Where does the gate charge come from? I think for charge injection you need some external drive with a common ground.
Regards, Dieter
The gate charge in the optofet and optomosfets comes from the photovoltaic action. It's like there is a small solar cell tied between the gate and source of the fets which receives infrared light from the isolated LED.
Internally, to rapidly turn off the mosfet, the gate does have a parallel load resistor to the source as well.
Note that the optofets have a linear transfer function related to led current by design. They are also good for linear isolated resistor control. They also have less parallel capacitance than the optomosfets.
The optomosfets don't usually do this with any real control. Once the LED reaches a specific current, the mosfet switches on like an avalanche. This is why these devices are also called optical solid state relays.
The FET version has better high frequency (RF range) AC performance as the Mosfet versions have built in 2 back to back mosfets for AC signal applications. As the protection diode in each mosfet may snap on through the drain-source capacitance when the voltage first swings in the opposite way when the switch is open. (This is so minor and happens potentially only once due to a charge held by only a few pf. If you can detect this, you must be relying on impedances in the multi megaohm region at RF frequency.) There is no such effect in the FET style optocouplers.
In other words, then H11FXM has no problems, but it's on resistance is high and varies with led current.
Say 10ohm at 1ma led current, or, 1 ohm at 10ma led current.