Testing NPN and PNP Transistors.
Transistors can be tested for junction breakdown with the aid of a multimeter switched to low resistance range, but remember when doing this that the polarity of the multimeter leads are reversed on the ohms ranges (check this by measuring the polarity with another meter on DC volts and you will see).
Some digital meters do not supply enough current to forward bias the diode junction on their normal resistance ranges, they usually have a special switch position for diodes and transistors.
Measure the resistance between each junction in turn, both forward and reversed. If the connections of the transistor are not known, the base lead on a good transistor can be identified, since ONLY the base to the 2 other connections with the same polarity or current direction.
An NPN transistor when tested will conduct to either emitter or collector when the base is POSITIVE.
A PNP transistor will conduct to either collector or emitter when the base is NEGATIVE.
Repeat the tests a few times to get the feel of things then try to identify the leads of an unmarked transistor. With the exception of some POWER transistors there should be no conduction between the emitter / collector connection of either polarity with nothing connected to the base connection (Some power transistors leak a bit).
What does the test achieve?
Tests with an ohm meter can identify junction faults. A good
transistor will have a very high resistance between emitter /
collector with either polarity. Measurements between the base and the
other 2 connections should show one conducting direction (low
resistance) and one non-conducting direction (high resistance).
Any variation from this pattern indicates a faulty transistor with
either an open circuit junction or excessive leakage in either
direction (it's getting hot).
The origin of some of the text on this page is from the RSGB with additions by the web master
To Semiconductor Devices - Measuring Gains