A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as “Zener knee voltage” or “Zener voltage”. A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged. A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage.

Advantages and Applications of Zener Diodes

  • A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as “Zener knee voltage” or “Zener voltage”.
  • Zener diodes are also used in surge protectors to limit transient voltage spikes.

Limitations of Zener Diodes

  • Zener diodes are limited to low current applications.
  • Zener diode’s regulated voltage varies within specified tolerance.
  • Zener regulator operating region is limited by series input resistor. For large variations in input voltage or load current results circuit damage.

Alternatives to Zener Diodes

  • A fair alternative to a Zener diode can be produced by placing several standard forward biased diodes in series; each will take on a voltage of approximately 0.7 V (Si) or 0.3 V (Ge) and the voltage across the series combination of n diodes will be 0.7n or 0.3n with a large dI/dV.
  • For high power applications, linear regulators or DC-DC converters can be used as alternates to Zener diodes.

Theory of Zener Diode

  • A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.
  • The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.
  • Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.
  • In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperature-critical applications.
  • Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.
  • Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode’s reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value.
  • In this circuit, a typical voltage reference or regulator, an input voltage, UIN, is regulated down to a stable output voltage UOUT. The intrinsic voltage drop of diode D is stable over a wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated like this, Resistor R is used to limit current through the circuit.
  • In the case of this simple reference, the current flowing in the diode is determined using Ohms law and the known voltage drop across the resistor R. IDiode = (UIN – UOUT) / RΩ
  • The value of R must satisfy two conditions:
    • R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6 device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then UOUT will be unregulated, and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher than nominal and could rise as high as UIN). When calculating R, allowance must be made for any current through the external load, not shown in this diagram, connected across UOUT.
    • R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX, then IDVB < PMAX.
  • A Zener diode used in this way is known as a shunt voltage regulator (shunt, in this context, meaning connected in parallel, and voltage regulator being a class of circuit that produces a stable voltage across any load). In a sense, a portion of the current through the resistor is shunted through the Zener diode, and the rest is through the load. Thus the voltage that the load sees is controlled by causing some fraction of the current from the power source to bypass it—hence the name, by analogy with locomotive switching points.
  • Shunt regulators are simple, but the requirements that the ballast resistor be small enough to avoid excessive voltage drop during worst-case operation (low input voltage concurrent with high load current) tends to leave a lot of current flowing in the diode much of the time, making for a fairly wasteful regulator with high quiescent power dissipation, only suitable for smaller loads.