将下列短文翻译为中文: (1) Three physical principles make a useful first description of the way a bipolar transistor works. (2) (a) The externally applied voltages control the ratio of the carrier densities on the two sides of each depletion layers. (3) (b) Away from the depletion layer the regions are neutral. (4) (c) Hence carriers move away from the junctions after injection by diffusion and the action of any fields built in by variable doping. (5) The applied voltages control the carrier flow solely through the injection process. (6) We can see these ideas from three aspects: the variation along a line from emitter to collector of the potential and the band structure, the fluxes of holes and electrons, and the carrier densities.