Working principle of Zener Diode
In this article, I will explain working principle of Zener Diode. In this I am explaining the working of Zener Diode, both in forward and reverse direction. I also illustrated it's working along with the V-I characteristics curve.
These special diodes behave in a forward or forward direction switched as normal silicon diodes. Their special properties they exhibit when operating in reverse or backward direction. Here, the diodes have a very limited breakdown voltage with a steep current increase. The diodes are doped so that under certain conditions a permanent operation in the area of the steep rise of the barrier characteristics is possible without destroying the part. These properties are especially simple circuits for voltage stabilization used.
The breakthrough in the stop band is obtained with the stabilization by a special diode doping. It can be diodes with breakdown voltages between 2 to 200 volts. From a certain reverse voltage suddenly many carriers are available. This observation can be explained by two effects, the Zener effect and avalanche effect.The Zener effect
In highly doped silicon diodes breakdown voltages in the off region is less than 5 volts, provides the Zener effect for the increase in current. In the barrier layer, the electric field strength is so large that they create the electron pair bonds are broken in the crystal lattice association a certain voltage. In the barrier layer, charge carriers that constitute the Zener effect. In this case, taking the electrical conductivity of the barrier layer and the differential junction resistance decreases.The avalanche effect
For breakdown voltages of greater than 6 volts results in the lightly doped silicon avalanche diode or avalanche effect the release of charge carriers in the barrier layer. The charge carriers are accelerated by the electric field of higher blocking voltage. In collisions with electrons of the atomic lattice bond more electrons are knocked free. These are also accelerated and can turn again to release electrons. This process of impact ionization is also called avalanche effect.
Both effects, the Zener and avalanche effect, overlap and destroy the semiconductor, unless certain limits are met. The operations in the barrier layer are reversible. In the breakdown region, a maximum current may not be exceeded. It is calculated dead of the semiconductor from the permissible power loss P. These values are determined by the ability to remove the heat energy from the crystal.
The figure illustrates the characteristic curve of some Zener diodes. In the passband, these diodes behave like normal silicon diodes. In the locking region where the cathode is positive with respect to the anode, a steep current increase can be observed depending on the type of diode from a certain voltage.
Diodes that are turned on by the Zener effect, have a flatter characteristic curve and the bend in the region of the breakdown voltage is less sharply defined. In Zener diodes with higher breakdown voltage of the avalanche effect is. The characteristic curve is steeper and has a sharp bend at UZ.
The picture is drawn for diodes whose maximum power is 1 watt. The working range of this Z-diode is between I and IZmin Zmax, is determined by the P-tot hyperbola. The minimum current IZ can not be determined by formulas zmin. It must be read from the characteristic curve or you can use a benchmark, which is at 10% of the maximum current IZmax.
The differential resistance of the zener diode is determined by the slope of the operating characteristic in the breakdown region.The temperature dependence of the Z-diode
Semiconductors are thermistors and have a negative temperature coefficient. Its resistance value decreases with increasing temperature, the voltage across the component is reduced. Similarly, the stabilization diodes that use the Zener effect behavior. Their temperature coefficient is negative. With increasing temperature, the Z voltage decreases. These diodes have a high degree of doping and consequently a very thin barrier layer. The critical field strength, which triggers the Zener effect is therefore achieved already at low voltage. An energy supply by increasing the temperature facilitates the breakup of the electron pair bonds in the crystal structure.
Z-diodes in which the avalanche effect is the main cause, however have a positive temperature coefficient. They are weakly doped, and their junction is therefore wider. The critical field strength for the Zener effect is higher. The blocking voltage is sufficient, however, to accelerate the wider boundary layer some minority carriers to the extent that they beat by impact ionization more free electrons. This avalanche effect causes the charge transport through the barrier layer.
An increase in temperature increases the non-directed motion of the electrons and therefore reduces its mean free path. This counteracts an increase of avalanche effect and increases the resistance of the barrier layer. Consequently, the Zener voltage increases with increasing temperature.
The transition region of the two trigger effects is 5 to 6 volts. Zener diodes with breakdown voltages that are largely independent of temperature. You also have the steepest characteristic curve in the workspace, and thus the smallest differential resistance and thus the best stabilization properties.