1.9 Reverse biased/breakdown

Before we leave diodes, it would be worthwhile exploring someother modes of operation, as well as some specific applications which will be of interest. We said that when the diode wasreverse-biased (p-region negative with respect to the n-region) that the only current which flows is the reverse saturationcurrent, resulting from the few thermally generated minority carriers which can fall down (or up) the barrier ( ).

There is now a sudden onset of current after the avalanche breakdown voltage has been exceeded. Do not be confused intothinking that this "breakdown" means that the diode has been damaged. The process of avalanching itself is notdestructive. But as you can see from , the diode current increases very rapidly once the breakdownthreshold has been exceeded. Thus, if there is not something in series with the diode to limit the maximum current through it,it could be damaged by overheating. Diodes in breakdown are used as voltage references (the voltage across them is more or lessindependent of the current running through them) but you will always find a series current limiting resistor used along withthem. Such diodes are called Zener Diodes (named after the grandfather of Will Rice's George Zener who graduateda few years ago...that is George did, not his grandfather) but the name is kind of a misnomer. The Zener Effect is also a reverse breakdown phenomena, but comes from direct fieldgeneration of extra carriers, rather than as a result of impact ionization. In truth, you can not tell the one effect from theother by looking at the diode I-V curve, and so all diodes used in reverse breakdown are called Zener Diodes. A circuit using aZener diode as a voltage reference is shown in .