working of transistor

GTO is a special thyristor which can be turned on by a positive gate signal and can be turned off by a neagative signal.Evidently the use of GTO in power electronic circuit eliminates the need of forced commutation circuit because turnoff is achieved by applying a negative circuit.

The two transistor analogy of a GTO

Two transistor analogy of transistor is shown in figure below

When a positive signal is applied,a GTO switches into conduction state like the ordinary thyristor.However in ordinary thyristor the current gains of NPN and PNP transistors are very high so that gate sensitivity for turn on is very high and on state voltage drop is low.However in GTO,the current gai of PNP transistor is low so that turn of fis possible if significant current is drawn from the gate.When a negative gate signal is applied the excess carriers are drawn from the base region of NPN transistor and collector current of PNP is diverted to the gate.Thus the base drive of NPN transistor is removed and this inturn removes the base drive of PNP transistor and turnoff is achieved