BJT Type and Lead ID Procedure

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tracecom

Joined Apr 16, 2010
3,944
After spending much of the morning trying to understand how to use my DMM to ID the type and leads of bipolar junction transistors, I decided to document the procedure. It is attached for review and corrections. Thanks.
 

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Thread Starter

tracecom

Joined Apr 16, 2010
3,944
I should have said in my first post that the procedure is based on what I have read. My only contribution is to try to simplify and proceduralize the information that I found.

How did you come to the conclusion in step 5 that the higher voltage is the base-emitter junction?
I read it somewhere today, but I don't recall exactly where. I have been looking at information on various web sites, in some books that I have, and even some you-tube videos. I tried it out on a 2N2222A, 2N3904, 2N2907A, 2N3055, 2N3906, 2N4403, BC547B, BC546, and BD139; it worked every time. As I recall the explanation, it was that the emitter was doped heavier than the collector, and thus had more charge carriers.

Do you think it's wrong? Thanks.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
Sounds reasonable to me. I'll have to try it out.
Here's the data from my (limited) testing. As you can see, sometimes there is only a tiny difference, but in this data, it's 100% correct in identifying the emitter.

I hope you will (and others) will try it, and will post your findings.
 

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Ron H

Joined Apr 14, 2005
7,063
The B-E junction is typically smaller (in area) than the B-C junction. (From this web site.)

Assuming equal doping of the junctions, and assuming you multimeter applies a current that is approximately constant, then the current density in the B-E junction will be higher than that of the B-C junction, so the B-E voltage will be higher.
The doping assumption is probably invalid, but It also probably doesn't change the relative fwd voltage drops (for equal areas) very much.
 
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