How will collector current be affected on increasing the thickness of base in a transistor?

Base controls the flow of majority charge carriers from emitter to collector. Base is made thin and hence, the measure of recombination of electrons and holes is minimum. Therefore, most of the holes  or electrons coming from the emitter diffuse across the base and reach the collector. Thus, the collector current is almost equal to the emitter current.

If the base is made thick, then most of the recombination of electrons and holes takes place in there and, the amount of collector current will enormously decrease as compared to the emitter current.