Solid state relays were introduced in the 1970s. Solid state relays (SSRs) are a new type of contactless switching device composed entirely of solid-state electronic components.

It utilizes the switching characteristics of electronic components such as switching transistors, bidirectional thyristors, and other semiconductor devices to achieve the purpose of connecting and disconnecting circuits without contact or spark, hence it is also known as a "contactless switch". Solid state relay is a four terminal active device, where two terminals are input control terminals and the other two terminals are output controlled terminals.

SSR is a substitute product for traditional electromagnetic relays, with more reliable performance.

Its switch does not contain any moving parts, hence it is named "solid-state relay".