Breakdown voltages can range from 1 to 100 V.īreakdown in Zener diodes is caused by two different, yet similar, means: the avalanche effect and the Zener effect. Therefore, in breakdown, the magnitude of the current is determined by the other elements of the circuit (effective resistence, current sources, etc.). At this voltage, known as the breakdown voltage, V Z, the diode will enter breakdown and allow nearly any amount of current through. As the reverse bias voltage increases, Zener diodes continue to conduct a constant amount of current (the saturation current), until a certain voltage is reached. Zener diodes are simply reverse-biased diodes that can withstand operating in breakdown. p - n junction diodes that are designed to be used in breakdown are called Zener diodes. This characteristic is called breakdown, and it will typically destroy p - n junction diodes. For p - n junction diodes made from silicon, the saturation current is on the order of a nanoampere, 10 -9 A.Īs long as the diode is reverse biased, the saturation current is generally independent of the magnitude of V however, if V becomes too large, the diode will break down and allow virtually any amount of current through. The result is a small current directed to the left, called the saturation current. However, some electrons will make it across the p side without recombining and enter the space charge region, where they will be pushed across by the electric field. As this electron moves to the right, it is likely to recombine with a hole. The diffusive force is negligible because the density of minority carriers is low (by definition!). For a free electron in the p-type material, the drift force is to the right. Now consider minority carriers in their respective material. Increased recombination due to diffusion, and carrier migration away from the space charge region due to drift, will combine to produce the net effect of a wider space charge region. The holes furthest from the space charge region will drift towards the anode. The opposite will then happen from the electrons on the n side: the holes closest to the center will diffuse into the space charge region where they will recombine with the diffusing electrons. At the same time, the holes on the p side will experience a drift force to the left and a diffusive force to the right. The electrons further away from the space charge region will experience a greater drift force than a diffusive force and will therefore drift to the right. The electrons that are close to the space charge region will experience the greatest diffusive force, since they are closest to the place of diffusion these electrons will diffuse into the space charge region. As before, they also experience a diffusive force to the left. Therefore, in the figure above, free electrons on the n side (negative charge) will experience a drift force to the right (towards the positive cathode). If the applied voltage is V, then the total potential difference across the diode becomes V reverse bias = v 0 + V (where v 0 is the barrier potential). Under reverse bias, the n side is held at a higher voltage than the p side.
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