Due to an increase in reverse biasing potential across the C-B junction the Input characteristics graph is shifted towards Y-axis or 𝐼𝐸 increases. According to the dopping base is lightly dopped as compared to the collector. In N dopped collector majority of charge carriers are free electrons, so free electrons will try to move to the P dopped Base leaving immobile positive ions near the junction. As the collector is heavily dopped and due to the presence of more free electrons, free electrons in the collector will try to fill these immobile positive ions.
Due to this the width of the depletion region across the collector decreases and as the base is lightly dopped so the depletion region is shifted more towards the base and the effective width which is responsible for base current will decrease due to the increase in reverse biasing potential across C-B. As the effective width of the base will decrease then the base current will also decrease. With the increase in reverse biasing potential, the concentration gradient of charge carriers(holes) in the effective width of the base will increase and more charge carriers from the emitter will try to move to this area and 𝐼𝐸 increases. This effect is known as BASE WIDTH MODULATION.