MOSFET (metal-oxide-semiconductor field-effect transistors) have an electrically-insulated gate between their source and drain terminals that controls the amount of electricity that can flow between them. They use electric fields to switch DC currents. They can switch at higher frequencies than a triac, but are not suitable for high-current switching.

Photocoupler uses light-emitting diodes (LED) and photodetectors to transfer signals and communicate feedback between isolated circuits quickly. They're suitable for high-voltage AC or DC signals and respond well to lower frequencies. They have long service lives and produce little electrical noise.

Phototriac uses light-emitting diodes (LED) and photodetectors to isolate input and output circuits to protect low-voltage devices from high-voltages. They eliminate electrical chatter and deliver reliable results. Available in zero crossing and non-zero crossing.

SCR (silicon controlled rectifier) is a solid-state unidirectional switch that only allows current to flow in one direction. They are used in high current and voltage applications to control AC circuits that change polarity.

Triac (triode for alternating current) allow current to flow in both directions and can switch AC or DC current. Once they begin to conduct, they will continue to do so until the current reverses or reduces below the holding current threshold. They can switch much higher currents than a MOSFET but are generally limited to 50A or less.