It is a reverse-biased heavily-doped silicon (or germanium) P-N junction diode which is operated in the breakdown region where current is limited by both external resistance and power dissipation of the diode. Silicon is preferred to Ge because of its higher temperature and current capability.
when a diode breaks down, both Zener and avalanche effects are present although usually one or the other predominates depending on the value of reverse voltage. At reverse voltages less than 6 V, Zener effect predominates whereas above 6 V, avalanche effect is predominant. Strictly speaking, the first one should be called Zener diode and the second one as avalanche diode but the general practice is to call both types as Zener diodes.
Zener breakdown occurs due to breaking of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage. It produces an extremely large number of electrons and holes which constitute the reverse saturation current (now called Zener current, Iz ) whose value is limited only by the external resistance in the circuit. It is independent of the applied voltage. Avalanche breakdown occurs at higher reverse voltages when thermally-generated electrons acquire sufficient energy to produce more carriers by collision.
Figure 4.1.1.19
(a) V/I Characteristic
A typical characteristic is shown by Fig. 4.1.1.19 in the negative quadrant. The forward characteristic is simply that of an ordinary forward-biased junction diode. The important points on the reverse characteristic are :
Vz = Zener breakdown voltage
Iz min = minimum current to sustain breakdown
Iz max = maximum Zener current limited by maximum power dissipation.
Since its reverse characteristic is not exactly vertical, the diode possesses some resistance called Zener dynamic impedance. However, we will neglect it assuming that the characteristic is truly
vertical. In other words, we will assume an ideal Zener diode for which voltage does not change once it goes into breakdown. It means that Vz remains constant even when Iz increases considerably.
The schematic symbol of a Zener diode and its equivalent circuit are shown in Fig. 4.1.1.20(a). The complete equivalent circuit is shown in Fig. 4.1.1.20 (b) and the approximate one in Fig.
4.1.1.20 (c) where it looks like a battery of Vz volts.
The schematic symbol of Fig. 4.1.1.20 (a) is similar to that of a normal diode except that the line representing the cathode is bent at both ends. With a little mental effort, the cathode symbol can be imagined to look like the letter Z for Zener.
Figure 4.1.1.20
(b) Zener Voltages
Zener diodes are available having Zener voltages of 2.4 V to 200 V. This voltage is temperature dependent. Their power dissipation is given by the product Vz Iz... maximum ratings vary from 150mW to 50 W.
(c) Zener Biasing
For proper working of a Zener diode in any circuit, it is essential that it must 1. be reverse-biased;
2. have voltage across it greater than Vz;
(d) Diode Identification
Physically, a Zener diode looks like any other diode and is recognized by its IN number such as IN 750 (10 W power) or IN 4000 (high power).
(e) Uses
Zener diodes find numerous applications in transistor circuitry. Some of their common uses are :
1. as voltage regulators;
2. as a fixed reference voltage in a network for biasing and comparison purposes and for
calibrating voltmeters;
3. as peak clippers or voltage limiters;
4. for metre protection against damage from accidental application of excessive voltage;
5. for reshaping a waveform.
