(a) In a transistor, the base is made extremely thin to reduce the combinations of holes and electrons. Under this condition, most of the holes (or electrons) arriving from the emitter diffuses across the base and reach the collector. Hence, the collector current, is almost equal to the emitter current, the base current being comparatively much smaller. This is the main reason that power gain and voltage gain are obtained by a transistor. If the base region was made quite thick, then majority of carriers from emitter will combine with the carriers in the base and only small number of carriers will reach the collector, so there would be little collector current and the purpose of transistor would be defeated.