A diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.). The diode has two electrodes, the positive pole, also known as the anode; the negative pole, also known as the cathode, when a forward voltage is applied between the two poles of the diode, the diode is turned on; when a reverse voltage is applied, the diode is cut off. The on and off of the diode is equivalent to the on and off of the switch.
The diode has unidirectional conductivity, and the direction of current is from the anode to the cathode through the tube when it is turned on. A diode is a two-terminal electronic component that only allows current to flow in one direction. Diodes are commonly used in alternating current (AC) to direct current (DC) power supplies.
Diode is one of the earliest semiconductor devices, and its application is very wide. Especially in various electronic circuits, diodes and resistors, capacitors, inductors and other components are used for reasonable connection to form circuits with different functions, which can realize rectification of alternating current, detection of modulation signals, amplitude limiting and clamping, and control of power supply voltage. Voltage stabilization and other functions.
The main classification of diodes
1. Point-contact diode
The point-contact diode is formed by pressing a metal needle on a single wafer of germanium or silicon material, and then through the current method. The PN junction contact area of the point-contact diode is small, and it cannot pass a large forward current and withstand a high reverse voltage, but its high-frequency performance is good, and it is suitable for use in high-frequency detection circuits and switching circuits.
However, compared with the junction type, the point contact type diode has poor forward and reverse characteristics, so it cannot be used for high current and rectification. Because the structure is simple, the price is cheap. It is the broad type for general purposes such as detection, rectification, modulation, frequency mixing, and limiting of small signals.
2. Surface contact diode
The surface contact diode is a special silicon diode, which not only can pass a large current, but also has stable and reliable performance, and is mostly used in switching, pulse and low-frequency circuits.
The "PN junction" of the surface-contact diode has a large area, which allows a larger circuit (several A to tens of A), and is mainly used in a rectifier circuit that converts alternating current into direct current. Surface-contact crystal diodes are more suitable for large The current switch can also withstand high reverse voltage and is suitable for use in rectifier circuits.
3. Planar diode
Planar diode is a special silicon diode, it can not only pass a large current, but also has stable and reliable performance, and is mostly used in switching, pulse and high frequency circuits. When a planar diode is used as a switch tube in a pulse digital circuit, the PN junction area is small, and when it is used for high-power rectification, the PN junction area is relatively large.
On a semiconductor single wafer (mainly N-type silicon single crystal), diffuse P-type impurities, and use the shielding effect of the oxide film on the silicon surface to selectively diffuse only a part of the N-type silicon single crystal to form a PN junction. Therefore, chemical corrosion for adjusting the area of the PN junction is unnecessary. Because the semiconductor surface is made flat, it is called a planar diode.
Because the surface of the PN bond is covered with an oxide film, it is very stable and durable. There are few types of planar diodes used for high current rectification, but there are many types used for small current switches.
4. Zener diode
Zener diodes are also called Zener diodes. Zener diodes refer to diodes that use the reverse breakdown state of the PN junction, and the current can change in a wide range while the voltage remains basically unchanged. The diode is made to stabilize the voltage. It is a special surface-contact semiconductor silicon diode, which has the function of stabilizing voltage.
The main difference between Zener diodes and ordinary diodes is that Zener diodes work in the reverse breakdown state of the PN junction. By limiting the magnitude of the reverse current during the manufacturing process and using it, it can be ensured that the Zener tube will not be damaged due to overheating in the state of reverse breakdown.
Zener diodes are different from ordinary diodes. Its reverse breakdown is reversible. As long as it does not exceed the allowable value of the Zener tube current, the PN junction will not be overheated and damaged. When the external reverse voltage is removed, the Zener tube will recover. The original performance, so the Zener tube has good repeated breakdown characteristics.
5. Photodiode
Photodiodes are also called photosensitive diodes. Its shell is equipped with a glass window to receive light. Its characteristic is that when light is irradiated on its PN junction, free electrons and holes can be generated in pairs, which increases the concentration of minority carriers in the semiconductor, and under a certain reverse bias voltage, reverse The current increases. Therefore its reverse current increases linearly with the increase of light intensity.
When there is no light, the volt-ampere characteristics of photodiodes are the same as ordinary diodes. As a light control element, photodiode can be used in various object detection, photoelectric control, automatic alarm and so on.
When made into a large-area photodiode, it can be used as an energy source and called a photocell. At this time, it does not need an external power supply, and can directly convert light energy into electrical energy.
6. LED
A light-emitting diode is a semiconductor solid-state display device that directly converts electrical energy into light energy, referred to as LED (Light Emitting Diode). Similar to ordinary diodes, LEDs are also made of a PN structure. The PN junction of the light-emitting diode is packaged in a transparent plastic case with square, rectangular and circular shapes. Light-emitting diodes have the advantages of low driving voltage, low working current, strong anti-vibration and impact capability, small size, high reliability, low power consumption and long life, and are widely used in signal indication and other circuits.
The principle of light-emitting diodes, which are commonly used in electronic technology, is opposite to that of photodiodes. Light-emitting diodes emit light when they are forward-biased and pass current, which is the result of energy released when electrons and holes are directly recombined. Its spectral range is relatively narrow, and its wavelength is determined by the basic materials used.
7. Schottky diode
Schottky diode is named after its inventor, Dr. Schottky (Schottky), and SBD is the abbreviation of Schottky Barrier Diode (Schottky Barrier Diode, abbreviated as SBD). SBD is not made by using the principle of contacting P-type semiconductor and N-type semiconductor to form PN junction, but using the principle of metal-semiconductor junction formed by contacting metal and semiconductor. Therefore, SBD is also called metal-semiconductor (contact) diode or surface barrier diode, which is a kind of hot carrier diode.
Schottky diodes have the advantages of high switching frequency and low forward voltage. Since there is no capacitance (PN junction), its switching speed is very fast.
The biggest disadvantage of Schottky diodes is their low reverse bias voltage and large reverse leakage current. For Schottky diodes made of silicon and metal, their reverse bias rated withstand voltage is only up to 50V. , and the reverse leakage current value has a positive temperature characteristic, which tends to increase rapidly as the temperature rises, so the hidden danger of thermal runaway should be paid attention to in the design of use.
