Thyristors

Definition:

Thyristor is a solid-state semiconductor device with four layers of  alternating P & N type materials,
A thyristor is the family name of the three-terminal electronic switch, this family includes :

• Silicon Controlled Rectifier [ SCR ]
• Gate turn off thyristor [ GTO ]
• Metal Oxide Silicon controlled Thyristor  [ MCT ] 
• Triacs

The SCR : 

electronic devices capable of carrying a large current and blocking large voltage hence are used in high power applications; however their switching

frequency is relatively low, usually used at power frequency applications, SCR is a four-layer semiconductor with 

three PN junction.

Gate turn off thyristor(GTO):

GTO is another member of the thyristor family. the same operating conditions required to turn on an SCR available to the GTO`s,however

the GTO can be turned OFF with negative gate current,When The gate turn OFF the current may last a few microseconds but it has very large

value that could reach one third of the ON-state anode current, the V & I of the GTO are similar to the SCR.

Triacs:

is a thyristor which is capable of carrying current in either direction.the triac is functionally equivalent to two thyristors connected back to back 

, They are usually used in high dimmer circuits and controlling induction motors.

Mos controlled thyristor (MCT):

these devices are equivalent to the GTO but without the high turn off gate current requirement. the MCT is turned on /off by establishing the proper voltage from gate to cathode as opposed to the gate current in GTO. The MCT is available at lower voltage and current ratings than the GTO.

why thyristors ??

WITH THE DEVELOPMENT IN THE TECHNOLOGY OF MANUFACTURING TRANSISTORS THEIR POWER HANDLING CAPACITY HAS BEEN LARGELY IMPROVED,

THUS LEAVING THE THYRISTOR USE TO VERY HIGH POWER APPLICATIONS