2N2222A - pins are EBC or CBE?

[CaptDon]

To the O.P., Many digital multimeters today use a somewhat standard method to measure things using the DIODE position. It is often a constant current source set to 1 milliamp. The current alone is deemed insufficient to damage solid state devices. Typical diodes will read something between .625 to .685 (or maybe 625 to 685 depending on your meter) and that is the actual measured voltage drop across the junction when forward biased. When reverse biased it will generally read like an open circuit although old germanium transistors often show a lot of leakage. Some transistors have internal diodes that may throw you off. Mosfets often have a body diode from S to D and that will throw you off unless you expect it. As you are testing mosfets they may appear shorted S to D if there is a residual gate charge remaining. Be aware that good meters limit the open circuit test voltage to 1vdc up to maybe 1.5vdc, this will not harm 99.99% of solid state devices. There are SOME meters that use as much as 9vdc and I have one that is 22vdc. This will destroy many fets and mosfets by breaking down the sensitive gate layer so be careful and know your meters O.C.V.!! As other posters have stated the forward biased E/B junction will tend to be a few millivolts higher than the B/C junction. It varies with design. I have good transistors with identical readings, although generally 'identical readings' indicate a transistor with some sort of failure mode present, usually shorted E to C or very high leakage and low gain. Meters with 1vdc O.C.V. won't test LED's very well. Cheers!!

[TimFox]

A simple test using not much more than a DMM:
First, using either the diode mode or a resistance mode, identify the two PN diodes in the NPN structure and their forward direction.
Then, use a 9 V battery in series with, say, 40 k ohms (anywhere from 30 to 50 would do).
Connect that to the 10 megohm input of the DMM in voltage mode, and you will measure close to 9 V.
Connect that to each of these diodes:
If you measure < 1 V, you are in the forward direction and should reverse the connections to the diode.
If you measure close to 9 V, you have found the C-B diode, whose breakdown voltage should be larger than 9 V.
If you measure between 5 and 7 V, you have found the B-E diode, and are applying between 100 uA and 50 uA (with 40 k)--disconnect quickly.
Really old germanium transistors had BE breakdown voltages comparable to the CB breakdown, but virtually all planar silicon transistors have this low breakdown voltage as a feature.

[Zero999]

If the diode check is inconclusive. I recommend the h_FE test, over breakdown voltage, because it doesn't damage the transistor and a much lower voltage is more convenient. All that's needed is a resistor 10k to 1M and a 1V to 5V power supply. Connect the base to the collector/emitter via the resistor, with the power between the collector and emitter and measure the current draw. The circuit will pull the most current, when the resistor is connected between the base and collector and the emitter is negative. Don't leave it connected for too long, as the h_FE increases with temperature which means the transistor can get too hot. It's better to start with lower voltages/higher value resistors.

[RoGeorge]

I recommend the h_FE test

That would make a very clean distinction (in case measuring Vbe and Vbc doesn't).   

I didn't try, but instead of a 3V power supply those 3V could be as well coming from the DMM set on measuring diodes.  Q1 will show open, and Q2 about 1V (on the DMM set on diodes range).

[David Hess]

I just did some testing and had no problem distinguishing the emitter from the collector by the emitter's slightly higher voltage drop.  It may be only 5 millivolts, but the readings were always stable.

A reverse breakdown test risks damaging some transistors, and some rare transistors have a high base-emitter breakdown anyway.

If the difference between forward-bias voltages is only 5 mV out of 600 to 700 mV, how do you know which one is which?  Certainly not by the datasheet.

I tested a bunch where I already knew the pinout, and the emitter junctions all had higher voltage drop than the collector junctions.