# Why does the emitter current increase with an increase in collector to base voltage in the common base configuration of a BJT?

It is said that because of the Early effect that with an increase in collector to base voltage there is decrease in base width due to which $$\\alpha\$$ (the current gain) increases due to lesser recombination of holes and electron, so with increase in $$\\alpha\$$ emitter current should decrease according to this equation:

$$I_E=\frac{I_C}{\alpha} \, .$$

Also, the concentration gradient of minority charge carriers in the base region increases, due to which more majority charge carriers (of emitter region) come from emitter to base region and hence increase the emitter current.

Both of these effects are in contradiction to each other, so does it happen that the latter effect is greater than the former?

• Please include a circuit diagram with all circuit questions. Dec 26, 2016 at 19:18
• Your present a false dichotomy. Base-width modulation due to $V_{BC}$ (aka Early Effect) is a latter modification to the Ebers-Moll model, but prior to the development of the Gummel-Poon model in 1970, which then included variation due to $V_{BE}$ (aka Late Effect.) Gummel-Poon (and its modifications, as well as the VBIC model and still more versions since) includes a more complete treatment of the physics. I'd recommend (relatively inexpensive) books such as Millman's 1979 edition of "Microelectronics" for a detailed walk-through.
– jonk
Dec 27, 2016 at 5:59
• Both descriptions state a larger collector potential yields a larger collector current Oct 1, 2018 at 2:33
• Possible duplicate of BJT gain.Width of base region vs collector current Dec 21, 2018 at 6:11
• Kartik, There is no better paper to read than J. M. Early's "Effects of Space-Charge Layer Widening in Junction Transistors", 1952.
– jonk
Dec 15, 2021 at 6:02