Diode, one of the electronic components, is a device with two electrodes, which only allows current to flow in a single direction. Many uses are to apply its rectifying function. And a varactor diode is used as an electronic adjustable capacitor.

The current directivity of most diodes is commonly called "rectifying". The most common function of diodes is to only allow current to pass in a single direction (called forward bias) and block it in the reverse direction (called reverse bias). Therefore, the diode can be thought of as an electronic check valve. However, in fact, the diodes do not show such perfect directionality of on and off, but rather more complicated nonlinear electronic characteristics - which are determined by the specific type of diode technology. In addition to being used as a switch, the diode has many other functions. Early diodes included "cat whisker crystals" & quot; Cat & # 39; s Whistler & quot; crystals "and vacuum tubes (called" thermal valves "in the UK). Most of the most common diodes today use semiconductor materials such as silicon or germanium.

Application of diode:

1. Rectification: by using the unidirectional conductivity of the diode, the alternating current with alternating direction can be converted into the pulsed DC with single direction.

2. Switch: under the action of forward voltage, the resistance of the diode is very small and it is in the on state, which is equivalent to an on switch; Under the action of the reverse voltage, the resistance is large and is in the off state, like an open switch. Using the switching characteristics of diodes, various logic circuits can be formed.

3. Amplitude limiting: after the diode is turned on in the forward direction, its forward voltage drop remains basically unchanged (0.7V for silicon tube and 0.3V for germanium tube). Using this characteristic, as a limiting element in the circuit, the signal amplitude can be limited within a certain range.

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

5. Wave detection: it plays the role of wave detection in the radio.

6. Varactor: used in TV tuner.

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

8. Voltage stabilization: the zener diode is essentially a surface junction silicon diode, and the zener diode works in the 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 the current limiting resistor is connected in series in the zener circuit, so that the current after the breakdown of the zener tube does not exceed the allowable value. Therefore, the breakdown state can last for a long time without damage. 9. Trigger: trigger diode, also known as bidirectional trigger diode (DIAC), is a two-terminal semiconductor device with three-layer structure and symmetry. It is commonly used to trigger the bidirectional thyristor and is used for overvoltage protection in the circuit.

Semiconductor triode is also called "crystal triode" or "transistor". A PNP (or NPN) structure is composed of two PN junctions which can influence each other on the single crystal of semiconductor germanium or silicon. The N region (or p region) in the middle is called the base region, and the regions on both sides are called the emitter region and the collector region. Each of these three parts has an electrode lead, which is called the base B, the emitter E and the collector C. It is a semiconductor electronic device that can amplify, oscillate or switch. It can also be said that it is the most important device in the electronic circuit. There are many kinds of triodes, and different models have different uses. Most of the triodes are plastic or metal packages. The appearance of the common triode is that the electrode with an arrow is the emitter, the NPN type triode with the arrow facing outward, and the PNP type triode with the arrow facing inward. In fact, the direction indicated by the arrow is the direction of the current.