Re: What fails a FET?


Andrew wrote:
lemonjuice wrote:
On 25 Oct 2006 13:34:03 -0700, "Andrew" <xxragexx@xxxxxxxxx> wrote:

I have had a circuit working for some time now that has worked great.
A particular portion uses a 2N7000 FET to invert a 5V signal, to 12V
(inverted). The input is from a servo driver, outputting about 4.8V,
directly to the gate. At the drain is a 10k resistor connected to the
power supply, which is ~11V to ~14V depending on battery charge. At
the resistor/drain node, the output is fed as clock to some CMOS chips.
This clock feed has a 470pF filtering capacitor to ground also, if
that is significant. The source is connected directly to ground.

At any rate, this setup has worked great, but I have since done another
revision of the board to clean up a few areas that should reduce noise.
No changes were made to the schematic or even component placement,
just routing.

Within the first hour of operating this new board (the old boards, 4 of
them, have at least 40 hours each, probably alot more on some of them,
without any single problem), the transistor I described earlier failed.
When viewing the (usually 4.8V peak) signal that is being inverted, at
the point that it enters my board, it was about 100mV peak. The square
nature of the signal was in tact, but the voltage was VERY low, so low
that it would not trigger the FET.

I measured the resistance between the Gate and Source and found that it
was low - about 150 ohms +/- 10 ohms. I replaced it, and the circuit
worked great again, for about another 20 minutes, then, the same thing
happened!! Measured the resistance of the transistor (which, in a
"good" circuit measures in the many many mega ohm range) and it was
again, in the 150 ohm range. This explains the 100mV peak, anyway.

So, I switched to a different board (same new rev, just a different
physical board). During comparisons of the "bad" board and "good"
board, somehow the second board's transistor blew too! Same problem.
I replaced it, and decided to run it with a new servo driver board
also, and now, for 2 hours it has been perfectly fine, but needless to
say I am skeptical. I am trying to find a cause for the failure of
this FET and I can't find one.

So, that brings me to the point of this post. What are things that can
cause a FET to fail? My guesses are over current the drain to source,
or over voltage the gate to source. Those are my guesses anyway, but
both are pretty impractical as the drain source current is limited by
the 10k resistor (and at 12V, is way way way under the 800mA rating at
Vgs = 5V), and the Vgs can be up to 20V and is at around 5V. Is there
a way to tell what would cause the gate to source resistance to fail in
the way I described?

My guesses as to the cause of the failure are that a) there is a
problem with the first board I tested, or b) there was some problem,
unrelated to my board, from the input 0-4.8V servo driver board that
blew the transistor. The power supply to the servo driver board is 6V,
though, so I don't know how it would get above 20V, which is the max
rating for Vgs.

Anyway, how can you kill a FET, and which way of killing one (if any)
will result in a very low gate to source resistance?

Thanks in advance.

Antistatic soldering stations?
ESD protection and overvoltage protection?
Clamping using reverse biased zeners will lower performance but it
will protect so will decoupling and the like.
Many things ... can kill a FET ... they like BJTs can melt. A
touch of a finger on the gate to internal ruptures can kill them...
High temperatures can open up a lead bond in a circuit leading the
circuit to fail then someone may try testing a FET... but that is
something really to avoid your very last shot because you can SO
easily kill it. As said above the thin oxide layer at the gate
electrode ensures high input impedance but also makes it vulnerable to
ESD. You need a low voltage ohm meter when testing it plus it has to
be set to the highest resistance reading.

lemonjuice

Thanks all for your input. I'm still a little new to effectively using
transistors and the things that can happen with them in the "real"
world. I didn't take any ESD care whatsoever when handling the
transistors and when soldering. It is poor practice that experience
has never forced me to change. Guess I've been lucky so far. I didn't
realize transistors were so sensitive to ESD. This could certainly be
what happened, and would explain what happened to the transistor on the
board that failed before any minutes/hours were even put on it (besides
testing - some of which was done with a DMM). I have plenty of room to
try a 10k resistor from gate to source (which would be gate to ground,
in my case) if the working board doesn't hold up. I'll be testing more
this week so I'll try to update everyone.

I'm curiou, though, is it possible to damage a transistor with ESD
while handling/soldering, then install it, have it work perfect for a
few hours, then fail suddenly because of the original ESD damage prior
to/during install? Or would a failure like this be unlikely to be ESD
and more likely to be a circuit problem?

I really don't think I am over-voltaging the gate. With 4.8V max going
in now, a 20V max Vgs, and no noise to speak of, I find it unlikely
that it was over-voltaged, but I could be wrong.

Thanks again for all your help!

One more thing I thought of, now that we are on the ESD topic (sort
of). With the current board (as opposed to previous) I am now using a
foam packing type insert to press the board against its plastic case,
instead of letting it free float. The entire plastic box that houses
the device can undergo some serious vibration. Is it possible that
this foam pad is generating ESD or something? I don't know anything
about ESD, I'm just thinking outloud. It is contacting the bottom of
the board, where there are no compontents, but all through hole
connections and vias (the transistor in question is through hole) are
being contacted by the foam piece.

Any chance this is related? Or very unlikely?

.

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