A talk about SCRs and thyristor in different sizes, packages and from different points in time. A Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor. It is the solid state equivalent of the thyratron (which I showed in this video. https://www.youtube.com/watch?v=odkbxWpQ0ck ) and hence it is also referred to as thyristor or thyroid transistor. In fact, SCR (Silicon Controlled Rectifier) is a trade name given to the thyristor by General Electric Company.