Basic knowledge of thyristor

(1) Thyristor [commonly known as: silicon controlled rectifier (SCR)] 

Thyristors first appeared in the 1960s. Its emergence has expanded the application of semiconductor devices from the weak current field to the strong current field.It has long enjoyed a dominant position in the field of power devices.At present, thyristors are still widely used in the field of low-frequency switching.The ability to achieve high voltage and high current is unmatched by other devices.

The 6-inch silicon single-device thyristor with a blocking voltage of 8000V and a rated current of more than 5000A has been available as early as 2008 and is used in high-voltage direct current transmission (HVDC).

The advantages of thyristors are: small size, light weight, noiseless, long life, large capacity, and simple use and maintenance.

The disadvantage is that the device current cannot be turned off at any time and can only rely on natural commutation.

That is, the direction of the current is changed, and the ability to withstand overvoltage and overcurrent is poor.

(2) Several basic concepts

a.Avalanche breakdown: In a PN junction with a low material resistivity, when the reverse voltage increases, the electrons and holes in the space series area gain energy increase during their interaction, and the electrons and holes in the crystal movement continue to collide with the crystal atoms. , causing the valence electrons bound in the covalent bonds to collide and generate new electron-hole pairs. Such a chain reaction causes the number of carriers in the head layer to increase avalanchely, and the current flowing through the PN junction increases day by day. , the breakdown caused by this kind of impact ionization is called avalanche breakdown.

b. Ohmic contact: The non-rectifying contact formed when metal and semiconductor come into contact. (Dependent on material, surface state, and doping concentration. The contact voltage drop is lower than the voltage drop falling anywhere else on the device under all operating conditions.)

c. Semiconductor doping: Doping a small amount of foreign atoms into a semiconductor in order to change the electrical properties of the semiconductor. Donor elements provide excess electrons in silicon, such as phosphorus P, arsenic AS, antimony Sb, etc., forming an N-type region in silicon. Acceptor elements provide excess holes in silicon, such as boron B, gallium Ga, aluminum Al, etc., forming a P-type region in silicon.

d. Base area: The common area between the two back-to-back PN junctions of the transistor is called the base area. Long base area: The N area in the middle of the PNP transistor in the thyristor structure is relatively long, which is called the long base area. Short base area: The middle P area of ​​the NPN transistor in the thyristor structure is relatively short, which is called the short base area.