Electronic – Strange behavior from n-channel MOSFET


I'm using BS170 N-channel enchantment enhancement mode MOSFET and have the following circuit:

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The idea behind it is that the MOSFET should act as a simple switch and turn the LED on and off depending on the status of its gate.

Well, my problem is that the LED is always on, even when the gate is connected only to ground, or directly to ground with no resistor. I've tried this with several transistors. At one point, I've had the gate floating and (as expected), I'd get various levels of light from the LED when I touch the gate. Also some of them can be now turned OFF by connecting the gate to the positive side of the power supply and with some, the LED actually is brighter when I connect the gate to the ground than when the gate is floating. From what I've read, such behavior is expected from depletion mode MOSETs, right?

Could I have killed the transistor or did I get bad parts? How susceptible to static electricity are they?

Best Answer

They are pretty susceptible to static, so I'd try a couple more.
Obviously make very sure you have it the right way round.
If they are cheap e.g. eBay parts (or from anywhere - we got sent the wrong part just last week from Farnell) then check they are not actually PNP parts (the fact that it turns off when you connect the gate to the positive supply sounds suspicious)
Also try and check that they are real parts - see if you can measure any conductance between drain and source or gate and source.