I am seeing a general pattern. Transistors or MOSFETs design for larger currents >2 amps usually have heatsink pads. Optoisolators or SSRs even with 8+ amp ratings do not.
Why?
Do optoisolators produce less heat i.e. less resistance?
Aren't optoisolators essentially optocoupled photo "transistors"?
Thanks
Those MOSFETs are usually switching at a much higher frequency. There are switching current losses, which create heat.
What are you planning to switch with optocouplers? Voltage, current, AC/DC?
General question whether optocouplers have less internal on state resistance than MOSFETs or transistors.
Optoisolators > 8 amps, there are none.
Simple reason, Power = Voltage * Amps
An 8-Amp SSR is usually larger than a MOSFET with more surface area to dissipate heat so they may essentially have a heatsink built-in. And some higher power SSRs are designed to be mounted on some kind of metal plate that can act as a heatsink.
A DC SSR is usually built-around a MOSFET (and an opto-isolator) and AC SSRs normally use TRIACs which also dissipate a bit of heat-power.
...I made some homemade SSRs with TRIACs. The TRIACs are mounted to the inside of a metal electrical box which doubles as a heatsink.
Hi,
Typical MOSFET Optocoupler.
Also Google;
photo mosfet relay
Tom... 
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