For a NPN bipolar transistor, what is the main stream of current in the base region?

due to diffusion of electrons

bcuz base has more concentration of holes and collector & emitter has low concentration of holes. so the main purpose of current in the base will be diffusion of holes bcuz diffusion occurs due to difference of concentration and goes from higher concentration to lower concentration

the Fermi level of metals is very near to conduction band or inside conduction band so the gap between conduction band and valence bend is near to zero
where as in semiconductor its near 4 e volts and in insulator its more than 5 e volt .