Learn about the Avalanche Diode – its construction, working principle, breakdown voltage, and applications in voltage regulation, surge protection, and waveform clipping.
A diode is a two-terminal electronic component widely used in various electrical and electronic circuits. It has two electrodes: the anode and the cathode. Most diodes are made from semiconductor materials such as silicon (Si) or germanium (Ge). The primary purpose of a diode is to allow the flow of electrical current in a single direction, acting as a one-way switch for current. Diodes are versatile and find applications in devices like rectifiers, voltage regulators, oscillators, signal mixers, and electronic switches. There are numerous types of diodes available, including Zener diodes, avalanche diodes, LEDs, laser diodes, Schottky diodes, and more.
This article provides an overview of the construction and operation of an avalanche diode. An avalanche diode is a type of diode designed for avalanche breakdown at a specific reverse bias voltage. Its junction prevents the current concentration, ensuring the diode remains protected during the breakdown.
What is an Avalanche Diode?
An avalanche diode is a special type of semiconductor diode designed to operate in the reverse breakdown region. Unlike regular diodes, which fail when reverse-biased beyond their limit, avalanche diodes are built to handle this condition safely. When the reverse voltage exceeds a specific threshold, called the breakdown voltage, the diode conducts a large current without getting damaged. This unique property makes avalanche diodes useful in voltage regulation and surge protection.
Avalanche Diode Symbol
The symbol of an avalanche diode resembles the standard diode symbol with a slight modification to indicate its specialized function. The symbol of the avalanche diode is,
Here’s a description of the symbol:
- Basic Structure: It consists of an arrow pointing toward a straight line, representing the anode and cathode, similar to a regular diode.
- Modification: A small zigzag or curved line is added near the cathode side to signify the diode’s ability to handle avalanche breakdown
Avalanche Diode Construction
The construction of an avalanche diode is specifically designed to handle high reverse voltage and enable the controlled avalanche breakdown process. Avalanche diodes are typically made using silicon or gallium arsenide. The diode has a P-type region and an N-type region, forming a P-N junction. This junction is designed with precise doping levels. The N-region is lightly doped to create a wide depletion region and the P-region is moderately doped. The proper doping ensures a high breakdown voltage and uniform electric field distribution across the junction. A thicker depletion region is created to spread the electric field evenly when the diode is reverse-biased. This prevents localized current concentrations, which could damage the diode.
Metal contacts are attached to the P-region (anode) and the N-region (cathode) for electrical connection. These contacts are designed to provide low resistance and ensure efficient current flow. The diode is enclosed in a protective casing made of plastic or ceramic. This casing ensures mechanical stability and protects the diode from environmental factors such as moisture and dust. Avalanche diodes are provided with heat-dissipating features, such as metal pads or fins, to handle the heat generated during the avalanche breakdown process.
Zener diodes are heavily doped, whereas avalanche diodes are lightly doped. This difference in doping levels gives each type of diode its unique characteristics and functionality.
Working of an Avalanche Diode
The operation of an avalanche diode is based on the principle of avalanche breakdown when it is reverse-biased. When a reverse voltage is applied to the avalanche diode, the majority carriers—free electrons in the n-type semiconductor and holes in the p-type semiconductor—are pushed away from the junction region. As a result, the depletion region at the P-N junction widens, and very little current flows due to the minority charge carriers. This small electric current because of minority carriers is called reverse leakage current. As the reverse voltage increases, the electric field across the depletion region becomes stronger. When this voltage reaches the breakdown voltage, the electric field becomes intense enough to accelerate the minority carriers.
The high-speed free electrons collide with the atoms in the semiconductor and release more electrons. This process is called impact ionization. It creates additional free electrons and holes. These newly freed charge carriers are further accelerated and create a chain reaction. The chain reaction leads to a sudden increase in current through the diode, known as the avalanche effect. This large current flows without damaging the diode because the device is designed to handle it safely, provided the external circuit limits the current. Unlike regular diodes, the junction of an avalanche diode distributes the current evenly, preventing damage due to hotspots or localized heating. The diode remains stable during the breakdown and resumes blocking current if the reverse voltage drops below the breakdown level.
Breakdown Voltage of the Diode
The breakdown voltage of an avalanche diode is the reverse voltage at which the diode undergoes avalanche breakdown. After avalanche breakdown, a large current flows without damaging the device. The avalanche breakdown voltage of the diode depends on doping levels, depletion region thickness, and material properties. The light doping increases the breakdown voltage and the diode can withstand higher reverse voltages before the avalanche effect occurs. The wider the depletion region, the higher the breakdown voltage. The depletion region thickness ensures the electric field spreads evenly across the junction.
Applications of Avalanche Diode
Avalanche diodes are used in various fields due to their ability to handle high voltages and currents in reverse bias. Common applications include:
- Voltage Clamping: Avalanche diodes protect sensitive circuits from voltage spikes by clamping the voltage to a safe level.
- Surge Protection: They are used in power supplies and telecommunication systems to protect against lightning-induced surges and power fluctuations.
- Waveform Clipping: Avalanche diodes help shape and stabilize waveforms in electronic circuits.
- Voltage Regulation: They are used in circuits requiring a stable reverse voltage, such as reference voltage sources.
Conclusion
Avalanche diodes can handle high reverse voltages. Their unique construction and working mechanism enable them to protect sensitive electronic devices from voltage surges and spikes.
