Thyristor is the generic name given to a family of semiconductors devices which are made up of four semiconductor layers. One member of this family is the two wire thyristor also known as PN-PN diode or trigger diode. This device's name in the IEEE standard for the graphic symbols is reverse blocking diode type thyristor.Its symbol is shown in figure below.
The PN-PN diode is a rectifier or a diode with an unusual voltage current characteristics in the forward biased region. The characteristics curve consists of three regions:
- The reverse blocking region
- The forward blocking region
- The conducting region
In the reverse blocking region the PN PN diode behaves as an ordinary diode and all current flow until the reverse breakdown voltage is reached.In the conducting region the PN PN diode again behaves as an ordinary diode , allowing large amounts of current to flow with very little voltage drop.
In a summary PN-PN diode is :
- Turns on when the applied voltage Vd exceeds Vbo
- Turns off when the current Id drops below Ih
- Blocks all currents flow in the reverse direction untill the maximum reverse voltage exceeds
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