IRL520N MOSFET say WHAT??????

Why when i give the gate 5v the mosfet switches on and STAYS ON? when i disconnect the 5v it stays on? is it not supposed to go off? like a transistor? or is this the big difference between a transistor and a mosfet

Without seeing your circuit it is hard to tell, but the input to the MOSFET is a capacitor with very low leakage. When you remove the 5 V, and have no other path to the source, the device can stay on indefinitely - it becomes an EEPROM, if you will.

Provide a 1K ohm or so resistor to ground and it will turn off like you expect.

KeithRB:
Without seeing your circuit it is hard to tell, but the input to the MOSFET is a capacitor with very low leakage. When you remove the 5 V, and have no other path to the source, the device can stay on indefinitely - it becomes an EEPROM, if you will.

Provide a 1K ohm or so resistor to ground and it will turn off like you expect.

awesome ok, thats so cool, i never actualy "really" understood what people meant when they said the gate behaves like a capacitor. You see you will never learn untill u sit down with a breadboard and tester!!!!!!!!!!! i love this stuff! XD

A MOSFET Gate doesn't "behave" like a capacitor, it is a capacitor. The MOS means Metal-Oxide-Silicon. You have a metal electrode on one side, a silicon dioxide insulating layer between, and the silicon semi-conducting channel. Any DC leakage is due to an imperfection in the device. And now-a-days, those leakages are very small.

The gate leakage for the IRF520N is specified as a maximum of 100nA at 16 V. I would suspect that in reality it is much lower. R=E/I so if it did leak 100nA at 16 V it would be equivalent to a resistance of 160 M ohms

Put another way capacitors store charge efficiently.

These days most MOSFETs are actually polysilicon-oxide-semiconductor, but the
important point is its conductor-insulator-conductor, which is a capacitor. In a
MOSFET the electric field(*) in this capacitor can also form a coducting channel in the
semiconductor between drain and source, thus connecting them together, so its
a capacitor with one plate that’s quite clever!

(*) hence “field-effect”.

MarkT:
Put another way capacitors store charge efficiently.

These days most MOSFETs are actually polysilicon-oxide-semiconductor, but the
important point is its conductor-insulator-conductor, which is a capacitor. In a
MOSFET the electric field(*) in this capacitor can also form a coducting channel in the
semiconductor between drain and source, thus connecting them together, so its
a capacitor with one plate that's quite clever!

(*) hence "field-effect".

Very nice. 8)

I knew someone was going to bring up the fact that the "M" in MOSFET isn't technically correct anymore!

KeithRB:
I knew someone was going to bring up the fact that the "M" in MOSFET isn't technically correct anymore!

But missed that the "T" in MOSFET does mean that MOSFET = TRANSISTOR, really. Answering question about if one should choose a MOSFET or a transitor just makes me cringe. =(