What is a Breakdown Diode?
When a diode is designed for a specific breakdown voltage, it is called a breakdown diode. The reverse-bias breakdown voltage of a junction can be varied by choice of junction doping concentrations. The breakdown mechanism is the Zener effect (tunneling) for abrupt junctions with extremely heavy doping.
However, the more common breakdown is avalanche (impact ionization), for lightly doped or graded junctions. The avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current. The breakdown mechanism involves the impact ionization. Whereas the Zener diode is a PN junction diode with a thin junction and heavy doping.
Zener Breakdown Diode
A Zener breakdown diode is a rectifying diode that handles breakdown due to reverse breakdown voltage without failing completely. When a heavily doped junction is reverse biased, the energy bands cross at relatively low voltages, due to the crossing of the bands, empty states align in the n-side conduction band opposite to the many filled states of the p-side valence band. If the barrier separating these two bands is quite narrow, then tunneling of electrons occurs, and tunneling of electrons from the p-side valence band to the n-side conduction band constitutes a reverse current from n to p, this is the Zener effect. Finally, net current develops and it increases with an increase in the electric field.
The Zener effect is the field ionization of the host atoms at the junction. That is, the reverse bias of a heavily doped junction causes a large electric field within depletion region, at a critical field strength, electrons participating in covalent bonds are torn from the bonds by an electric field and accelerated to the n side of the junction.
In reverse bias Zener breakdown diodes, initially, the current is very small. A leakage current is a small current that will be flowing through the diode. The moment it hits the breakdown voltage, the current increases suddenly. This is used in the application of voltage regulation. Once the voltage across a Zener reaches the breakdown voltage, it is said to be Zener voltage. The VI characteristic curve is shown below.
Avalanche Breakdown Diode
An avalanche diode is a semiconductor device designed to operate in a reverse breakdown region. Avalanche diodes are lightly doped, and the width of the depletion layer in the avalanche diode is very wide compared to the Zener diode, because of this wide depletion region reverse breakdown occurs at higher voltages in avalanche diodes. The breakdown voltage of avalanche diodes is carefully set by controlling the doping level. Avalanche diodes are used to protect electrical systems from excess voltages.
The avalanche breakdown occurs due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. The VI characteristic curve of Avalanche diode in comparison with Zener diode is shown below. In the graph reverse breakdown voltage for the avalanche diode is more than zener diode.
Differences between Zener and Avalanche Breakdown Diode
There are two types of breakdowns that occur in PN junction diodes. Below given table shows the difference between those breakdowns which are Zener breakdown and avalanche breakdown.
The avalanche breakdown occurs in junctions that are lightly doped and have wide depletion width.
The zener breakdown occurs in junctions that are heavily doped and have narrow depletion width.
The high reverse potential is required
Low reverse potential is required
Increase in temperature increases the breakdown voltage.
An increase in temperature decreases the breakdown voltage.
Impact ionization occurs.
Field ionization occurs