With no external voltage applied there exists an insulating depletion region between all of the junctions of dissimilar materials. This is formed when the dissimilar materials first contact: the free carriers in one material (either positive or negative) diffuse into the second material and recombine with an opposite carrier. Since the free carriers have crossed the junction the dopants (e.g phosphorus & boron) which these carriers originally belonged to become ionised. This creates electric fields which prevent further diffusion. This area of ions is void of free carriers, and is thus an insulator. This insulating layer is known as the depletion region. The electric fields (shown by green arrows) in a BJT with no external voltage are shown below: This is the part I don't understand However, if a voltage is applied between the base-emitter and collector-emitter terminals, a current can flow between these terminals. But why? what has happened to the depletion regions which originally stopped the current from flowing? Thanks!