Transistor emitter-collector polarity

[saul]

Hi,

Using the method described in http://www.smallbearelec.com/HowTos/FuzzFaceFAQ/FFFAQ.htm I tried sampling a large bag of germanium transistors that I have, to check what is their leakage current and there is some transistor issue that I don't understand.
When the base is open (in leakage current testing) why is the battery polarity important? When measuring the leakage I got different readings after swapping the battery polarity. What is the physical difference between the collector and emitter in a bipolar transistor?

Thanks,
Saul

[R.G.]

... there is some transistor issue that I don't understand.
When the base is open (in leakage current testing) why is the battery polarity important? When measuring the leakage I got different readings after swapping the battery polarity. What is the physical difference between the collector and emitter in a bipolar transistor?

In general, they are doped differently and are optimized for completely different things. This is less true the earlier the transistor was made. The earliest germaniums had no intentional difference between the two junctions, largely because (a) no one knew it made a difference and (b) even if they knew, they had no ability to make them different. After planar processing was invented by Fairchild, things got under way and it became possible to make the base-collector junction different from the base-emitter.

In all modern transistors, the base-collector is doped to withstand the transistor's main voltage and currents. The collector base junction in modern silicons will withstand MORE than the rated BVceo because leakage into the base region gets multiplied by the beta and the transistor dies suddenly at high voltages. The base emitter is doped to about 5-9V, and optimized to be easy to get charge carriers into and out of to turn the transistor on and off. It has a much higher reverse leakage than the collector base.

And that is the answer to your question. If the collector-base and base emitter are intentionally different, the collector base will have a lower leakage because that's what the designers were trying to do. I say "intentionally" because the earliest transistors were literally assembled with sandwiches of indium/germanium/indium and baked on what could have been a cookie sheet until the indium diffused into the germanium. Quality control was something like a stopwatch and a thermometer. Only later did we as a race discover that cleanliness, diffusion rates, planar processing, masking, etc made a difference.

So yes, it makes a difference, and that's why. Ask questions where I've confused things instead of clearing them up.