GTO ( Gate Turn Off)
GTO stands for gate turn off thyristor . it is four layer PNPN device. It can be triggered into conduction like a conventional thyristor by a pulse of positive gate current.GTO can be easily turned off by a negative gate pulse of appropriate magnitude. The GTO is a three terminal devices with anode cathode and gate terminals. The basic structure and symbol of GTO is shown in figure. The two way arrow convention on the gate lead distinguishes the GTO from an ordinary thyristor . The need of forced commutation circuit because turn off is achieved by applying a negative circuit.
The gate turn off thyristor has highly doped N spots in the P layer at the anode. The plus again indicating high doping level. The gate cathode structure is inertdigitated i e each electrode is composed of a large number of narrow channels closely located.
Function of GTO except for turn off are same as those of conventional thyristors therefore were mainly describe the turn off operation here. When a GTO is in the on state the central base regions are filled with holes supplied from the anode and electrons supplied from the cathode. If reverse bias is applied to make the gate negative in respect to the cathode part of holes in the p base layer are extracted through the gate suppressing the injections of electrons from the cathode. In response to this suppression more hole current is extracted through the gate further suppressing the electron injection. In the course of this process the cathode emitter junction is put into a reverse bias state entirely GTO turned off.
GTO is analogy of two transistors ( PNP and NPN ) as shown in figure suppose that GTO thyrisotr is divided into npn transistor Trl on the cathode side and pnp transistor Tr2 on the anode side and that they are connected as shown in figure in this figure the current amplification factor of transistor Trl is called a and that of transistor Tr2 a2. If reverse current IGQ flows through the gate base current iB at transistor Trl is reduced when IGQ is increased. The relationship between GTO thyristor anode current (Ia) and cathode current (Ik) is expressed by the followings equations:
La = Ik + IGQ
A GTO thyristor can carry out the turn off if an adequate magnitude of reverse bias current is supplied t the gate. Actually however sheet resistance exists in the trl base region making it difficult to turn off the on state current flowing at he emitter junction that is far from the gate.
figure Two transistor analogy of GTO