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How to Tell the Transistor or FET - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

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.


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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