Correct...all EXISTING thyristers have 3 terimals and switch 2 ways. In the most simplistic terms, a fourth terminal, on a properly constructed sandwich, should, in theory, allow 3 way switching ability, which seems to be necessary on the monolithis chip to co-ordinate all three of the components, (sensor, vscel and transistor) VERY SIMPLE explanation. The full explanation is many chapters or a least a CD to explain fully.