Zener diodes are silicon rectifiers in which the reverse current remains small until the breakdown voltage is reached and then increases rapidly with little further increase in voltage.

 The schematic symbol for a zener diode is shown in Fig.  11; a typical zener characteristic curve is shown in Fig. 12 in comparison with that of a rectifying diode.

The breakdown voltage is a function of the diode material and construction, and can be varied from one volt to  several hundred volts for various current and power ratings, depending on the junction area and the method of cooling. Zener diodes are useful as stabilizing devices and as reference voltage sources.

 Current stability in a transistor can be achieved by use of a compensating diode. Because the forward characteristic of a compensating diode is similar to the transfer characteristics of a transistor, the diode can maintain transistor bias voltages within 0.015 volt of a desired value despite supply-voltage variations up to 40 per cent and simultaneously compensate for a wide range of ambient-temperature variations.

On to NPN and PNP Devices

 

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