Types and Applications of Diodes

Ⅰ. Types of diodes

1. According to usage

(1) LED

It is made of gallium phosphide and gallium arsenide phosphide materials, small in size, and can be driven forward to emit light. The working voltage is low, the working current is small, the light is uniform, the life is long. It can emit red, yellow and green monochromatic light.

(2) Varactor Diode

Low-power diodes used for automatic frequency control (AFC) and tuning are called varactor diodes. There are many other names for Japanese manufacturers. Varactor diodes change the capacitance of their PN junctions by applying a reverse voltage. Therefore, it is used for automatic frequency control, scanning oscillation, frequency modulation and tuning, etc. Usually, silicon diffusion type diodes are used, but specially manufactured diodes such as alloy diffusion type, epitaxial bonding type, and double diffusion type can also be used. This is because these diodes have a particularly large rate of change in capacitance with respect to voltage. The junction capacitance changes with the reverse voltage VR, and the varactor diode replaces the variable capacitance, which is used as a tuning circuit, an oscillating circuit, and a phase-locked loop. It is often used in channel switching and tuning circuits of TV tuners, and is mostly made of silicon materials.

(3) Zener Diode

It is a product that replaces the Zener electronic diode. It is made into a diffusion type or alloy type of silicon, and it is a diode with a sharp change in the reverse breakdown characteristic curve. It is made for use as control voltage and standard voltage. The terminal voltage (also known as Zener voltage) when the diode is working is from about 3V to 150V, and can be divided into many levels at intervals of 10%. In terms of power, Zener diodes also have products from 200mW to more than 100W.

(4) Switching Diode

There are logic operations used at small currents (about 10mA) and switching diodes for core excitation used at hundreds of milliamperes. Switching diodes for small currents are usually contact-type and key-type diodes. There are also silicon diffused, mesa, and planar diodes that may work at high temperatures.

The specialty of switching diodes is their fast switching speed. The switching time of the Schottky diode is extremely short, so it is an ideal switching diode. 2AK type point contact is used for medium-speed switching circuits; 2CK type plane contact is used for high-speed switching circuits; it is used for switching, limiting, clamping or detection circuits. Schottky (SBD) silicon high current switch, small forward voltage drop, fast speed and high efficiency.

(5) Damping Diode

It has a high reverse operating voltage and peak current, and a small forward voltage drop. High-frequency high-voltage rectifier diodes are used in TV line scanning circuits for damping and boost rectification.

(6) PIN Diode

This is a crystal diode with a layer of intrinsic semiconductor (or semiconductor with low concentration of impurities) sandwiched between the P region and the N region. The I in PIN is an English abbreviation of "intrinsic" meaning. 

When its operating frequency exceeds 100MHz, due to the storage effect of minority carriers and the transit time effect in the "intrinsic" layer, its diode loses its rectification effect and becomes an impedance element. Also, its impedance value varies with bias voltage. At zero bias or DC reverse bias, the impedance of the "intrinsic" region is very high; at DC forward bias, due to carrier injection into the "intrinsic" region, the "intrinsic" region exhibits low impedance state. Therefore, we can use the PIN diode as a variable impedance element. 

It is often used in high-frequency switching (ie microwave switching), phase shifting, modulation, limiting and other circuits.

(7) Rectifier Diode

In principle, getting an output DC from an input AC is rectification. Taking the size of the rectified current (100mA) as the boundary, usually the output current greater than 100mA is called rectification. Surface junction type, working frequency less than KHz, maximum reverse voltage from 25 volts to 3000 volts, A to X, 22 levels in total.

The classification is as follows: 

① silicon semiconductor rectifier diode 2CZ type 

② silicon bridge rectifier QL type 

③ 2CLG type used in high-voltage silicon stacks for TV sets with a working frequency of nearly 100KHz

(8) Detection Diode

As far as the principle is concerned, taking out the modulation signal from the input signal is called detection, and the size of the rectified current (100mA) is used as the boundary, usually the output current is less than 100mA is called detection. Germanium material point contact type, working frequency up to 400MHz, small forward voltage drop, small junction capacitance, high detection efficiency, 2AP type with good frequency characteristics. Diodes for detection like the point-contact type can be used in circuits such as limiting, clipping, modulation, mixing, and switching in addition to detection. There are also two diode assemblies with good consistency for FM detection.

(9) Avalanche Diode 

It is a transistor that can generate high-frequency oscillations under the action of an applied voltage. Its working principle of generating high-frequency oscillation is luan: it uses avalanche breakdown to inject carriers into the crystal, because it takes a certain time for the carriers to cross the chip, so the current lags behind the voltage. If the transit time is properly controlled, a negative resistance effect will appear in the relationship between current and voltage, resulting in high frequency oscillations. It is often used in oscillation circuits in the microwave field.

(10) Amplifying Diode

Amplifying diodes rely on the amplification of negative resistive devices such as tunnel diodes and body effect diodes, and the parametric amplification of varactor diodes. Therefore, amplifying diodes generally refer to tunnel diodes, body effect diodes, and varactor diodes.

(11) Limiting Diode

Most diodes can be used as limiters, and there are also special limiter diodes such as protective instrumentation and high frequency zener tubes. In order for these diodes to have a particularly strong effect of limiting sharp amplitudes, we usually use diodes made of silicon. There are usually such components for sale: according to the need to limit the voltage, several necessary rectifier diodes are connected in series to form a whole.

(12) Double Base Diode

The three-terminal negative resistance device with two bases and one emitter is used in the relaxation oscillation circuit and the timing voltage readout circuit. It has easy frequency adjustment and good temperature stability.

(13) Mixer Diode

When using the diode mixing method, we often use Schottky type and point contact type diodes in the frequency range of 500 to 10,000 Hz. 

(14) Modulation Diode

Usually refers to a diode dedicated to ring modulation. It is an assembly of four diodes with good forward characteristic consistency. Even though other varactors have modulation uses, they are usually used directly for frequency modulation.

(15) Step Recovery Diode

It is also a type of diode with a PN junction. Its structural feature is that it has a steep impurity distribution area at the boundary of the PN junction, thus forming a "self-help electric field". Since the PN junction conducts with minority carriers under forward bias, and has a charge storage effect near the PN junction, its reverse current needs to go through a "storage time" before it can be reduced to a minimum value (reverse saturation current value). The "self-help electric field" of the step recovery diode shortens the storage time, makes the reverse current cut off quickly, and generates abundant harmonic components. Comb spectrum generation circuits can be designed using these harmonic components. Fast turn-off (step recovery) diodes are used in pulse and harmonic circuits.

(16) Tunnel Diode

It is a crystal diode with tunnel effect current as the main current component. Its base materials are gallium arsenide and germanium. The N-type region of its P-type region is highly doped (that is, high-concentration impurity). Tunneling currents arise from quantum mechanical effects in these degenerate semiconductors. 

The following three conditions are met for the occurrence of tunnel effect: 

① Fermi level is located in the conduction band and full band; 

② The width of the space charge layer must be very narrow (less than 0.01 microns); the holes in the P-type and N-type regions of degenerate semiconductors. There is a possibility of overlap with electrons at the same energy level. Tunnel Diode are two-terminal active devices. Its main parameters are the peak-to-valley current ratio (IP/PV), where the subscript "P" stands for "peak" and the subscript "V" stands for "valley". Tunnel Diode can be used in low-noise high-frequency amplifiers and high-frequency oscillators (the operating frequency can reach the millimeter wave band), and can also be used in high-speed switching circuits.

(17) Schottky Barrier Diode

It is a "metal-semiconductor junction" diode with Schottky characteristics. Its forward starting voltage is low. In addition to materials, the metal layer can also use gold, molybdenum, nickel, titanium and other materials. Its semiconductor material is silicon or gallium arsenide, mostly N-type semiconductors. This device is conducted by majority carriers, so its reverse saturation current is much larger than that of PN junctions conducted by minority carriers. Since the storage effect of minority carriers in the Schottky diode is very small, its frequency response is only limited by the RC time constant, so it is an ideal device for high frequency and fast switching. Its operating frequency can reach 100GHz. Also, MIS (Metal-Insulator-Semiconductor) Schottky diodes can be used to make solar cells or light-emitting diodes.

(18) Frequency Multiplier Diode

As far as the frequency multiplication effect of the diode is concerned, there are frequency multiplication relying on varactor diodes and frequency multiplication relying on step (that is, abrupt change) diodes. The varactor diode used for frequency multiplication is called a varactor. Varactors work on the same principle as varactor diodes for automatic frequency control, but are constructed to withstand high power. The step diode is also called a step recovery diode, and the reverse recovery time trr is short when switching from on to off. Therefore, its feature is that the transition time for rapidly turning OFF is remarkably short. If a sine wave is applied to the step diode, the output waveform is suddenly pinched off due to the short tt (transition time), so many high-frequency harmonics can be generated.

(19) Transient Voltage Suppression Diode

TVP tube, for rapid overvoltage protection of the circuit, is divided into bipolar type and unipolar type, classified according to peak power (500W-5000W) and voltage (8.2V-200V).

2. According to characteristics

Point contact diodes are classified as follows according to their forward and reverse characteristics.

1. General Point Contact Diodes

This kind of diode, as the title says, is usually used in detection and rectification circuits, and is an intermediate product of forward and reverse characteristics. SD34, SD46, 1N34A, etc. belong to this category.

2. High Reverse Resistance Point Contact Diode

The forward voltage characteristics are the same as those of general-purpose diodes. Although the withstand voltage in the reverse direction is also particularly high, the reverse current is small, so its specialty is high reverse resistance. It is used in circuits with high input resistance and circuits with high load resistance. As far as germanium material high reverse resistance diodes are concerned, SD54, 1N54A, etc. belong to this type of diodes.

3. High Conductivity Point Contact Diode

It is the opposite of the high reverse resistance type. Although its reverse characteristics are very poor, it makes the forward resistance small enough. For high-conduction point-contact diodes, there are SD56, 1N56A, etc. For high conduction bond type diodes, it can get better characteristics. These diodes have a high rectification efficiency at particularly low load resistances.

4. High Reverse Withstand Voltage Point Contact Diode

This is a product with high maximum peak reverse voltage and maximum DC reverse voltage. It is used for detection and rectification of high voltage circuits. This type of diode generally has poor forward characteristics. Among the point-contact germanium diodes, there are SD38, 1N38A, OA81 and so on. This germanium material diode has a limited withstand voltage.

Ⅱ. The functions of diodes

1. Freewheeling: it plays the role of freewheeling in the inductance of switching power supply and in inductive loads such as relays.

2. Detection: It plays the role of detection in the radio.

3. Display: used on VCD, DVD, calculator and other displays.

4. Limiting: After the diode is forward-conducting, its forward voltage drop remains basically unchanged (0.7V for the silicon tube and 0.3V for the germanium tube). Through this characteristic, the diode is used as a limiting element in the circuit, which can limit the signal amplitude within a certain range.

5. Trigger: trigger diode, also known as bidirectional trigger diode (DIAC), is a three-layer structure and has a symmetrical two-terminal semiconductor device. It is often used to trigger bidirectional thyristor, and it is used for overvoltage protection in the circuit.

6. Variable capacity: used in the tuner of the TV.

7. Switch: The diode has a small resistance under the action of forward voltage and is in a conducting state, which is equivalent to a switched on switch. Under the action of reverse voltage, the resistance is very large, and it is in a cut-off state, just like a disconnected switch. Through the switching characteristics of diodes, various logic circuits can be formed.

8. Rectification: Using the unidirectional conductivity of the diode, the alternating current with alternating directions can be converted into pulsed direct current with a single direction.

9. Regulator: The Zener diode is essentially a junction silicon diode, and the Zener diode works in a reverse breakdown state. In the manufacturing process of the diode, it has low-voltage breakdown characteristics. The reverse breakdown voltage of the Zener diode is constant, and a current-limiting resistor is connected in series in the voltage stabilizing circuit so that the current after the Zener diode breaks down does not exceed the allowable value. Therefore, the breakdown state can last for a long time without damage.

Ⅲ. Applications of diodes

1. Electronic circuit application

In almost all electronic circuits, we use semiconductor diodes. In electrical circuits, semiconductor diodes can protect and prolong circuit life. The development of semiconductor diodes has made integrated circuits more optimized. It has played a positive role in various fields. Diodes have many functions in integrated circuits, maintaining the normal operation of integrated circuits. The following briefly introduces the role of diodes in the following four circuits.

(1) Limiting circuit

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

(2) Voltage regulator circuit

We usually need to use Zener diodes in voltage regulator circuits. It is a junction-type silicon semiconductor diode manufactured by a special process. The impurity concentration of this special diode is relatively high, and the charge density in the space charge region is large, which is easy to form a strong electric field. When the reverse voltage across the Zener diode is added to a certain value, the reverse current increases sharply, resulting in reverse breakdown.

(3) Varactor circuit

In varactor circuits, we often use varactor diodes to realize automatic frequency control, tuning, frequency modulation and scanning oscillation of the circuit.

(4) Switching circuit

In the digital and integrated circuits, the unidirectional conductivity of the diode is used to realize the conduction or disconnection of the circuit, and this technology has been widely used. The switching diode can protect the circuit very well, prevent the circuit from being burned out due to problems such as short circuit, and can also realize the function of a traditional switch. The switching speed of switching diodes is very fast, which is unmatched by traditional switches.

2. Industrial product application

After years of unremitting efforts of scientists, the application of semiconductor diode light-emitting has been gradually promoted. Light-emitting diodes are widely used in indicator lights of various electronic products, light sources for optical fiber communications, indicators and lighting of various instruments. Many characteristics of light-emitting diodes are incomparable to ordinary light-emitting devices. They mainly have the following characteristics: safety, high efficiency, environmental protection, long life, fast response, small size, and firm structure. Therefore, light-emitting diodes are a light source that meets the requirements of green lighting.

Light-emitting diodes are widely used in many fields. Here are some main applications:

(1) Application in coal mines

Because light-emitting diodes have the characteristics of high efficiency, low energy consumption, long life, and strong luminosity compared with ordinary light-emitting devices, light-emitting diodes are used in equipment such as miner's lamps and underground lighting. Although light-emitting diodes are not yet fully popularized, they will be widely used in the near future. Light-emitting diodes will replace ordinary light-emitting devices in coal mine applications.

(2) Application in automobiles and large machinery

Light-emitting diodes are widely used in automobiles and large machinery. Light-emitting diodes are used in direction lights, interior lighting, mechanical equipment instrument lighting, headlights, turn signals, brake lights, tail lights, etc. in automobiles and large mechanical equipment. This is mainly due to the fast response and long service life of light-emitting diodes (generally, the life of light-emitting diodes is longer than that of automobiles and large machinery).

(3) Urban decorative lights

In today's prosperous commercial era, neon lights are an important symbol of urban prosperity. But there are many shortcomings in neon lights, such as not long enough life and so on. Therefore, we replace neon lights with light-emitting diodes. Because compared with neon lights, light-emitting diodes not only have a long life, but also have the characteristics of energy saving, simple driving and control, and no maintenance. The replacement of neon lights by light-emitting diodes will be the inevitable result of the development of lighting equipment.

(4) Application in electronic products

Light-emitting diodes are generally used as screen backlights or for display and lighting applications in electronic products. Light-emitting diodes are used as screen backlights in displays ranging from large LCD TVs and computer displays to media players MP3, MP4, and mobile phones.