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  I_GQ flows through the gate base current iB at transistor Trl is  reduced when I_GQ is increased. The  relationship  between GTO  thyristor  anode current (I_a)  and cathode current  (I_k) is expressed by the followings  equations:

L_a = I_k + I_GQ

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