What is an Avalanche Diode?
An avalanche diode is a type of semiconductor diode which is designed to experience avalanche breakdown at a specified reverse bias voltage. The pn junction of an avalanche diode is designed to prevent current concentration and resulting hot spots so that the diode is undamaged by the avalanche breakdown.
The avalanche breakdown that occurs is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.
The construction of the avalanche diode is similar to the Zener diode, and indeed both Zener breakdown and Avalanche breakdown are present in these diodes. Avalanche diodes are optimized for avalanche breakdown conditions, so they exhibit small but significant voltage drop under breakdown conditions, unlike Zener diodes that always maintain a voltage higher than breakdown.
This feature provides better surge protection than a simple Zener diode and acts more like a gas discharge tube replacement. Avalanche diodes have a small positive temperature coefficient of voltage, where diodes relying on the Zener effect have a negative temperature coefficient.
The normal diode allows an electric current in one direction i.e. forward direction. Whereas, avalanche diode allows the current in both direction i.e. forward and reverse direction but it is specially designed to work in reverse bias condition.
What is Reverse Bias Condition in Diode?
When a positive terminal of a battery is connected to N-region (cathode) and negative terminal to P-region (anode), the junction is said to be reverse biased.
Now if a diode is lightly doped (i.e. impurities concentration is less), then the width of the depletion region is increased so breakdown voltage occurs at a very high voltage.
At a very high reverse bias voltage, the electric field becomes strong in the depletion region and a point is reached where the acceleration of minority carrier is so much that, when they collide with the semiconductor atoms in the depletion region, they break the covalent bonds.
So electron-hole pairs are generated. The newly generated electron-hole pairs are also accelerated by the electric field resulting in more collisions and further production of charge carriers and carrier multiplication takes place.
This continuous process increases the reverse current in the diode, and hence the diode comes into breakdown condition. This type of breakdown is known as avalanche (flood) breakdown and this effect is known as the avalanche effect.
What is the Depletion Region in Diode?
The Depletion region is an insulating region where the flow of charge carriers is decreased. Free electrons from N-side of the P-N junction diode diffuse into the P-side where they recombine with holes and atoms become negatively charged.
It is called negative immobile ions. Similarly, holes from P-side diffuse into N-side where they recombine with free electrons and atoms become positively charged. It is called positive immobile ions.
The recombination of electrons and holes decays exponentially with time and thus there exists a wall near the junction with negative immobile ions on the p side and positive immobile ions on n side. Due to this recombination process, a depletion layer is formed at both sides of the junction.
This layer contains only immobile ions and they have opposite polarity across the junction. This layer does not contain any free charge carriers therefore it is known as depletion region, depletion layer or an insulating region.
What are Immobile Ions and Mobile Ions?
Immobile ions are fixed ions of impurity atoms and not able to move. Mobile ions are free ions and carrying electric charges.
What is a Covalent Bond?
The sharing of an electron between two atoms forms the covalent bonds.
The covalent bond is one in which electrons are equally shared between bonded atoms. The electrons surrounding each atom in a semiconductor are due to a covalent bond.
For Example, silicon and germanium are purely covalently bonded because electrons are shared equally.
Applications of Avalanche Diode
Some of the applications of an Avalanche Diode include:
- The Avalanche diode is used for the protection of the circuit. When reverse bias voltage increases then up to certain limit diode starts an avalanche effect at a particular voltage and diode breakdown due to avalanche effect.
- It is used to protect the circuit against unwanted voltages.
- It is used in surge protectors to protect the circuit from surge voltage.
Symbol of Avalanche Diode
The symbol of the Avalanche diode is the same as that of the Zener diode.
Difference Between Zener Breakdown and Avalanche Breakdown
The difference between Zener breakdown and avalanche breakdown has been summarised in the table below:
| Sr. no. | Zener Breakdown | Avalanche Breakdown |
| 1 | Zener Breakdown occurs in a heavily doped P-N junction diode. | Avalanche Breakdown occurs in a lightly doped P-N junction diode. |
| 2 | Zener has a narrower (thin) depletion region. | Avalanche has a wider (thick) depletion region. |
| 3 | The Electric field set up across the depletion region is stronger. | The Electric field set up across the depletion region is weaker. |
| 4. | Breakdown occurs due to the breaking of covalent bonds by the strong electric field across the junction. | Breakdown occurs due to the collision of accelerated charge carriers with the adjacent atoms and due to carrier multiplication. |
| 5 | Breakdown occurs at a breakdown voltage of less than 4 V. | Breakdown occurs at a breakdown voltage of more than 6 V. |
| 6. | The breakdown voltage decreases as junction temperature increases. | The breakdown voltage increases as junction temperature increases. |
| 7. | The temperature coefficient is negative. | The temperature coefficient is positive. |
I-V Characteristic of Avalanche Diode
The I-V characteristics are the variation of current for the applied voltage. The figure above shows the combine characteristics of Zener breakdown (Zener diode) and Avalanche breakdown.
It is noted that both breakdowns occur when a diode is in reverse bias condition.
Zener breakdown occurs at a breakdown voltage of less than 4 V which is denoted as a Vz and avalanche breakdown occurs at a breakdown voltage of more than 6 V which denoted as a VBR.
What is Avalanche Breakdown Voltage?
When we increase the reverse bias voltage across a P-N junction, the reverse saturation current remains constant up to a certain point. If further increase this reverse bias voltage it will breakdown the junction i.e. electrical breakdown and reverse current rises sharply to a high value.
This critical value of reverse bias voltage at which reverse current rises unexpectedly and Avalanche breakdown occurs is known as the avalanche breakdown voltage. Typically avalanche breakdown occurs at a breakdown voltage more than 6 V.
Is Avalanche Breakdown Reversible?
Avalanche breakdown is not reversible while Zener breakdown is reversible.
Avalanche Breakdown occurs due to the collision of accelerated charge carriers with the adjacent atoms and due to carrier multiplication. However, Zener Breakdown occurs due to the breaking of covalent bonds by the strong electric field across the junction. It means that in Zener breakdown power rating of the diode is not increased.
Avalanche breakdown can be reversible if we put a series resistor in a diode.
How Zener Breakdown is Reversible and Avalanche Breakdown is Not Reversible?
If a P-N junction is in Zener breakdown condition and if we now reduce the external reverse bias voltage, the P-N junction is not damaged and returns to its initial state. However, if a P-N junction is in avalanche breakdown condition and if we reduce the reverse bias voltage, the P-N junction cannot return to its initial state. It means that in avalanche breakdown condition the P-N junction is permanently damaged.
