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as far as i know hfe is parameter which is decided by the designer and that comes out more or less by the fab process. Since the base width is the decisive factor i think it could be factor as its difficult to control its width to perfection.
So the prob is the thinner base becomes more imperfect the device will be.. even if the fab proces also improves
this was jus a interpretation..
please confirm its correct..
(1) smaller Wb (base width) or
(2) higher ratio of emitter to base doping densities
will have higher βf or hfe.
from (1), it explain why npn has higher beta compare pnp. Wb of npn is fixed by process, Wb of pnp depends on how wide the designer draw it.
from (2), since its doping dependance, you may say it's from stastical distribution.