- Avalanche Diode Definition: An avalanche diode is a semiconductor that allows electrical current in both forward and reverse directions but is specifically optimized to handle high reverse voltages without damage.
- Working Principle: The avalanche diode operates on the principle of avalanche breakdown, where accelerated charge carriers gain enough energy to ionize other atoms, thus creating a chain reaction that significantly increases current flow.
- Applications: Avalanche diodes are primarily used for protecting circuits by clamping high voltages and preventing voltage spikes.
- Reverse Bias Configuration: In reverse bias, the diode’s configuration involves connecting the N-side to a positive voltage, setting the stage for avalanche breakdown at high voltages.
- Comparison to Zener Diodes: While both can handle breakdowns, avalanche diodes are suited for higher voltage operations and feature a positive temperature coefficient, contrasting with the negative coefficient of Zener diodes.
What is an Avalanche Diode?
An avalanche diode is a semiconductor diode designed to operate in controlled avalanche breakdown at a specified reverse voltage. Its pn junction spreads reverse current more evenly than an ordinary rectifier junction. The diode can therefore survive breakdown only while current, pulse energy, power dissipation and junction temperature remain within its data-sheet ratings.
Avalanche breakdown begins when carriers accelerated by the reverse electric field gain enough energy to create electron-hole pairs by impact ionisation. The new carriers accelerate and create more pairs, producing carrier multiplication and a steep rise in reverse current. Voltage still rises with current because a real diode has dynamic resistance, so a data sheet specifies breakdown voltage at a test current and clamping voltage at a higher pulse current.
The construction resembles that of a Zener diode. Both Zener breakdown and avalanche breakdown can contribute in the same junction; the dominant mechanism changes gradually with doping and breakdown voltage. In either case, the terminal voltage at a working current is higher than the voltage measured at the lower breakdown test current.
Avalanche-rated TVS diodes are widely used for surge protection, but no diode type is automatically the better protector. Selection depends on the normal working voltage, required clamping voltage, surge waveform, peak current, pulse power, capacitance, temperature and source impedance. Avalanche-dominant breakdown generally has a positive voltage temperature coefficient, while low-voltage Zener-dominant breakdown generally has a negative coefficient.
Like other pn-junction diodes, an avalanche diode conducts readily when forward biased and blocks most current when reverse biased below breakdown. Above its rated reverse breakdown voltage, it conducts a controlled reverse current. An ordinary diode may also enter avalanche breakdown, but it must not be operated there unless its manufacturer explicitly gives an avalanche rating.
What is Reverse Bias Condition in Diode?
In reverse bias, the diode’s N-region (cathode) connects to the positive terminal of the battery, and the P-region (anode) to the negative terminal.
A relatively lightly doped junction has a wider depletion region. It therefore needs a higher reverse voltage to create the field conditions for avalanche breakdown than a heavily doped junction needs for Zener-dominant breakdown.
As reverse voltage increases, the electric field in the depletion region accelerates the small population of carriers present there. Above a threshold, a carrier can transfer enough energy in a collision to excite an electron into the conduction band, leaving a hole and creating an electron-hole pair.
This process generates electron-hole pairs that are accelerated by the electric field, causing more collisions and further increasing the number of charge carriers. This phenomenon is known as carrier multiplication.
Repeated impact ionisation increases the reverse current sharply. This carrier-multiplication process is avalanche breakdown. External circuit impedance and the diode’s ratings must limit the current and dissipated energy.
What is the Depletion Region in Diode?
The depletion region is the space-charge region around a pn junction that has very few mobile majority carriers. When the junction forms, electrons diffuse from the N-side and recombine with holes on the P-side. Their departure exposes fixed positive donor ions on the N-side, while recombination on the P-side leaves fixed negative acceptor ions.
These fixed charges create the junction’s built-in electric field. The field opposes further majority-carrier diffusion and establishes an equilibrium depletion width. Reverse bias pulls mobile electrons and holes away from the junction, so the depletion region widens.
The depletion region contains few mobile majority carriers, with fixed negative acceptor charge on the P-side and fixed positive donor charge on the N-side. Minority carriers and thermally generated carriers can still cross it and produce a small reverse leakage current.
The region contains exposed ionised dopants and very few mobile carriers compared with the neutral material outside it. It is called the depletion layer or space-charge region. Describing it as an ideal insulator is only an approximation.
What are Immobile Ions and Mobile Ions?
Immobile ions are ionised donor or acceptor atoms fixed in the semiconductor crystal lattice. The mobile charge carriers are electrons and holes, not freely moving impurity ions.
What is a Covalent Bond?
A covalent bond forms when neighbouring atoms share a pair of valence electrons.
In a pure silicon or germanium crystal, each atom shares valence electrons with neighbouring atoms in a repeating lattice. In compound semiconductors, the sharing can be unequal, so the bonds can also have an ionic component.
Silicon and germanium are common elemental semiconductors whose crystal bonding is predominantly covalent.
Applications of Avalanche Diode
Common applications of avalanche-rated diodes include:
- Clamping short overvoltage transients on power rails, signal lines and control inputs.
- Absorbing specified surge energy in automotive load-dump, inductive-switching and industrial protection circuits.
- Setting a voltage threshold in avalanche-noise sources, voltage references and active-clamp networks when the selected device is rated for that use.
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-dominant breakdown occurs in a more heavily doped P-N junction diode. | Avalanche-dominant breakdown occurs in a more lightly doped P-N junction diode. |
| 2 | The junction has a narrower depletion region. | The junction has a wider depletion region. |
| 3 | A very strong field exists across the narrow depletion region. | Carriers gain ionising energy while crossing the wider depletion region. |
| 4. | Electrons tunnel between energy bands under the strong electric field. | Accelerated carriers create electron-hole pairs by impact ionisation and carrier multiplication. |
| 5 | This mechanism dominates at a lower breakdown voltage, usually below about 5 to 6 V. | This mechanism dominates at a breakdown voltage above about 5 to 6 V. |
| 6. | For Zener-dominant devices, breakdown voltage usually decreases as junction temperature increases. | For avalanche-dominant devices, breakdown voltage usually increases as junction temperature increases. |
| 7. | The voltage temperature coefficient is generally negative. | The voltage temperature coefficient is generally positive. |
I-V Characteristic of Avalanche Diode
An I-V curve shows current as applied voltage changes. The figure above illustrates forward conduction and reverse characteristics of Zener breakdown. The label “Zener diode” commonly covers devices in which Zener tunnelling, avalanche multiplication or both mechanisms determine the reverse breakdown region.
It is noted that both breakdowns occur when a diode is in reverse bias condition.
Zener tunnelling is normally dominant below about 5 to 6 V, while avalanche multiplication is normally dominant above that range. The transition is gradual, and both mechanisms can contribute near the crossover. A data sheet states the applicable breakdown or working voltage and the current at which it was measured.
What is Avalanche Breakdown Voltage?
As the reverse bias voltage across a P-N junction increases, only a small leakage current flows before breakdown. Near the breakdown region, impact ionisation multiplies carriers and reverse current rises steeply. The external circuit must limit that current.
Avalanche breakdown voltage is the reverse voltage specified at a defined test current. Avalanche multiplication usually dominates above about 5 to 6 V, but the exact value and test conditions depend on the junction design. It is not the same as clamping voltage, which is measured at a larger pulse current.
Is Avalanche Breakdown Reversible?
Avalanche and Zener breakdown are both electrically reversible when a diode is designed for breakdown operation and stays within its current, power, pulse-energy and temperature ratings. Reducing the reverse voltage below breakdown then returns the device to its blocking state.
Avalanche breakdown results from impact ionisation and carrier multiplication. Zener breakdown results mainly from quantum tunnelling through a narrow depletion region. Neither mechanism increases the diode’s power rating; the package and junction thermal limits still govern safe operation.
A series resistor is one way to limit current, but it is not the only way. Source impedance, a current-limited supply or another protection network can perform the same function. A TVS design must also check reverse standoff voltage, breakdown voltage, clamping voltage, peak pulse current and pulse-power ratings.
How Zener Breakdown is Reversible and Avalanche Breakdown is Not Reversible?
A breakdown-rated P-N junction returns to its low-current reverse-blocking state after either Zener-dominant or avalanche-dominant operation if its ratings were not exceeded. An ordinary rectifier junction can develop local current crowding and permanent damage in reverse breakdown because it was not designed for that use. An avalanche-rated diode can also fail if excessive current, energy or heat exceeds its data-sheet limits.
