Hi i bought some MOSFETs IRFP2907 to be specific and i get some weird behavior from them. I have been testing them by connecting the drain to the positive side of a 9V battery and the source via a led and a 1K resistor to the negative. the LED lights up no matter what i connect the gate to, even if i leave it disconnected. this also works if i connect it the other way, source -> drain instead. is the MOSFET broken? i bought 2 of them and both display the same behavior.

The thing that bothers me is that if i touch the gate with a wire connected to the positive end of the battery it will stay "on" even after i remove the connection. it can only be turned of by touching it with a wire connected to the negative end. shouldnt it turn of as soon as there is no connection at the gate? i have tried running this in a circuit simulator i found online here is the circuit i have and it confirms that it should not have this weird memory effect.

any explanations/help? is the MOSFET i chose not a n-type? and would that even cause this? this is my circuit sorry didn't find the in-site editor

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    \$\begingroup\$ Sorry, I can't see that circuit. I don't have Java enabled in my browser. Draw the circuit using the built-in schematic system in this website instead, so everyone can see it. \$\endgroup\$ – Majenko Nov 22 '14 at 20:02
  • \$\begingroup\$ Your question is ambiguous, because in the first paragraph you say, "nothing works", and in the second, you are talking about how it works. \$\endgroup\$ – gbarry Nov 22 '14 at 20:07

The gate has a very high impedance and will store a charge, because it has some amount of capacitance there. If you wait long enough, the charge will drain off and the light will go out.

Memory effect, indeed. You just discovered how dynamic RAM (the kind that has to be refreshed) works!

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  • \$\begingroup\$ thank you that clarifies things, how would i go ahead to make a switch that would deactivate as good as instantly after gate potential has been removed? thanks \$\endgroup\$ – maxx Nov 22 '14 at 20:11
  • \$\begingroup\$ Connect the gate to ground to switch off. \$\endgroup\$ – Majenko Nov 22 '14 at 20:17

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