There are two types of Optical-coupled MOSFETs (OCMOS FET: Solid State Relay: SSR): "make" type ("a" contact: Normally open) and "break" type ("b" contact: Normally closed). The differences between these two types by operation is explained below.
1. "Make" Type ("a" Contact: Normally Open)
Th "make" type contains a couple of enhancement-type MOSFETs. They shut off by application of no bias, and conduct by application of forward bias.
The initial state is "OFF".
2. "Break" Type ("b" Contact: Normally Closed)
The "break" type contains a couple of depletion-type MOSFETs. They conduct by application of no bias, and shut off by application of reverse bias.
The initial state is "GO".