strong electric field. When the reverse voltage at both ends of the Zener diode is added to a certain value, the reverse current increases sharply, resulting in reverse breakdown.

(3) Limiter circuit

In electronic circuits, limiter circuits are commonly used to process various signals. It is used to allow the signal to selectively transmit a portion of the signal within a preset level range. Most diodes can be used as limiting, but there are times when a special limiter diode is required, such as when protecting instruments.

(4) Switching circuits

In the digital, integrated circuits using the diode's unidirectional conductivity to achieve the circuit on or off, this technology has been widely used. The switching diode can protect the circuit, preventing the circuit from being burned out due to short-circuit and other problems, and can also realize the function of a traditional switch. Another characteristic of switching diodes is that they switch very quickly. This is unmatched by conventional switches.

Industrial product applications

After years of tireless efforts by scientists, the application of semiconductor diode light-emitting has been gradually increasing. Light-emitting diodes (LEDs) are now widely used in a variety of electronic product indicators, optical fiber communication light sources, various instrumentation indicators, and lighting. LEDs have several unique features that make them superior to ordinary light-emitting devices. These features include safety, high efficiency, environmental friendliness, long life, fast response, small size, and solid structure. Therefore, LEDs are an excellent source of light that meets the requirements of green lighting.

Light-emitting diodes are commonly used in many fields, the following are some of their main applications:

(1) Automobiles and large machinery applications

Light-emitting diodes are widely used in automobiles and large machinery. Automobiles and large machinery and equipment in the directional lights, interior lighting, mechanical equipment instrumentation lighting, headlights, turn signals, brake lights, tail lights, etc. are used in light-emitting diodes. Mainly because of the light-emitting diode's fast response, and long service life (the general life of light-emitting diodes than cars and large machinery life).

(2) Decorative lights in cities

In today's busy commercial era, neon lights are an important symbol of urban prosperity, but neon lights have many shortcomings, such as life is not long enough. Therefore, using light-emitting diodes to replace neon lights has many advantages, because light-emitting diodes compared with neon lights in addition to a long lifespan are energy-saving, easy to drive and control, no need for maintenance and other characteristics. Light-emitting diodes instead of neon lights will be the inevitable result of the development of lighting equipment.

(3) Applications in electronics

Light-emitting diodes in electronic products are generally used as a screen backlight or for display, and lighting applications. From large LCD TVs, computer monitors to media players MP3, MP4 and mobile phones and other displays will be used as a light-emitting diode screen backlight.

(4) Coal mine applications

As light-emitting diodes than ordinary light-emitting devices with high efficiency, low energy consumption, long life, luminosity and other characteristics, the miner's lamp underground lighting and other equipment use light-emitting diodes. Although not yet fully popularised, shortly will be commonly used, light-emitting diodes will replace ordinary light-emitting devices in coal mine applications.