ZENER DIODE By Dr. APARAJITA Assistant Professor (guest faculty) B.N. COLLEGE, PATNA UNIVERSITY

Introduction A Zener diode is a reverse biased properly doped crystal diode having a sharp breakdown voltage and operated in breakdown region.It is reverse biased heavily doped silicon or germanium P-N junction diode. But ordinary PN junction diode connected in reverse biased condition is not used as Zener diode practically. A Zener diode is a specially designed, highly doped PN junction diode.

Breakdown mechanism/ Junction breakdown If the reverse bias applied to a P-N junction is increased a point is reached when the junction breaks down and reverse current rises sharply to a value limited only by the external resistance connected in series with the junction. This critical value of the voltage is known as breakdown voltage( vBR). After this breakdown, very little further increase in voltage is required to increase the current to relatively high values. The junction itself offers almost zero resistance at this point. The breakdown voltage depends on the width of the depletion region, which in turn depends on doping level.

There are two mechanism of breakdown under reverse biasing. 1. ZENER BREAKDOWN 2. AVALANCHE BREAKDOWN

1. Zener diode This breakdown occurs in junctions which being heavily doped, have narrow depletion layers. The breakdown voltage sets up a very strong electric field ( about108 v/m) across this narrow layer. This field is strong enough to break or rupture the covalent bonds thereby generating electron-hole pairs. High field at the junction is due to small reverse potential difference. After electron-hole pair the reverse current then increases rapidly and is no longer limited by the diode.Even a small further increase in reverse voltage is capable of producing large no of current carriers.Thats why junction has very low resistance in the breakdown region. This condition is known as Zener breakdown. This mechanism occurs in diodes whose breakdown voltage VB is less than -6v. The magnitude of zener breakdowm voltage decreases with increase of temperature of junction.For that smaller reverse voltage is required. 2. Avalanche Breakdown This form of breakdown occurs in junctions which,being lightly doped, have wide depletion layers where the electric field is not strong enough to produce avalanche breakdown. Reverse current is due to minority carriers gain K.E. When Ir is sufficiently high they may knock out electrons from covalent bonds of semiconductor material as a result of collisions with the ions in depletion layer, thereby producing new electron-hole pairs. These newly generated charge carriers are also accelerated by the electric field resulting in more collisions and hence further production of charge carriers. This leads to an avalanche of charge carriers and consequently, to a very low resistance. This leads to multiplication of reverse current and unless limited by some external resistance may destroy the diode. The avalanche breakdown occurs when breakdown voltage VB is very large (exceed 6v). The magnitude of avalanche breakdown voltage increases with the increase of junction temperature. Increase in temp leads to increase of collisions of electrons with crystal atom, So energy of crystal atom is lost. For that more reverse voltage is required to avalanche breakdown.

Working Principle of Zener Diode When a PN junction diode is reverse biased, the depletion layer becomes wider. If this reverse biased voltage across the diode is increased continually, the depletion layer becomes more and more wider. At the same time, there will be a constant reverse saturation current due to minority carriers. After certain reverse voltage across the junction, the minority carriers get sufficient kinetic energy due to the strong electric field. Free electrons with sufficient kinetic energy collide with stationary ions of the depletion layer and knock out more free electrons. These newly created free electrons also get sufficient kinetic energy due to the same electric field, and they create more free electrons by collision cumulatively. Due to this commutative phenomenon, very soon, huge free electrons get created in the depletion layer, and the entire diode will become conductive. This type of breakdown of the depletion layer is known as avalanche breakdown, but this breakdown is not quite sharp. There is another type of breakdown in depletion layer which is sharper compared to avalanche breakdown, and this is called Zener breakdown. When a PN junction is diode is highly doped, the concentration of impurity atoms will be high in the crystal. This higher concentration of impurity atoms causes the higher concentration of ions in the depletion layer hence for same applied reverse biased voltage, the width of the depletion layer becomes thinner than that in a normally doped diode.

Due to this thinner depletion layer, voltage gradient or electric field strength across the depletion layer is quite high. If the reverse voltage is continued to increase, after a certain applied voltage, the electrons from the covalent bonds within the depletion region come out and make the depletion region conductive. This breakdown is called Zener breakdown. The voltage at which this breakdown occurs is called Zener voltage. If the applied reverse voltage across the diode is more than Zener voltage, the diode provides a conductive path to the current through it hence, there is no chance of further avalanche breakdown in it. Theoretically, Zener breakdown occurs at a lower voltage level then avalanche breakdown in a diode, especially doped for Zener breakdown. The Zener breakdown is much sharper than avalanche breakdown. The Zener voltage of the diode gets adjusted during manufacturing with the help of required and proper doping. When a zener diode is connected across a voltage source, and the source voltage is more than Zener voltage, the voltage across a Zener diode remain fixed irrespective of the source voltage. Although at that condition current through the diode can be of any value depending on the load connected with the diode. That is why we use a Zener diode mainly for controlling voltage in different circuits. Zener Diode Circuit Zener Diode is nothing but a single diode connected in a reverse bias, we have already stated that. A diode connected in reverse bias position in a circuit is shown below,

The circuit symbol of a Zener diode is also shown below.

Characteristics of a Zener Diode Now, discussing about the diode circuits we should look through the graphical representation of the operation of the zener diode. Normally, it is called the V-I characteristics of a Zener diode.

The above diagram shows the V-I characteristics of a zener diode. When the diode is connected in forward bias, this diode acts as a normal diode but when the reverse bias voltage is greater than zener voltage, a sharp breakdown takes place. In the V-I characteristics above Vz is the zener voltage. It is also the knee voltage because at this point the current increases very rapidly. Application of Zener Diode Following are the applications of Zener diode: Zener diode as a voltage regulator: Zener diode is used as a Shunt voltage regulator for regulating voltage across small loads. The breakdown voltage of Zener diodes will be constant for a wide range of current. Zener diode is connected parallel to the load to make it reverse bias and once the Zener diode exceeds knee voltage, the voltage across the load will become constant. Read More: Zener Diode as a Voltage Regulator Zener diode in over-voltage protection: When the input voltage is higher than the Zener breakage voltage, the voltage across the resistor drops resulting in a short circuit. This can be avoided by using the Zener diode. Zener diode in clipping circuits: Zener diode is used for modifying AC waveform clipping circuits by limiting the parts of either one or both the half cycles of an AC waveform.