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I’m reading a GTO document and I’m not able to understand this part:

"The anode current is crowded into higher and higher density filaments in most remote areas from the gate contact. This is the most critical phase in the GTO turn off process since highly localized high temperature regions can cause device failure unless these current filaments are quickly extinguished."

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A current filament is the conductive current path in a GTO device.

Most power GTO's have a large number of smaller thyristors in parallel on the actual device, and as a negative pulse is delivered to the gate to switch them off, they don't all switch off at the same time. As individual parts of the device switch off, higher and higher currents exist through the parts that are still on. This is what normally causes a GTO to fail, burning up the individual thyristors that make up the overall die.

Some serious negative currents are normally injected into the gate to force the conduction regions to shrink as quickly as possible. For high current devices, it is common to have turn off pulses be at least 5 to 10% of the forward current, and not unusual to have the turn off pulses be up to 25 to 30% of the forward current to switch them off as quickly as possible.

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It means that conduction through the semiconductor layer is not uniform, but is concentrated into thin high-current streams, like thin wires running through the semiconductor layers, as the GTO turns off.

Since the total current does not drop much until the GTO turns off the current density in those areas of the semiconductor is very high, and there is a potential for damage unless the GTO turns off relatively quickly. Presumably that is accomplished by keeping the load characteristics and turn-off pulse within bounds.

A somewhat similar thing can happen when triggering a thyristor 'on'- if the turn-on pulse is insufficient to quickly turn on the whole chip, 'hot spots' can develop during the turn-on as the conduction spreads across the chip and that can cause damage.

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