Differently from photocouplers, MOSFET relays can switch either DC or AC loads and its output switching current is not affected by input current. So MOSFET relays can be operated by very small currents. MOSFET relays have much smaller leakage current (1 nA) when in off-state compared to photocouplers and phototriacs.  Also, its small heat generation does not require heat sinks.