Zener diodes are often used in monolithic integrated circuit IC design. Typically an NPN transistor emitter-base junction is used as a zener diode. The breakdown voltage of such a structure is in the vicinity of 5 to 10 volts. Such a diode will be controlled by its uppermost periphery, which occurs at the semiconductor surface under the passivating oxide. It has been found that surface effects act to alter the diode breakdown characteristic with time and tend to make the diodes noisy in the breakdown region.
From a circuit standpoint, it is often useful to employ a zener diode to produce a constant potential that is less than the transistor emitter-base diode breakdown. It would be extremely useful to be able to incorporate zener diodes having a breakdown in the 2 to 6 volt range into IC designs. Accordingly, many IC manufacturing processes incorporate a series of extra steps in which the zener diodes are produced. Some of these processes employ two-step epitaxial depositions with diffusions applied between depositions. Other processes involve extra diffusion steps to make the zener diodes. In these special process approaches care is taken to ensure that the zener diode is such that breakdown is confined to a region below the semiconductor surface. It is clear that any process employing extra steps is expensive and subject to yield reductions. Since there is more complexity, more things can go wrong and more process controls and testing must be employed.