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COLLECTOR region thickness in BJT

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captain_haddock

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Could anyone explain clearly why COLLECTOR region in BJT is thicker than the other region. thank you
 

The C-B junction is what has to stand off the max
working voltage. There is a max field before onset of
avalanche multiplication of leakage and, possibly,
base punchthrough if the depletion region on the
collector side pushes back too hard. Both need to
be prevented and this is a combination of doping
and distance.
 

The critical thickness in bipolar transistor is the base - the thinner the base, the faster the carriers transit the base, the higher the Ft.
Carrier transport in the base is not very efficient - it is usually diffusion, or drift in the built-in electric (or quasi-electric) field caused by non-uniform doping or by grading of Ge profile in SiGe HBTs.
But to have low base resistance, base has to be heavily doped.
To sustain high voltage, collector region has to have lower doping density.
As a result, base-collector p-n junction is asymmetric, with depletion extending mostly into collector region.
So, it has to be thicker (than say base) to allow for the depletion thickness to sustain the high applied voltage.
Collector thickness is not as critical for high-speed operation as the base thickness, since there s a very high electric filed in the depletion region (in the collector) with carriers traveling at the maximum, saturation velocity.

Emitter is not as critical as base, since contact to the emitter is at the top, over the whole (almost) emitter area, while the base contacts are located on the periphery.
In bipolar transistors, emitter is more heavily doped than the base, to have high injection efficiency and beta, while in heterojunction bipolar transistors (HBTs), where base bandgap is engineered to be lower than the emitter bandgap (using SiGe allow in the base), and base can be doped much more heavily than the emitter without impacting negatively the emitter injection efficiency.
 

in that situation how on earth "hole" is playing in the game.

My opinion thing is that we can disregard "hole" thing completely, but I can be grossly wrong since this is "bipolar" device.

timof said:
The critical thickness in bipolar transistor is the base - the thinner the base, the faster the carriers transit the base, the higher the Ft.
Carrier transport in the base is not very efficient - it is usually diffusion, or drift in the built-in electric (or quasi-electric) field caused by non-uniform doping or by grading of Ge profile in SiGe HBTs.
But to have low base resistance, base has to be heavily doped.
To sustain high voltage, collector region has to have lower doping density.
As a result, base-collector p-n junction is asymmetric, with depletion extending mostly into collector region.
So, it has to be thicker (than say base) to allow for the depletion thickness to sustain the high applied voltage.
Collector thickness is not as critical for high-speed operation as the base thickness, since there s a very high electric filed in the depletion region (in the collector) with carriers traveling at the maximum, saturation velocity.

Emitter is not as critical as base, since contact to the emitter is at the top, over the whole (almost) emitter area, while the base contacts are located on the periphery.
In bipolar transistors, emitter is more heavily doped than the base, to have high injection efficiency and beta, while in heterojunction bipolar transistors (HBTs), where base bandgap is engineered to be lower than the emitter bandgap (using SiGe allow in the base), and base can be doped much more heavily than the emitter without impacting negatively the emitter injection efficiency.
Click to expand...
 

I intentionally did not mention "electron" or "hole", because electrons' role in NPN bipolar transistor is the same as role of holes in PNP transistor, and vice versa.

To answer a generic question how holes (or electrons) playing a role in bipolar transistor would require a long explanation (that can also be found in textbooks).
Please ask specific questions if you want short and specific answers.
 

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