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Guest Post subject: How to tell the transistor or FET Posted:
Tue Jul 26, 2005 9:16 pm Dear members, I would like to know how
to tell TR or FET with SOT-23 Package in the PC Board. If it is TR,
how to tell NPN or PNP. If it was FET, Ntype MOSFET or Ptype MOSFET.
Top Guest Post subject: Posted: Wed Jul 27, 2005
11:53 am BJTs are easy: A BJT will look like two diodes (Base-Emitter
and Base - collector). Find the common lead. That's your base. If
the transistor conducts ('+' to '-') from the base to the other
two, it's an NPN. If it conducts the other way it's a PNP. If it
conducts both ways or neither, pitch it. Telling the difference
between the emitter and collector is a tad harder. If your multimeter
has a "transistor tester" the direction with the largest beta is
correct. The (forward-biased) resistance of the base-collector
junction is usually lower than that for the base-emitter junction.
The difference is quite small, though. Before discarding
them, be aware that some transistors have a diode between collector
and emitter, while others have resistors between the base and emitter.
Then there are digital transistors ... You can generally get
a good idea of a MOSFET's condition by making a simple resistance
measurement between the gate and source leads. The MOSFET's gate
oxide is quite thin and fragile, so if the MOSFET fails one of the
consequences will be that it will normally damage the gate oxide
(even if the gate itself wasn't abused by excessive voltage). A
failed MOSFET will normally measure <1k ohms between gate and
source. These failed MOSFETs will also normally exhibit low resistance
from drain to source as well. A good MOSFET's gate to source resistance
will measure somewhere in the megohms to infinity range.
There are exceptions to this however. Sometimes the failure (especially
in the case of power MOSFETs used in such a way that if they fail
large currents flow) is so catastrophic that internal fusing can
occur. Sometimes enough transient power can be dissipated in the
device to physically blow the plastic package apart, physically
separating one of the leads from the die. In these types of failures
the above mentioned resistance test read infinity even though the
device is definitely broken. In my experience these types of failures
are relatively uncommon compared to more mundane failures which
simply result in low resistance shorts between the three pins. When
they do fail like this it is often obvious since the plastic package
has literally blown apart or has otherwise suffered evident trauma.
In one case I found a 2N7000 MOSFET that failed by means of
a parametric shift. The drain to source became very leaky (comparable
to a 100k ohm resistor) even though the gate was fully intact. The
device still worked fine, besides the extra leakage. The leakage
current could be reduced by driving the gate with negative gate-source
potential, but still not completely turned off. I have no idea what
might have happened to this device to cause this failure. Nevertheless
I think this is a very rare failure mode which you shouldn't normally
need to worry about. Posted
11/12/2012
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