Transistors: Collector vs Emitter (where does the symmetry get broken)

[aeftimia]

I thought I had a good understanding of transistors, but I have seen articles on how to tell the collector from the emitter. I had always pictured transistors as a perfectly symmetrical device. Either side could be the collector, and the opposite side was the emitter. Clearly, there is some subtlety I am missing.

So what physically makes the collector and emitter distinct?

 

[yungman]

iThey is no way it is symetrical. Collector is very lightly doped and emitter is heavily doped. That's the reason collector base junction can take a larger reverse voltage. You can make the device work like a transistor if you reverse the C and E, but it don't not work very well.

 

[Bassalisk]

If it was perfectly symmetrical, both active and reverse bias regions would be identical.

Collector is very lightly doped because, after reverse biasing the base-collector, you can "collect" charge carries over that very large depletion region.

Remember: lighter doping, wider depletion region.

 

[uart]

If it was perfectly symmetrical, both active and reverse bias regions would be identical.

Collector is very lightly doped because, after reverse biasing the base-collector, you can "collect" charge carries over that very large depletion region.

Remember: lighter doping, wider depletion region.

The lighter doping (and hence wider depletion region on the collector side of the CB junction) is, as yungman said, to increase the reverse breakdown voltage of the CB junction.

Even low power general purpose BJT's are usually designed for a CB reverse bias voltage of around 30 too 40 volts, with higher voltage devices requiring several hundred volts or more. Remember that under normal forward bias (active region) operation the CB junction is in fact reverse biased. If the same doping was used on the collector as was used on the emitter, most transistors would be unable to operate at much more than about 4 to 5 volts (Vce).

For the emitter, the higher levels of doping are used to increase the injection efficiency. For example, in an npn transistor, the heavy n+ doping in the emitter causes the vast majority of current across the BE junction to be electrons emitted from the n side (the emitter) rather than holes emitted from the base. Any holes emitted from the base are just "wasted" current, contributing to base current but contributing nothing collector current, and hence greatly reducing current gain.

 

[es1]

It's helpful to look at the cross section of an actual BJT like in the image below:
http://www.allaboutcircuits.com/vol_3/chpt_2/8.html#03302.png

As you can see there are asymmetries in both the geometry and the doping.