SOLUTION

In an ideal transistor, it is assumed that the base width remains constant. But in a real bipolar junction transistor, the base-collector region is reverse biased, therefore, as the reverse bias across the base-collector junction is increased, the base-collector junction depletion-layer width increases.

in the case of PN junction, the junction potential is directly proportional to the square of the width of the depletion layer. In equation form, we express this as

V_j ∝ W²

From this, we conclude that the junction width can be varied by varying the applied voltage. 

In a bipolar junction transistor, we have its emitter-base junction forward biased. This voltage creates a narrow depletion layer between the base and the emitter. Similarly, its collector-base junction is reverse biased, which creates a wider depletion layer between the base and the collector. The depletion region reduces the effective width of the base and hence reduces the chances of recombination in the base. This, in turn, increases the collector current with applied voltage. This effect is called the Early effect. It is also known as the base-width modulation or base-narrowing effect as the base width gets modulated or narrowed with the applied voltage.