Gate Turn-Off Thyristor, GTO

- description and information about the Gate Turn-Off Thyristor,GTO, its operation, applications, structure and circuit design considerations.

The Gate Turn-Off Thyristor, GTO is a variant of the more standard form of thyristor. Rather than the gate being used to turn the thyristor on, within a gate turn-off thyristor, GTO, the gate pulse turns the device off.

These gate turn-off thyristors are useful in a number of areas, particularly within variable speed motor drives, high power, inverters and similar areas. Although they are not nearly as well known as the more standard forms of thyristor, the gate turn off thyristor, is now widely used as it is able to overcome many of the disadvantages of the traditional thyristor. As a result the gate turn-off thyristor is used in virtually all DC to AC and DC to DC high voltage conversion units

The first thyristors were developed in the mid-1950s and established their place in the market as a high current high voltage switch. The gate turn-off thyristor, GTO was not developed until later and only entered the market around 1973. Accordingly it is not as well known in many circles as the more familiar thyristor, although within its own area, it is widely known and used.

Gate turn-off thyristor basics

The gate turn off thyristor is behaves somewhat differently to a standard thyristor which can only be turned on and cannot be turned off via the gate. The gate turn off thyristor, GTO can be turned-on by a gate signal, and it can also be turned-off by a gate signal of negative polarity.

The device turn on is accomplished by a "positive current" pulse between the gate and cathode terminals. As the gate-cathode behaves like PN junction, there is a relatively small voltage between the terminals.

The turn on phenomenon in GTO is however, not as reliable as that of a standard thyristor and small positive gate current must be maintained even after turn on to improve reliability.

Gate turn-off thyristor structure

Like the standard thyristor, the gate turn-off thyristor is a four layer device having three junctions. Again the layers are p-n-p-n with the outside p layer providing the anode connection, and the outside n layer providing he cathode connection.To attain high emitter efficiency, the cathode layer is highly doped to give an n+ region. This has the drawback that it renders the junction nearest to the cathode (normally referred to as J3) with a low breakdown voltage - typically 20- 40 volts.

The doping level of the p region for the gate is graded. This is to provide good emitter efficiency for which the doping level should be low, while providing a good turn off characteristic for which a high doping level is needed.

The gate electrode is often interdigitated to optimise the current turn=off capability. High current devices, i.e. 1000A and above may have several thousand segments which are all connected to the common gate contact.

Another key parameter for a gate turn-off thyristor is the maximum forward blocking voltage. This is determined by the doping level and thickness of the n type base region. As many devices may need to block voltages of several kilovolts, the doping level of this region needs to be kept relatively low.

Gate turn-off thyristor structure
Gate turn-off thyristor structure

Gate turn off thyristor operation

Many aspects of the Gate turnoff thyristor, GTO are very similar to that of the ordinary thyristor. It can be thought of as being one PNP and one NPN transistor being connected in a regenerative configuration whereby once turned on the system maintains itself in this state.

Equivalent circuit of a gate turn off thyristor
Equivalent circuit of a gate turn off thyristor

When a potential is applied across the gate turn-off thyristor between the anode and cathode, no current will flow because neither device is turned on. Current would only flow if the voltage exceeded the breakdown voltage and current would flow as a result of avalanche action, but this mode would not be wanted for normal operation. In this non-conducting state the gate turn-off thyristor is said to be in its forward blocking mode.

To turn the device on it is necessary to inject current into gate circuit of the device. When this is done, it turns on TR2 in the diagram. This pulls the collector of this transistor down towards the emitter voltage and in turn this turns on the other transistor - TR1.

The fact that TR1 is now switched on ensures current flows into the base of TR2, and thus this feedback process ensures that once the gate turn-off thyristor like any other thyristor is turned on it remains on.

The key capability of the gate turn-off thyristor is its ability to be turned off by the use of the gate electrode on the device. The device turn off is achieved by applying a negative bias to the gate with respect to the cathode. This extracts current from the base region of TR2. The resulting voltage drop in the base starts to reverse bias the junction and thereby stopping the current flow in this transistor - TR2.

This then stops the injection into the base region of TR1 and this prevents current flow in this transistor.

In terms of the physics of the turn off phase, it is found that during the turn off phase of the GTO, current is crowded into higher and higher density current filaments in areas that are most remote from the gate region. These high current density areas become hot, and can cause device failure if the current is not extinguished quickly.

When the current filaments are extinguished, the overall current flow stops and the depletion layers around the junctions grow - the gate turn-off thyristor enters its forward blocking state again.

The gate turn off thyristor is similar to the ordinary thyristor in many ways, but its capability of being able to be turned off by voltages on the gate provide more capability for the device and enable the gate turn off thyristor to be used in areas where the standard thyristor cannot be used. Accordingly the gate turn off thyristor is a useful tool for many applications.

By Ian Poole

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