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I am new to electronics and I was playing around with MOSFETs. I have an IRFZ44N MOSFET from International Rectifier that is showing unusual behavior.

The gate of the MOSFET wasn't connected to anything, but current was flowing from drain to source. The amount of current flowing was about 0.7 mA which is enough to power a LED.

Suspecting that I somehow killed the MOSFET, I bought another one and the same thing happened. The new one also allowed 1 mA of current from drain to source without any gate voltage. The voltage between gate and source is 5 V in both cases.

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I have been trying to do this thing for a whole day and I am stuck. Are both of my MOSFETs damaged, or am I missing something crucial?

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  • \$\begingroup\$ Try connecting the gate to ground. Does the LED go out? If so then it's working just as the answers and comments say it will. \$\endgroup\$ Mar 4, 2023 at 12:29
  • \$\begingroup\$ @PeterJennings Connecting the gate to the ground made the LED shine more brightly \$\endgroup\$
    – izack
    Mar 4, 2023 at 13:19

3 Answers 3

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That is not unusual for a FET with a gate left floating. You can expect anything to happen when gate is left unconnected.

Don't leave the gates unconnected.

The gate may have accumulated charge so there is enough gate voltage to turn the FET on.

So basically, contrary to what you say, the FET is not without gate voltage. It clearly has voltage accumulated by touching or leakage currents. It's just that you did not apply any specific voltage in the circuit, so you don't know what the voltage is.

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  • \$\begingroup\$ when i connected it to the 555 timer output [5 volt] with frequency of 1Hz, ~50% duty cycle, during the on cycle of the 555 timer, the LED lights up fully. But during off cycle, I expect the LED not to light, but small amount of current is flowing through the LED like the above thing. What should I do? \$\endgroup\$
    – izack
    Mar 4, 2023 at 12:34
  • \$\begingroup\$ You have no schematics or don't show how the LED and FET and 555 are connected, so it's impossble to know what you should do. And that is a new and different question, and as this question is now answered, make a new question with enough info to be able to anwer it. For example, your FET seems to control a LED and the LED has no resistor, so it is unknown what prevents the LED from burning up. \$\endgroup\$
    – Justme
    Mar 4, 2023 at 13:14
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Are both of my mosfets damaged? or am I missing something crucial?

A floating gate will inevitably acquire charge because it's basically just a capacitor. Once a volt or two has become charged on the gate with respect to source, the MOSFET will begin to switch on. A few more volts of charge build-up and, hey-presto, the MOSFET starts to conduct heavily from drain to source.

Never leave the gate floating (except in exceptional circumstances) in case sufficient charge is acquired to cause gate-source voltage breakdown (circa 20 volts).

So, if you had 1 nA of leakage current into the gate and, the gate-to-source capacitance is 1 nF (fairly typical of many MOSFETs) you can use the following formula to estimate how quickly the gate-source voltage builds-up: -

$$I = C\dfrac{dv}{dt}\hspace{1cm}\rightarrow\hspace{1cm}\text{1 nA} = \text{1 nF }\cdot\dfrac{dv}{dt}$$

Hence, with 1 nA flowing it will take 1 second to acquire 1 volt between gate and source i.e. not very long to acquire enough voltage to fully turn-on the MOSFET.

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For an NPN type MOSFET To turn the MOSFET on, you need to raise the voltage on the gate. To turn it off you need to connect the gate to ground.

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