Semiconductor divaces

Module 1
Power Semiconductor Devices
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Lesson 5
Gate Turn Off Thyristor (GTO)
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Instructional objective
On completion the student will be able to • • • • • • Differentiate between the constructional features of a GTO and a Thyristor. Explain the turn off mechanism of a GTO. Differentiate between the steady state output and gatecharacteristics of a GTO and a thyristor. Draw and explain the switching characteristics of a GTO. Draw the block diagram of a GTO gate drive unit and explain the functions of different blocks. Interpret the manufacturer’s data sheet of a GTO.

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Introduction
The thyristor has reigned supreme for well over two decades in the power electronics industry andcontinues to do so at the very highest level of power. It, however, has always suffered from the disadvantage of being a semi-controlled device. Although it could be turned on by applying a gate pulse but to turn it off the main current had to be interrupted. This proved to be particularly inconvenient in DC to AC and DC to DC conversion circuits, where the main current does not naturally becomes zero. Abulky and expensive “commutation circuit” had to be used to ensure proper turning off of the thyristor. The switching speed of the device was also comparatively slow even with fast inverter grade thyristor. The development of the Gate Turn off thyristor (GTO) has addressed these disadvantages of a thyristor to a large extent. Although it has made a rather late entry (1973) into the thyristorfamily the technology has matured quickly to produce device comparable in rating (5000V, 4000Amp) with the largest available thyristor. Consequently it has replaced the forced commutated inverter grade thyristor in all DC to AC and DC to DC converter circuits. Like thyristor, the GTO is a current controlled minority carrier (i.e. bipolar) device. GTOs differ from conventional thyristor in that, theyare designed to turn off when a negative current is sent through the gate, thereby causing a reversal of the gate current. A relatively high gate current is need to turn off the device with typical turn off gains in the range of 4-5. During conduction, on the other hand, the device behaves just like a thyristor with very low ON state voltage drop. Several different varieties of GTOs have beenmanufactured. Devices with reverse blocking capability equal to their forward voltage ratings are called “symmetric GTOs”. However, the most poplar variety of the GTO available in the market today has no appreciable reverse voltage (20-25v) blocking capacity. These are called “Asymmetric GTOs”. Reverse conducting GTOs (RC-GTO) constitute the third family of GTOs. Here, a GTO is integrated with ananti-parallel freewheeling diode on to the same silicon wafer. This lesson will describe the construction, operating principle and characteristic of “Asymmetric GTOs” only.

5.2 Constructional Features of a GTO
Fig 5.1 shows the circuit symbol and two different schematic cross section of a GTO.

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A Anode Short.

p+

n+

p+ n p

n+

p+

Anode Contact J1Buffer Layer J2 J3

p+ n np
n+ n+

G K (a) G
n+

n+

G C (b) (c) C

Fig. 5.1: Circuit symbol and schematic cross section of a GTO (a) Circuit Symbol, (b) Anode shorted GTO structure, (c) Buffer layer GTO structure. Like a thyristor, a GTO is also a four layer three junction p-n-p-n device. In order to obtain high emitter efficiency at the cathode end, the n+ cathode layer is highlydoped. Consequently, the break down voltage of the function J3 is low (typically 20-40V). The p type gate region has conflicting doping requirement. To maintain good emitter efficiency the doping level of this layer should be low, on the other hand, from the point of view of good turn off properties, resistively of this layer should be as low as possible requiring the doping level of this region to...