In the schematic attached, there's a PN junction with a source , the barrier potential , contact potentials on the P-side and on the N-side. indicates the barrier potential of the PN junction with no source connected, the barrier potential with this source present and 'a' is a fraction in . How can we interpret the whole circuit in the point of view of erergy-providing and energy-consuming? 1) There are voltage drops , . Does that mean a test positive unit charge will "loose" energy while going from the (+) side to the (-) side? If so, in what means does the test charge get it's energy? What kind of form of energy is it transferred to? 2) Across the depletion region from P to N-side, there is an electric field pointing to the P-side. I guess there must be a reverse electric field acting on a test positive charge for it to pass through the build-in field. Where does that reverse electric field come from? 3) If the external source > (the barrier potential of the PN junction with no source connected), will there still be a barrier voltage inside the PN junction? Thank you!