A GTO is a thyristor that can be turned off by applying a current to the gate in the reverse direction to that required to turn it on. GTOs are, by nature, relatively slow switches. Typical transition times from on to off state and vice versa are in a range of 10 - 30 microseconds while typical on-off switching frequency range of 200 - 500 Hz. GTO thyristors can be classified as follows:

1. Asymmetric GTOs are divided in two categories:
• Buffer layer GTOs have exceptionally low on-state and dynamic losses.
• Fine pattern types (5SGF) are optimised for fast switching and transparent emitter (5SGT) for low on-state losses.
• Standard GTOs have excellent trade-off between on-state and switching losses.
2. Symmetric GTOs feature:
• low-on-state and turn-off losses,
• full reverse voltage,
• low gate trigger current, and
• duitable for drives and traction applications.