Electrical – Faulty P-CH MOSFET


Last month I bought a couple of IRF5305 (click for datasheet) on Ebay. Today I soldered a couple of them in my PCB and had some issues with them. My FET schematic looks like the following:


simulate this circuit – Schematic created using CircuitLab

According to the datasheet the gate-source threshold voltage lies somewhere between 2V and 4V. Now when I put logic '1' on the gate the gate-source voltage is around 1.7V. In this situation I would expect that the MOSFET will not conduct. However, in practice it did conduct and I could even power a small blue LED as load.

Now I played a little bit with the voltage on the gate, and it seems to be turned off once I put around 3.8V on the gate. So right now I'm guessing that either I'm doing something wrong, or that these 'chinese' MOSFETS are out of spec. I hope someone can help me out here.

Best Answer

A MOSFET does not suddenly turn on or off at the Vgs(th) value. Look more closely at the datasheet and it shows that at Vgs = -2V (the minimum), there is in fact 250 microamp of drain current (the test condition).

Vgs with test condition

What you are seeing is conduction in the sub-threshold region.

A zero current (or close to it) of -1.2V seems reasonable for a device rated at -2V.

You should always set Vgs to zero volts (for an enhancement mode device) to guarantee the part is fully turned off. The circuit below does the job. Note the control is inverted.


simulate this circuit – Schematic created using CircuitLab

Edited for MKeith's comment.

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