When the PN junction plus the forward voltage, that is, the positive pole of the power supply P area, the negative then connected to the N zone, the applied electric field and the PN junction in the opposite direction of the electric field. Under this applied electric field, the equilibrium state of the PN junction is broken. Most carriers in the P region and most of the carrier electrons in the N region move to the PN junction. When the P zone hole enters the PN junction , And the original part of the negative ions and then, so that the P area of the space charge reduction.
Similarly, when the N-region electrons enter the PN junction, the partial ions are neutralized, so that the amount of space charge in the N region is reduced, resulting in a narrowing of the PN junction, i.e., the depletion region is thickened by the thickness, The carrier is increased, and the resistance is reduced. The reduction of the barrier greatly increases the number of carriers in the P and N regions that can cross the barrier to form a diffusion current. In this case, the drift current formed by a small number of current flows in the opposite direction to the diffusion current, which is small and negligible compared to the forward current. The current in the PN junction is determined by the dominant current. In the external circuit to form a current into the P area, known as the forward current. When the applied voltage changes slightly (such as O.1V), it can cause a significant change in current, so the current is rapidly increasing with the applied voltage. At this time, the positive PN junction appears as a very small resistance.