Breakdown Voltage of Diode
what is breakdown voltage ?
The voltage across reverse Biased PN junction increase to a certain value, called breakdown voltage of diode, the high reverse current is caused which may damage the junction diode.
There are two mechanism by which breakdown can occur when PN junction is Reverse biased:
- Avalanche breakdown.
- Zener breakdown.
It is a phenomenon that occurs in a both insulating and semiconducting material. It is a form of electric current multiplication that can allow very large current within the materials which are otherwise good insulators.
It is a type of electron Avalanche. The avalanche process occurs when the career in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collision with a bound electron.
Avalanche breakdown usually destroys ordinary diodes, but Avalanche diode is designed to break down this way at low voltage and can survive the reverse current.
When a PN junction is Reverse Biased, the minority carrier (in p region and n region) flowing through the junction acquire kinetic energy under the effect of the electric field across the junction.
The kinetic energy increases with the increase in reverse voltage. At a particular high reverse voltage, the kinetic energy of minority carrier become so high that they are able to Knock Out electron from the covalent bond of the semiconductor material.
These liberated electrons in turn liberate more electron from the covalent bond. The effect is accumulative and hence a large amount of current is caused which breakdown the crystal structure of the material.
This mechanism is known as Avalanche breakdown. This type of breakdown occurs in a lightly doped PN junction.
Breakdown voltage of zener diode
When a PN junction is Reverse Biased, the depletion layer is widened and the potential barrier is increased with the increase in reverse voltage. The electric field across the junction is also increased to a high value.
This large electric field breaks some of the covalent bonds in p and n reason, leading to the production of more and more electron-hole pairs.
These electron-hole pairs diffuse through a junction and hence a large rivers current flow through the junction diode. This mechanism of increasing reverse current is known as Zener breakdown.
The reverse voltage at which Zener breakdown occurs is known as Zener voltage and the current corresponding to it is known as Zener current. This type of breakdown occurs in heavily doped PN junction.
Zener diode is a special kind of diode that allows current to flow in the forward direction in the same manner as an ideal diode, but also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage of Zener diode, Zener knee voltage or Zener voltage. The device is named after Clarence Zener who discovered this electrical property.
A properly doped PN junction diode that works even in the Breakdown region is called a Zener diode.
Zener diode is also known as the breakdown diode or breakdown voltage of diode. This diode works on either the Zener breakdown mechanism or on the Avalanche breakdown mechanism. Zener diode is manufactured by either Silicon or Germanium.
However, silicon is preferred in manufacturing a Zener diode because of its higher operating temperature and current capability.
circuit symbol and equivalent circuit
The Zener diode consists of dynamic resistance rz and d.c.battery having voltage equal to the Zener potential (Vz). However, for all applications, rz may be neglected as compared to the large external resistance. Hence the equivalent circuit of Zener diode as shown in the figure.
Volt-ampere characteristics of Zener diode
Breakdown diode operates over the Breakdown region and current is limited by the external resistance. The breakdown voltage Vz depends on the concentration of dopants. The depletion layer of heavily doped diode is very thin and hence general voltage will be very low.
Zener diode characteristics with enlarge the Breakdown region are shown in the figure. it is clear that there is some specific current near the knee, the voltage across the diode begins to differ significantly from Vz. This current is known as breakdown current. it is denoted by Izk.