Help ! Why I can not make N-ch MOSFET switch to "ON" ?

I am just a new bird in Electronic test, I am developing the flying probe test program, encountering a headache that I can not make this N-Ch MOSFET switching to "ON", while other part number of N-Ch MosFET works, I don't know why ?

Flying probe machine give 10MA current from G to S; S connected with GND; D connected with test voltage. normally the FET should get "ON", but this not !

I test following two parts:
-the Not working N-ch FET is IRFL4315
-the working N-ch FET is 2N7002K

Could somebody tell me the reason ?

Thank you very much !

A Mosfet is turned on with a voltage from its gate to source, not with a current. What voltage did you use? The drain has a load connected between it and the positive supply voltage, didn't you use a load? What voltage did you use? What load resistance? Didn't you read the datasheets for the different Mosfets?

The gate to source voltage needed by a IRFL4315 Mosfet is 3V for all of them to barely turn on or 10V for all of them to turn on well.
The gate to source voltage needed by a 2N7002 Mosfet is 1V for all of them to barely turn on or 5V for all of them to turn on well.
Some of the Mosfets turn on well with a lower voltage, maybe your 2N7002 is a very sensitive one.

Audioguru said:

A Mosfet is turned on with a voltage from its gate to source, not with a current. What voltage did you use? The drain has a load connected between it and the positive supply voltage, didn't you use a load? What voltage did you use? What load resistance? Didn't you read the datasheets for the different Mosfets?

The gate to source voltage needed by a IRFL4315 Mosfet is 3V for all of them to barely turn on or 10V for all of them to turn on well.
The gate to source voltage needed by a 2N7002 Mosfet is 1V for all of them to barely turn on or 5V for all of them to turn on well.
Some of the Mosfets turn on well with a lower voltage, maybe your 2N7002 is a very sensitive one.

Click to expand...

Audioguru:
Thank you very much for your kindness to answer my question !
Because I am using flying probe machine to test assemblied PCB, for the FET test, flying prober uses 3 probes to contact FET's 3 pins, I can only select different current to apply to G and S pins, the current selection is 1ua,10ua,1ma,10ma,100ma. I can not set its Vgs directly. I tried all the current, none can make it ON.

You meantioned load resistor, flying probe machine uses anothr 2 probes to contact FET's D and S pin directly, D pin injected with VCC and S connected with GND. I don't know what the load resistor it is.

in this way, FET 2N7002 test works well just in 10ma G-S current, while IRFL4315 not work in all G-S current setting ( I also tried 100ma), I feel very strange and don't know why ?

Very sorry to bother you ! thanks !

Your "flying probe machine" feeds current into the base of a bipolar transistor, not voltage into the gate of a Mosfet. Then the hFE (current gain) of the bipolar transistor is tested.
The base-emitter voltage needed for a bipolar transistor is 1.5V maximum which is not enough for your IRFL4315 Mosfet but is enough for your 2N7002 Mosfet.

Audioguru said:

Your "flying probe machine" feeds current into the base of a bipolar transistor, not voltage into the gate of a Mosfet. Then the hFE (current gain) of the bipolar transistor is tested.
The base-emitter voltage needed for a bipolar transistor is 1.5V maximum which is not enough for your IRFL4315 Mosfet but is enough for your 2N7002 Mosfet.

Click to expand...

Audioguru：
I am really touched by your so fast reply to me !

Ok, I will go to machine next monday to have a further try based on your suggestions, hope I can find a way to solve it and I will come to you the results, thanks !

Sorry for my borhering and hope you have had a good xmas holiday !
Good night !