1. Gate Turn-Off Thyristor
M.S.P.V.L Polytechnic College,
Pavoorchatram.

2. Introduction
A Gate Turn-Off Thyristor (GTO) is a special type of thyristor, a high-power semiconductor device.
GTOs, as opposed to normal thyristor, are fully controllable switches which can be turned on and off by their third lead, the GATE lead.
GTO thyristor can be fabricated with a reverse conducting diode in the same package. These are known as RCGTO, for Reverse Conducting GTO.

3. Circuit diagram of GTO

4. GTO Diagram and Symbol

5. Reverse BIAS
GTO thyristor are available with or without reverse blocking capability.
Reverse blocking capability adds to the forward voltage drop because of the need to have a long, low doped P1 region.
GTO thyristor capable of blocking reverse voltage are known as Symmetrical GTO thyristor, abbreviated S-GTO

6. Functions of GTO
Gate turn-off thyristor (GTOs) are four-layer PNPN devices that act as switches, rectifiers, and voltage regulators.
Like other thyristor, GTOs can be turned on by the application of a positive gate signal (g > 0); however, unlike other more conventional devices that can be turned off only at a zero crossing of current, GTOs can be turned off at any time by the application of a gate signal equal to zero. 

7. Functions of GTO

8. Functions of GTO
Performance specifications for gate turn-off thyristor (GTOs) include peak repetitive off-state voltage, peak repetitive reverse voltage, peak cycle surge-on current, maximum root mean square (RMS) current, and holding current.
Peak repetitive off-state voltage (VDRM) is the maximum instantaneous value of the off state.

9. Cross section of GTO

10. Operations of GTO
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.

11. Structure of GTO

12. Operations of GTO
However in GTO, the current gain of PNP transistor is low so that turn of its 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.

13. Operations of GTO
Thus the base drive of NPN transistor is removed and this in turn removes the base drive of PNP transistor and turnoff is achieved.

14. The End
…… Thank You …….