Zener diode
When a typical semiconductor diode is reverse biased, a minimal amount of saturated reverse current flows through it due to the flow of minority carriers, i.e., thermal electrons and holes; This current is not dependent on the applied reverse bias voltage. When this reverse bias voltage exceeds a certain value, the reverse current suddenly increases rapidly. This phenomenon is called semiconductor diode breakdown. This results in a rapid increase in the diode’s dissipated power, i.e., the rate of heat generation, potentially damaging the diode.
The tolerance of some specially prepared semiconductor diodes is increased so that high reverse current in reverse bias does not damage the diode i.e., it does not burn out. This type of diode has some important practical applications. This diode is commonly known as a Zener diode.
Explanation of Zener effect
- When the reverse bias voltage of a diode reaches a high enough value, electrons and holes are accelerated as minority carriers. Because of this high speed, they can break the bonds between semiconductor crystals, creating new electron-hole pairs. They are also faster, so they can form more electron-hole pairs. Thus, the number of charge carriers increases exponentially and the reverse diode current reaches very high values. This phenomenon is called avalanche breakdown and the diode is called an avalanche diode.
- In a semiconductor diode, i.e., if both parts of the p-n junction are doped very heavily, the thickness of the depletion region is greatly reduced. A very small reverse bias is then applied, but a very high electric field is created between the two ends of the depletion region. This electric field directly breaks the bonds in the semiconductor crystal and frees large amounts of charge carriers within the crystal. This results in diode breakdown due to relatively low reverse bias voltage. That is, the diode reaches a reverse current multiplier state at a low constant breakdown voltage. This condition is called Zener breakdown. Such a diode is known as a Zener diode.
- In any such semiconductor diode, the avalanche action and Zener action are simultaneous in reverse bias. In general, if the breakdown voltage is close to 6 V, the avalanche action and Zener action become equivalent and the breakdown voltage does not change significantly with temperature. Hence, diodes with breakdown voltages close to 6 V are most suitable for use at different temperatures.
Such semiconductor diodes are called Zener diodes in practical terms, whichever is dominant, such as avalanche action or Zener action.
Characteristics curve
The ampere-volt characteristic curve of a forward-biased Zener diode is similar to the characteristic curve of a typical semiconductor diode. But when the reverse bias voltage reaches a certain value (at VZ), the reverse current increases at a faster rate. There is no significant change in voltage. This part of the characteristic curve is roughly indicated by a vertical line AB. In an ideal Zener diode, the voltage increase is zero as the current increases. In real cases, this increase is between 1% and 5%.
Rating of a Zener diode
Each Zener diode has a voltage and a capacitance reference. This is called the voltage rating VZ, and the watt rating PZ, of the diode. The maximum reverse bias that the Zener diode operates at, i.e., VZ, is indicated by this voltage rating. The power rating or watt rating PZ means that the diode will burn out if the power dissipated by increasing the current through the diode exceeds the value of PZ.
Hence, the maximum safe reverse current through the diode, Imax = PZ / VZ. The point P in the figure indicates this value.
For example, for a Zener diode with a 4.7 V – 1 W rating,
Imax = 1 W / 4.7 V = 0.21 A = 210 mA
Care must be taken when using Zener diodes in the circuit that the reverse diode current value should never exceed Imax.
Circuit symbol
A Zener diode is introduced in the circuit by slightly changing the n-terminal to the circuit symbol of the common semiconductor diode.
Use of Zener diode—as a voltage regulator
Limit voltage regulation of load resistance
A Zener diode is used to obtain a constant voltage across a resistor connected to an unsteady dc voltage source.
Circuit connection
To keep the terminal voltage across a load resistor connected to a supply line of variable value input voltage fixed to a specified value, a Zener diode of equal voltage rating is connected in reverse bias in parallel with the load resistor.
Explanation: As the input voltage increases, the current through the Zener diode Rs also increases. This results in an increase in the voltage drop across the resistor RS, but does not change the terminal voltage across the Zener diode. This can be attributed to the fact that even if the current through the Zener diode increases, there is no change in the Zener breakdown voltage. Similarly, as the input voltage decreases, the voltage drop across the resistor Rs decreases for the same reason, but the voltage across the Zener diode remains the same. That is, it is seen that no change in the threshold voltage of the Zener diode occurs with any fluctuation in the input voltage. By exploiting this property of the Zener diode, it is used as a voltage regulator in the circuit. The method is called voltage regulation.
Incidentally, if the Zener diode is rated (VZ – PZ), the maximum safe current through it, Imax = PZ / VZ. In the discussed circuit the resistor RS is chosen in such a way that Zener current cannot exceed Imax even for the highest value of unstable input voltage.
Load regulation
In the circuit in the figure, a milliammeter is placed to measure the current IL through the load resistance RL, and a voltmeter is placed to measure its terminal voltage VL. RL is varied step by step keeping the supply voltage Vi constant and the values of IL and VL are recorded in each case.
In the IL ~ VL diagram obviously, the segment AB represents the regulated voltage. If the Zener diode behaved ideally the line AB would be horizontal, in reality, point B is slightly below. As can be seen from the BC portion, if the value of IL is too high i.e., if the value of Zener current IZ is too low, VL is no longer in control. Since the flow of current IL is maximum at point B in the control region, the ratio VL / IL at that point indicates the minimum value of load resistance RL. Changing the value of the load resistor over a long range beyond that minimum value does not significantly change its marginal potential difference VL.
If the voltage at point A is VNL and the voltage at point B is VL in the figure,
As VNL = VL in an ideal Zener diode this percentage regulation is zero. In reality its value is between 1% and 5%. [NL means no load or zero flow]
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