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heavy or lightly doped?

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forkschgrad

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why should the collector be lightly doped

most books tell us that emitter is the most heavily doped part of a BJT and heavy doping means lower resistance. Then, an emitter to resistor connection must be as short as possible to have a lower impedance. That is so, i think, to have a lower output impedance such as in a current mirror.

But someone told me that the emitter must be lightly doped to have a lower resistance. His sample is a PNP current mirror. Why should it be lightly doped? It confuses me. I guess he refers to lightly doped N type but heavily doped P type. Can anyone tell me the relationship between NPN and PNP current mirrors. Tnx.
 

is it correct to say that the emitter of an NPN is a heavily doped N type but lightly doped P type?
 

The emitter of an NPN is always highly doped and usually as thin as possible. The Base is lightly doped P for higher BVCBO BVCEO etc. The collector is heavily doped N at the collector contact then is graded as it goes to the base.
 
how about PNP? if we say that emitters must be heavily doped then a PNP has an emitter of heavily doped P type... how does resistance decreases if P type is dominant?
 

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