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we make Emitter to emit more electrons(for current flow)so,it is highly doped when compared to other two....purpose of collecter is to collect the electrons coming from Emitter via Base.Since Colectro duty is to only collect not emit it is moderately doped....Morever it should have less surface area when compared to that of Emitter to reduce power dissipation..
hope it pays...
Note: Base should always have less doping and less suraface area.The latter is to reduce the transit time (an important parameter which decides delay and frequancy of operation) and i am not clear abt the former...but if also having it's role
For less power dissipation collector area should be more. In other words, when area is more, resistance is less for a given current and hence power disspation is less.
I hope I do make a clear explaination in the following:
1) If the collector doping is very high (E.g. 1e19), the collector resistance is low. However, the breakdown voltage of the BJT will be very small, which is BAD especially for POWER AMPLIFIER application.
2) If the collector doping is very low (e.g. 1e15), the collector resistance is very HIGH, which limits the available maximum collector current. However, the breakdown voltage for BJT is INCREASED.
You may ask why the breakdown voltage depends on the collector doping?
The idea is, when you apply a collector voltage, most of the applied voltage dropped across the base-collector junction, which is in reverse-bias. And more importantly, the HIGHER the collector doping you have, more voltage drop acorss the BASE region [in case you don't understand, using simple potential divider, collector (collector doping > base doping) has small resistance comparing with base], which lowers the emitter-base barrier and makes the BJT go to the so called "punchthrough".
Therefore, a straight forward method is using moderately doping.
In real devices, the collector region near the base is lightly doped to increase the breakdown voltage. And there is a heavy doped collector "pick up" region to reduce the collector resistance.
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