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Gate Turn Off Thyristor
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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 gate characteristics of a GTO and a
• 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
• Interpret the manufacturer’s data sheet of a GTO.
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The thyristor has reigned supreme for well over two decades in the power electronics industry
and continues 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. A bulky 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 thyristor family 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, they are 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 been manufactured. 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 an anti-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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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 highly doped. 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
be high. Therefore, the doping level of this layer is highly graded. Additionally, in order to
optimize current turn off capability, the gate cathode junction must be highly interdigitated. A
3000 Amp GTO may be composed of upto 3000 individual cathode segments which are a
accessed via a common contact. The most popular design features multiple segments arranged in
concentric rings around the device center.
The maximum forward blocking voltage of the device is determined by the doping level and the
thickness of the n type base region next. In order to block several kv of forward voltage the
doping level of this layer is kept relatively low while its thickness is made considerably higher (a
few hundred microns). Byond the maximum allowable forward voltage either the electric field at
the main junction (J2) exceeds a critical value (avalanche break down) or the n base fully
depletes, allowing its electric field to touch the anode emitter (punch through).
The junction between the n base and p+ anode (J1) is called the “anode junction”. For good turn
on properties the efficiency of this anode junction should be as high as possible requiring a
heavily doped p+ anode region. However, turn off capability of such a GTO will be poor with
very low maximum turn off current and high losses. There are two basic approaches to solve this
In the first method, heavily doped n+ layers are introduced into the p+ anode layer. They make
contact with the same anode metallic contact. Therefore, electrons traveling through the base can
directly reach the anode metal contact without causing hole injection from the p+ anode. This is
the classic “anode shorted GTO structure” as shown in Fig 5.1 (b). Due to presence of these
“anode shorts” the reverse voltage blocking capacity of GTO reduces to the reverse break down
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voltage of junction J3 (20-40 volts maximum). In addition a large number of “anode shorts”
reduces the efficiency of the anode junction and degrades the turn on performance of the device.
Therefore, the density of the “anode shorts” are to be chosen by a careful compromise between
the turn on and turn off performance.
In the other method, a moderately doped n type buffer layer is juxtaposed between the n-
base and the anode. As in the case of a power diode and BJT this relatively high density buffer
layer changes the shape of the electric field pattern in the n-
base region from triangular to
trapezoidal and in the process, helps to reduce its width drastically. However, this buffer layer in
a conventional “anode shorted” GTO structure would have increased the efficiency of the anode
shorts. Therefore, in the new structure the anode shorts are altogether dispensed with and a thin
p+ type layer is introduce as the anode. The design of this layer is such that electrons have a high
probability of crossing this layer without stimulating hole injection. This is called the
“Transparent emitter structure” and is shown in Fig 5.1 (c).