Transistors

BJT or Bipolar Junction Transistors are either PNP or NPN semiconductors.

Identifying Transistors

They are easy to identify by using the Diode Tester on my multimeter. I will need to find the base, emitter and collector.

In order to first find the base i take my positive probe and place it on a pin. I then alternate my black probe between the 2 remaining legs. If i don't get a positive reading i know that my positive probe is on the wrong leg. I then alternate legs until eventually i get a positive reading on both legs when i alternate the black probe. If i still dont get a reading i will then switch my positive probe for my negative probe and alternate my positive probe on the pins. Once i find the base, i can identify what type of transistor it is. Either PNP or NPN. To find this out (for example) if my positive probe is on the base and i get a reading when i alternate my negative probe on both legs I have a NPN transistor. This is because i have 2 negatives and 1 positive leg. If my negative probe is on the base and i alternate my positive probe on the other 2 legs and get a reading, i have a PNP transistor. Now that i have identified what kind of transistor i have, i can then work out which pin is the Emitter, and which is the collector. If i am using a NPN type transistor and i have a reading of 0.763V on one pin and 0.767V on the other, the pin with 0.767 is the emitter. This is because the emitter has a higher reading then the collector.

This is due to the disparity in doping concentration between the emitter and collector regions of the transistor. The emittor is a much more heavily doped piece of semiconductor then the collector, causing its juction with the base to produce a higher voltage voltage drop (ref http://www.allaboutcircuits.com/vol_3/chpt_4/3html)

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