why the base of the transistor is lightely doped?

Question

is there is possibility of interchanging the collector & emitter terminals of the transistor

Answer 1

The base of a transistor is lightly doped so that the mean-free-path of the carrier can exceed the base thickness. The idea of the transistor is to have current injection into the base determined by the base-emitter potential, but that current should not flow out the base lead but travel directly into the collector. That will happen if the average length of path prior to recombination is longer than the travel through the base. The smaller number of doping sites accomplishes this.

In some transistors (grown-junction, eg) the collector-base and emitter-base junctions are symmetrical, and they can be interchanged. In most transistors, such as planar transitors, the juctions do not have the same geometry. Usually the collector-base junction is larger in area so as to collect the max amount of carriers from the base. In this case, the transistor will work very poorly or not at all if the collector and emitter junctions are interchanged.