I can turn on/off an AO3400 even across my body by touching the gate with my hand and the +/- pole of a 3V battery with the other, and it stays on even if I leave my finger by the gate capacitence. Now I want to invert a reed relay's output for a door open detector, because the MCU in deepsleep mode still consumes about 80uA on a LOW input thanks to the pull-up resistor. With this method the batterylife will be extended a little. So my question is, will this work on a long term or maybe the MOSFET won't open after a while. I read triacs getting less sensitive with time, but I didn't find anything about MOSFETs. Thanks for your answer.
ESD is a major concern if you have a FET gate directly exposed. Static discharge is likely to erode the device each time you tough it and eventually it will fail. You can get protection devices intended for USB data lines that would minimise this effect. That said, the threshold voltage of a FET does not tend to vary with age, in my experience.