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How to tell the transistor or FET - RF Cafe Forums

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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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