Here is a basic solid-state relay that uses two back-to-back MOSFETs for on/off load control from either an AC or DC supply (either polarity).
As shown below, two N-MOSFETs are connected source-to source so that they can block voltage in either direction when OFF.
When their gates are biased ON they will switch a load from either an AC or bipolar DC supply (V_Source) since MOSFETs conduct equally well in either direction when ON.
MOSFETs have an efficiency advantage over SCRs for switching...