MOSFETs turn on without any voltage applied to the gate

Question

I have some MOSFETs that turn on without any voltage applied to the gate. They turn on even without the gate connected to the circuit; I tested them in a simple switch circuit using an LED:

I also checked them with a multimeter. It gives a reading only between the source and the drain, the current flows only from the source to the drain.

These are the part numbers: k2519, k1985, k3115, 2d0n60f, and irf740. There is many others with the same problem. I have checked all of the datasheets, they say they are n-channel MOSFETs, so I don't know what the problem is.

Here you can see the LED is on without the gate even connected (the green wire). The orange wire is the drain, the blue wire is the source.

Answer 1

The gate to the MOSFET is a capacitor. Capacitors store charges. If the gate previously had a small charge, that would stay, keeping the MOSFET on (or off), unless the charge is removed ("bled" off) by a connection to the circuit through a pull-down or pull-up resistor.

However, the circuit as-is might make a useful bottle detector! If the blue wire is connected to the gate,

• Get a plastic bottle, such as an empty medicine vial or empty, dry, soda bottle.
• Rub the plastic container on a piece of cloth, such as an article of clothing.
• Wave the bottle back and forth near that gate wire.

Voila, the LED, she brightens and dims... but beware of a spark from bottle to gate, lest the MOSFET, liala tamut, RIP.

Answer 2

Rule 1 for MOS: gotta tie that gate to a known voltage to get the result you expect.

The MOS gate is essentially one side of a capacitor. Without anything tied to it, it's a floating signal that can pick up stray voltage from, well, just about anything. So even though you haven't applied a voltage explicitly, your FET's gate nevertheless has a voltage on it.

If this behavior is a concern as you do your bench work, add a pull-down resistor to the FET between gate and source to bias the gate to 'off'. The resistor can be a pretty high value: 10k ~ 100k would work.

Also, check your drain and source connection. The pinout is:

• pin 1 - gate
• pin 2 - drain
• pin 3 - source

Rule 2 for MOS: The FET will conduct through its body diode if the FET is connected in 'reverse', regardless of the gate voltage. In this case for an n-FET, if Vsource > Vdrain + 0.5V or so, it behaves as a diode.

Try this:

• disconnect your MOSFET from your system.
• Tie gate to source.
• Test source to drain with a voltmeter, in both directions using 'diode' test.

If the FET is good, you should see conduction only when (+) is on source (reverse direction - body diode.) If you see continuity in both directions, congratulations, your FET is toast.

Answer 3

That's normal and even expected if you left the gate unconnected, so there is no problem. It just does not work the way you expect it to work.

FET gates must not be left unconnected.

The gate has extremely high impedance and any leakage currents and electric charges can push the gate voltage high enough for it to turn on and conduct.

If you want the FET to stay off, add a reasonable sized resistor like 10k from gate to ground to keep the gate voltage discharged.

Answer 4

You have wrongly connected Drain and Source. Simply swap the connection, and if everything else is wired correctly - including gate pull-down resistor - then your circuit will work properly.

Answer 5

Remember that a mosfet is controlled by the gate-source voltage. To keep an NMOS (enhancement type- the usual) off, connect a resistor between gate and source pins. Then there won’t be any confusion. No worries about what’s ground etc.

To identify which is drain and which is gate, use a multimeter in diode mode. Once you find a combination of two pins that shows typical silicon diode forward voltage (0.6-0.7V), the pin you touched with the red (+) probe is the source, and the one touched with the (-) probe is the drain. That’s true for NMOS. For PMOS, the drain and source are swapped vs. the foregoing.

Answer 6

P-FET has inner (body) diode that connects its Drain to Source, while in N-FET, body diode connects Source to Drain. That may answer your question.