Weird/interesting FET problem

[hammil]

So I have this p-channel FET I got from Farnell (an IRLIB9343 to be specific) but I can't get it to work as I would have thought

I can't seem to confirm the pinout of the p-channel MOSFET in TO-220, so I just assumed it was the same as the n-channel, Gate Drain Source when viewed from the front.

I had an adjustable negative supply connected to the gate pin - from the output of an inverting amplifier, and had it at 0 initially. I then connected the source pin to ground... and the amplifier started to oscillate dramatically. Like, saturating either way. And it was conducting about 15mA through the gate pin to the source, ground.

However.. just shorting the amplifier's output did nothing. Which makes sense, since there was no voltage difference between them.

Now.. unless I've got the pinout wrong, which from some passive measurements I can't really see how it could be the case, the gate pin should be very high impedance and not conduct at all. I find it difficult to see how I could have broken it, since the maximum possible voltage I could have put across any of the pins is 15V, and the most current I could put through it was about  2A, which is well below the maximum rating in the datasheet.

Is this a defective part, perhaps? Either way, I'd be interested to know how it managed to cause the amplifier to oscillate like that.

[Icarus]

Source should go positive for this type of mosfet

[muvideo]

Hello, as for the oscillation, you should post the schematic, but in general the
gate of a mosfet is capacitive, and a capacitive load on the amplifier output can
easily make it unstable.

As for the part, it's the first time I see a datasheet without pinout, dont know
if it's funny or depressing, but surely it will be the usual GDS as you said.

Checking a mosfet with a dmm is an easy task, many words for a very easy
test sequence (for a P mosfet):

- put the dmm in diode test mode
- put the negative meter's lead on the source and with the positive lead touch
the gate, you should not see any conduction.
- hold the negative lead on the source and put the positive on the drain, here
you measure the S-D junction, it's like a normal diode, you will read 0.5-0.6V.
- swap meter leads: positive on the source and negative on the drain, no
conduction
- hold the positive lead on the source and touch the gate with the negative one
no conduction here, but doing this you have charged the Gate to the test voltage
of the meter, if the threshold voltage of the mosfet (check Vth) is low enough it will
stay in conduction for few seconds
- quickly move the negative lead from the gate to the drain, you should see some
conduction or even a dead short for few seconds.

Sometime the mosfet contains a protective zener that connects the gate to the
source, but usually it's in the datasheet part drawing, and this seem not the case
with yor part.

If your mosfet is gone, probably you will find a perforated gate, maybe it was subject
to excessive voltage gate to source, also static electricity can be a culprit, for example
touching the part when it's connected to a power supply not grounded.

Fabio.

Edit: for an N-channel mosfet the test is the same but with lead polarity swapped.
As for the current you measured, how did you was measuring it? If the amplifer
was in autooscillation it's normal that you have measured AC current flowing
trough gate and source, since as I said before the gate to source (and also to drain)
behaves like a capacitor.

[muvideo]

The pinout is on pg7.

http://www.irf.com/product-info/datasheets/data/irlib9343.pdf

Ops, i missed it, my apologies to IRF 

Fabio.