biasingcurrentsemiconductorstransistors
I know that due to Early effect in a npn transistor the effective base width reduces.However I can't understand how this will lead to an increase in emitter current.
Some sources say that the charge concentration gradient increases due to this Early effect and hence the increase in emitter current but how can the charge gradient increase since that is determined by doping.
. Can any one please explain it then.
Any help will be appreciated
Thank you.
Dave is correct... I'll try to clarify some more.
In an NPN:
BJTs are designed with light doping in the base and a very narrow base width to maximize the diffusion of the emitter current to the collector. As a result, the base current needed to develop the Vbe for a given rate of emitter current injection is very small compared to the emitter and collector current, and so BJTs have high current gain.
Here's an online reference that goes into some detail: Modern Semiconductor Devices for Integrated Circuits, Ch. 8
You seem to think that because the depletion area is large for the base collector junction that there must be a large field there. That is the opposite of how it works. The voltage difference between the emitter, base, and collector is dropped exclusively in the depletion region. Thus the larger the depletion region, the more spread out the field is and so the less strong the field is. In fact, back injection is one of the reasons for hetero junction transistors. In the non depletion regions, diffusion is the dominant mechanism. Thus by creating a large electric field that essentially acts like a reverse flow barrier, the BJT forces diffusion to occur. The reason the current flows from C->E (NPN) or E->C (PNP) is because the BJT uses different doping levels to create depletion widths for different purposes. The reverse biased C-B depletion region, being large, is dropped almost exclusively on the collector's side. That means there's very little length for diffusion. That small length means that the Collector becomes an excellent source of minority carriers for the base compared to the emitter, whose large electric field is almost insurmountable. When those minority carriers cross, they drive diffusion in the base. Once the base provides that minority carrier to the emitter, it can't get it back, so it must get another one from the collector.
In essence the collector current is dependent only on the amount that flows out of the emitter, not on the field, which is designed to drop almost entirely over the collector.
Please leave a comment on this if you don't understand, because I needed about 10 pictures to get this down in college.
Edit: The BJT picture to rule them all!
Best Answer
following up on comment of "LvW", the mechanism would predict the highest_Beta transistors will have the least Early Effect (have the flattest I_out).
And because there is a small region of bulk silicon between the base contact for metal and the actual base region that is sandwiched between emitter and collector, with the Ibase * R_bulk_base == voltage varying as Ibase varies, it sees the LOWER NOISE transistors (which have lower R_bulk_base) will have lower Early Effect.
I realize the parameters we use are Abstractions, are parts of modeling.
SUMMARY: I'm thinking high_beta low_noise transistors will be the best current sources.