# BJT gain.Width of base region vs collector current

We know that in active region Ic=beta(Ib),so it means that as Ib decreases the Ic should decrease proportionally,but the early effect states that as we increase Vce the width of the base region decreases hence less recombination ( less base current as less electrons travel into base ,for PNP) therefore the Ic increases.. Aren't these two concepts contradicting or am i missing something ?Explain for common emitter bias. Please help me clear my doubts.Thanks in advance.

## 1 Answer

At first, you have to realize that the emitter current - and accordingly the collector current - is controlled by the base-emitter voltage Vbe (see Shockleys famous exponential equation). I am aware that in some textbooks the BJT is described as a current-controlled device, but that is a simplified description which cannot explain all observable effects. The base current does exist (unfortunately) and it is taken into account for calculating the base bias circuitry - but it is kind of by product. (Barrie Gilbert: ...the base current is purely incidental - it is best viewed as a „defect“).

Now - what happens if the voltage Vce across the BJT increases? This increase results in a broader depletion layer between collector and base (Vcb increases). As a consequence, the space charge region between base and emitter (forward biased by Vbe) decreases. Therefore, the electrical field strength within this area is increased (constant base-emitter voltage) - and more charged carriers are able to move to the collector. This effect somewhat increases the collector current (and reduces the base current for a constant emitter current because of Vbe=const.).

Note: This happens even in the case that the base current is kept constant (Ib=const.). Hence, this is one of some proofs that it is the voltage Vbe which controls Ic.

• You say that the base current decreases but collector current increases ,doen't that defy the relation of Ic=beta(Ib)? – vrnsgh Apr 30 '16 at 8:57
• No - because B (resp. beta) are no real constant factors. Both parameters depend somewhat on Ic as well as on Vce. But because of the large tolereances the actual values are, normally, unknown - and we treat these factors as if they were constant. – LvW Apr 30 '16 at 9:00
• so you mean that more(less) recombination in the base region is actually decreasing(increasing) alpha (in case of common base) rather than changing Ib ?!?! – vrnsgh Apr 30 '16 at 9:08
• i have one more query, does more recombination in base region mean more base current value in the circuit (for compensation of the electrons which have recombined with the holes)?that means that base current is increasing but the collector current is decreasing (due to less holes from emitter reaching the collector)?this is my main doubt here! – vrnsgh Apr 30 '16 at 9:10