ZENER DIODE
A Specially designed silicon diode which is optimised to operate in the breakdown region is known as zener diode. The ordinary rectifier and small-signal diodes are never intentionally operated in the breakdown region because this may damage them. On the other hand, zener diodes are only operated in the breakdown region.
Therefore, zener diodes are cryptically designed to have a sharp breakdown voltage. By varying the doping level of silicon diode, a manufacturer can produce zener diodes with breakdown voltages from about 2 to 200 V. It is similar to the symbol of an ordinary crystal diode except that its bar is just turned into Z-shape.
CHARACTERISTICS OF A ZENER DIODE
The V-I characteristics of a zener diode have the following points :
(i) Its characteristics are similar to an ordinary rectifier diode with the exception that it has a sharp (or distinct) breakdown voltage called zener voltage (Vz).
(ii) It can be operated in any of the three regions i.e. forward, leakage or breakdown. But usualy it is operated in the breakdown region.
(iii) The voltage is almost constant (Vz) over the operating region.
(iv) Usually, the value of Vz at a particular test current Izr is specified in the data sheet.
(v)During operation it will not burn as long as the external circuit limits the current flowing through it below the burn out value i.e. IZM (the maximum rated zener current).
AVALANCHE BREAKDOWN AND ZENER BREAKDOWN
The avalanche breakdown and zener breakdown are two different mechanisms by which a pn junction breaks. These are distinguished by the following explanation :
Avalanche Breakdown
For thicker junctions the breakdown mechanism is by the process of avalanche breakdown. In this mechanism, when the electric field existing in the depletion layer is sufficiently high, the velocity of the carriers (minority carriers) crossing the depletion layer increases. These carriers (electrons and holes) collide with the crystal atoms. Some collisions are so violent that electrons are knocked of the crystal atoms, thus creating electron-hole pairs. As the pair of electron-hole is created in the midst of the high field, they quickly separate and attain high velocities to cause further pair generation through more collisions. This is a cumulative process and as we approach the breakdown voltage, the field becomes so large that the chain of collisionss can give rise to an almost infinite current with very slight additional increase in voltage. This process is known as avalanche breakdown. Once this breakdown occurs, the junction can not regain its original position. Thus, the diode is said to be burnt-off.
Zener Breakdown
Zener breakdown takes place in a very thin junction, ie. when both sides of the junction are very heavily doped and consequently the depletion layer is narrow. In the zener breakdown mechanism, the electric field becomes as high as 107 V/m in the depletion layer with only a small applied reverse bias voltage. In this process it becomes possible for some electrons to jump across the barrier from the valence band in p-material to some of the unfilled conduction band in n-material. This process is known as zener breakdown.
In this process, the junction is not damaged. The junction regains its original position when the reverse voltage 1S removed. This process is used in the zener diodes.
However, if the number of electrons jumping across the barrier (i.e. flow of current) increases beyond rated capacity of the zener diode, then avalanche breakdown takes place which destroys the junction.
Thus, it is concluded that zener breakdown does not result in the destruction of the diode, as long as current through the diode is limited by the external circuit to a level within its power handling on capacity. Whereas, the avalanche breakdown destroys the diode.
RATING OF ZENER DIODE
Manufacturers provide a data sheet for various zener diodes bearing different numbers. This data sheet shows the ratings of different diodes which include zener voltage, tolerance range or zener voltage, zener current limits, maximum power dissipation, maximum operating temperature, maximum zener resist (or impedance) etc. Some of the important ratings are discussed below :
1. Zener Voltage (V): The voltage at which a zener diode breaks in the reverse bias condition is known as zener voltage. In fact, it is the voltage at which a zener diode is to operate. Commenially available zener diodes are having zener volage ratings from 3 V to 200 V. The vaue of breakdown (or zener) voltage depends upon doping- more the doping, lesser the breakdown voltage.
2. Tolerance: The range of voltages about the breakdown voltage in which a zener diode conducts in reverse direction is called tolerance.
In fact, during manufacturing, it is very difficult to have exact doping for every zener of the same number (ie. type). Therefore, breakdown voltages of zeners of the same number also differ slightly. This range of breakdown voltages for the same type of zener diode is described as tolerance. For example, consider a particular type of zener marked 9 V, 10% tolerance. These zener diodes may have breakdown voltage from 8.1 V (9-0.9) to 9.9 V (9 +0.9) instead of sharp 9 V for all.
Efect of temperature, it may be noted that the breakdown voltage of zener diode depends upon operating temperature. It decreases with the increase in junction temperature. 'This is because of the increased reverse current (i.e. increase in minority carriers) that flows with increasing temperature. The decrease is about 2 mV/°C rise in temperature.
3. Power Rating (PZM): The maximum power which zener diode can dissipate (or handle) without damage is known as its power rating (PZM). Commercially available zener diodes have power ratings from 1/4W to more than 50 W. 4
Power rating is the product of maximum current IZM which a zener diode can handle and e ra or operating volage of a zener diode (Vz).
PZM = IZM VZ
Sometimes, a data sheet includes the maximum current rating of a zener diode.
4. Maximum Current Rating (IZM): The maximum value of current which a zener diode can handle as its rated voltage without damage is known as its maximum current rating (IZM).
5. Zener resistance (RZR): When a zener diode is in current operated in the breakdown region, as produces a sligh increase is called voltage. This shows that a zener diode has a small ac resistance called zener resistance (often called zener impedance ZZR). Sometimes, manufacturers specify this resistance in their data sheets.
The opposition offered to the current flowing through as zener resistance the zener diode in the operating region is known as zener resistance (Rz) or zener impedance (ZZR). The zener resistance at test current Izr is represented as (RZR).
ZENER AS A VOLTAGE REGULATOR
Due to the unique characteristics mentioned above, zeners are used as a 'constant voltage source'. They act as voltage regulators, i.e., regulate an unregulated D.C. power supply and maintain a constant voltage across the load. They find applications in automatic stabilizers to stabilize the supply to a refrigerator, TV, etc.
When the zener breaks down, the voltage Vz across it remains constant though current through it (IZ) may change. The extra current passes through the diode and the extra voltage drops across regulating resistance R. So in all cases the total current, IT = IZ + IL .
APPLICATIONS OF ZENER DIODES
Important applications of Zeners are described below:
1. As Voltage Regulator: This is the most important field, where Zeners are used. They provide constant voltage across the load despite fluctuations in the supply. They, therefore, constitute an important part of all voltage stabilizers used for refrigerators and other equipment.
2. Protection of Meters: Zeners are employed for the protection of multimeters against overloads. In case of sudden overload, the extra current finds its path through the Zener, which is connected in parallel to the meter. No need-to mention that the Zener should be connected in reverse bias.
3. For Wave Shaping: Zener diodes are also used to obtain non-sinusoidal waves, which are required in the operation of television and other devices. Zeners can be used in the circuit for obtaining a non sinusoidal output from a sinusoidal input.
Note that
1. A Zener diode is an ordinary diode with proper doping.
2. Due to proper doping, it has a very sharp break down characteristic.
3. It is always used in reverse biased mode.
4. When forward biased, it behaves as an ordinary (semiconductor) diode.
5. A Zener diode does not burn until the current flowing through it becomes more than the 'Burn out current'.
6. A Zener diode can be used for rectification in its forward region but due to higher cost than the ordinary diodes, it is used for voltage regulation only.
7. They are available from 2V to 200V.
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