Gate Turn Off Thyristor (GTO)

Gate Turn Off Thyristor (GTO) is a current-controlled minority carrier i.e. bipolar device. GTO differs from conventional thyristors 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 needed to turn off the device with typical turn off gains.GTO is a four-layer, three-terminal current-controlled minority carrier device.

Different varieties of GTO have been manufactured. Devices with reverse blocking capability equal to their forward voltage ratings are called symmetric GTO.GTO is integrated with an anti-parallel freewheeling diode on to the same silicon wafer.

Constructional Features of GTO

Constructional Features of GTO

GTO is 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 breakdown voltage of the junction J3 is low typically 20-40V. The p-type gate region has a conflicting doping requirements. To maintain good emitter efficiency the doping level of this layer should low, on the other hand, from the point of view of a 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.

In order to optimize current turn off capability, the gate cathode junction must be highly interdigitated. A 3000 Amp GTO may be composed of up to 3000 individual cathode segments which are accessed via a common contact. The most popular design features multiple segments arranged in concentric rings around the device center.

Forward blocking voltage of the device is determined by the doping level and the thickness of the n-type base region next. Maximum allowable forward voltage either the electric field at the main junction J2 exceeds a critical value i.e. avalanche break down or the n base fully depletes, allowing its electric field to touch the anode emitter.

Anode junction J1 is the junction between the n base and p+ anode. For good turn on properties 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 method to solve poor turn off capability of GTO.

First method: A heavily doped n+ layers are introduced into the p+ anode layer that makes 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. Due to the presence of these anodes short the reverse voltage blocking capacity of GTO reduces to the reverse break down voltage of junction J3. Therefore, the density of the anode shorts are to be chosen by a careful compromise between the turn on and turn off performance.

Second method: A moderately doped n-type buffer layer is juxtaposed between the n- type base and the anode. 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 the 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 introduced 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.

Operating Principle of GTO

GTO is a p-n-p-n structure can be though of consisting of one p-n-p and one n-p-n transistor connected in the regenerative configuration.

GTO is a p-n-p-n structure that can be thought of as consisting of one p-n-p and one n-p-n transistor connected in the regenerative configuration.

GTO structure one can write as

IC1 = ∝P IA  + ICBO1

IB1 = iC2 = ∝n IK + ICBO2

IK  = IA  + IG  and  IA = iB1 + iC1

formula of anode current in GTO

Applied forward voltage VAK less than the forward break over voltage both ICBO1 and ICBO2 are small. Further if IG is zero IA is only slightly higher than ICBO1 + ICBO2. Under this condition both ∝n and ∝P are small and (∝n + ∝P) <<1. The device is said to be in the forward blocking mode. Normally, this is done by injecting current into the p base region via the external gate contract.

To turn the device on either the anode voltage can be raised until ICBO1 and ICBO2 increases by avalanche multiplication process or by injecting a gate current.

In saturation all junctions assume a forward bias and total potential drop across the device becomes approximately equal to that of a single p-n diode. The anode current is restricted only by the external circuit. Once the device has been turned on the external gate current is no longer required to maintain conduction, since the regeneration process is self-sustaining. Reversion to the blocking mode occurs only when the anode current is brought below the holding current level.

To turn off a conducting GTO the gate terminal is biased negative with respect to the cathode. The holes injected from the anode are, therefore, extracted from the p base through the gate metallization into the gate terminal.

Voltage drop in the p base above the n emitter starts reverse biasing the junction  J3 and electron injection stops here. The process originates at the periphery of the p base and the n emitter segments and the area still injecting electron shrinks. The anode current is crowded into higher and higher density filaments in most remote areas from the gate contact.

Cathode current has ceased the anode to gate current continues to flow as the n base excess carriers diffuse towards J1. This tail current then decays exponentially as the n base excess carriers reduce by recombination. Once the tail current has completely disappeared does the device regain its steady-state blocking characteristics. Anode Shorts or transparent emitter helps reduce the tail current faster by providing an alternate path to the n base electrons to reach the anode contact without causing an appreciable hole injection from the anode.

Steady state and dynamic characteristics of a GTO

Steady state output and gate characteristics

The latching current and forward leakage of a GTO is considerably higher than a thyristor of a similar rating. In fact, if the gate current is not sufficient to turn on a GTO it operates as a high voltage low gain transistor with considerable anode current. It should be noted that a GTO can block rated forward voltage only when the gate is negatively biased with respect to the cathode during forward blocking state.

The latching current and forward leakage of a GTO is considerably higher than a thyristor of a similar rating. In fact, if the gate current is not sufficient to turn on a GTO it operates as a high voltage low gain transistor with considerable anode current. It should be noted that a GTO can block rated forward voltage only when the gate is negatively biased with respect to the cathode during forward blocking state.

Low-value resistance must be connected across the gate cathode terminal that increasing the value of this resistance reduces the forward blocking voltage of the GTO. Asymmetric GTOs have small reverse break down voltage. This may lead the device to operate in reverse avalanche under certain conditions. This condition is not dangerous for the GTO provided the avalanche time and current are small. The gate voltage during this period must remain negative.

The zone between the minimum and maximum curves reflects parameter variation between individual GTOs. These characteristics are valid for DC and low-frequency AC gate currents. They do not give the correct voltage when the GTO is turned on with high dia/dt and dIG/dt . VG in this case is much higher.

The zone between the minimum and maximum curves reflects parameter variation between individual GTOs. These characteristics are valid for DC and low-frequency AC gate currents. They do not give the correct voltage when the GTO is turned on with high dia/dt and dIG/dt . VG in this case is much higher.

Dynamic characteristics of a GTO

Dynamic characteristics of a GTO

When the GTO is off the anode current is zero and VAK  = Vd. To turn on the GTO, a positive gate current pulse is injected through the gate terminal. A substantial gate current ensures that all GTO cathode segments are turned on simultaneously and within a short time. There is a delay between the application of the gate pulse and the fall of anode voltage called the turn-on delay time td.

After time delay td the anode voltage starts falling while the anode current starts rising towards its steady value IL . Within a further time interval  tr they reach 10% of their initial value and 90% of their final value respectively.  tr is called the current rise time. Both td and maximum permissible on state diA/ dt are very much gate current dependent.

A minimum ON time period  tON (min) is required for homogeneous anode current conduction in the GTO. This time is also necessary for the GTO to be able to turn off its rated anode current. The anode voltage,current or the gate voltage does not change appreciably from their on state levels for a further time period called the storage time(tS).

At the end of current fall time tf the anode current reaches 10% of its initial value after which both the anode current and the gate current continues to flow in the form of a current tail for a further duration of  ttail.

GTO should not be retriggered within a minimum off period off (min) to avoid the risk of failure due to localized turn ON. GTOs have typically low turn off gain in the range of 4-5.

Summary

  • GTO is a four-layer, three-terminal current-controlled minority carrier device.
  • GTO can be turned on by applying a positive gate current pulse when it is forward biased and turned off by applying a negative gate current.
  • GTO has a shorted anode and highly inter-digitized gate cathode structure to improve the gate turn off performance.
  • The forward i-v characteristics of a GTO is similar to that of a thyristor.
  • GTO have relatively larger holding current and gate trigger current.
  • A GTO can block rated forward voltage only when the gate cathode junction is reverse biased.
  • GTOs have relatively low turn off current gain.
  • A GTO can block rated forward voltage only when the gate cathode junction is reverse biased.

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