GTO (GATE TURN OFF THYRISTOR)

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A GTO is a more versatile power
semiconductor device. It is like a conventional thyristor but with added features in it.

A GTO can be easily turned off by a negative gate pulse of appropriate amplitude. Thus a GTO is a device that can be turned on by a positive gate current and turned off by a negative gate current at its gate cathode terminals. Self- turn off capability of GTO makes it the most suitable device for inverter and chopper applications.

The four layers of GTO have different doping levels indicate by P+nP+n+ Transistor Q1 is P+ NP+ type and Q2 is NP+ N+. Emitter of Q1 as anode and Emitter of Q2 as cathode.

Turns on process:
A GTO is turned on by applying a positive gate current Ig in the direction shown. Current gains a1 and a2 begins to rise and when a1 + a2 = 1 saturation level is reached and GTO is turned ON.
Turn off process:
For initiating the turn off process, Q2 must be brought out of saturation. The gate current ig1 is given in opposite side. So when negative gate current ig1 flows
between gate cathode terminals. Net gate current is reversed & excess carriers are drawn from base P+ region of Q2 and collector current ic1 of Q1 is diverted into the external gate circuit. This removes base drive of transistor Q2 this further removes base current ib1 of transistor Q1 and the GTO is eventually turned off.

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