The diodes designed to work in breakdown region are called zener diode. If the reverse voltage exceeds the breakdown voltage, the zener diode will normally not be destroyed as long as the current does not exceed maximum value and the device closes not over load.
When a thermally generated carrier (part of the reverse saturation current) falls down the junction and acquires energy of the applied potential, the carrier collides with crystal ions and imparts sufficient energy to disrupt a covalent bond. In addition to the original carrier, a new electron-hole pair is generated. This pair may pick up sufficient energy from the applied field to collide with another crystal ion and create still another electron-hole pair. This action continues and thereby disrupts the covalent bonds. The process is referred to as impact ionization, avalanche multiplication or avalanche breakdown.
There is a second mechanism that disrupts the covalent bonds. The use of a sufficiently strong electric field at the junction can cause a direct rupture of the bond. If the electric field exerts a strong force on a bound electron, the electron can be torn from the covalent bond thus causing the number of electron-hole pair combinations to multiply. This mechanism is called high field emission or Zener breakdown. The value of reverse voltage at which this occurs is controlled by the amount ot doping of the diode. A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage.
At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown. Since the Zener effect (avalanche) occurs at a predictable point, the diode can be used as a voltage reference. The reverse voltage at which the avalanche occurs is called the breakdown or Zener voltage.
A typical Zener diode characteristic is shown in fig. 1. The circuit symbol for the Zener diode is different from that of a regular diode, and is illustrated in the figure. The maximum reverse current, IZ(max), which the Zener diode can withstand is dependent on the design and construction of the diode. A design guideline that the minimum Zener current, where the characteristic curve remains at VZ (near the knee of the curve), is 0.1/ IZ(max).
Fig. 1 - Zener diode characteristic
The power handling capacity of these diodes is better. The power dissipation of a zener diode equals the product of its voltage and current.
PZ= VZ IZ
The amount of power which the zener diode can withstand ( VZ.IZ(max) ) is a limiting factor in power supply design.
When zener diode is forward biased it works as a diode and drop across it is 0.7 V. When it works in breakdown region the voltage across it is constant (VZ) and the current through diode is decided by the external resistance. Thus, zener diode can be used as a voltage regulator in the configuration shown in fig. 2 for regulating the dc voltage. It maintains the output voltage constant even through the current through it changes.
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| Fig. 2 | Fig. 3 |
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The load line of the circuit is given by Vs= Is Rs + Vz. The load line is plotted along with zener characteristic in fig. 3. The intersection point of the load line and the zener characteristic gives the output voltage and zener current.
To operate the zener in breakdown region Vs should always be greater then Vz. Rs is used to limit the current. If the Vs voltage changes, operating point also changes simultaneously but voltage across zener is almost constant. The first approximation of zener diode is a voltage source of Vz magnitude and second approximation includes the resistance also. The two approximate equivalent circuits are shown in fig. 4.
If second approximation of zener diode is considered, the output voltage varies slightly as shown in fig. 5. The zener ON state resistance produces more I * R drop as the current increases. As the voltage varies form V1 to V2 the operating point shifts from Q1 to Q2.
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