What is the avalanche diode?

What is an avalanche diode?

An avalanche diode is a special type of diode whose operating characteristics are primarily based on the avalanche breakdown effect. This device has a wide range of applications in the field of electronics, and its unique characteristics and functions enable it to play a key role in various circuit designs.

Characteristics of Avalanche DIODES

• Avalanche Breakdown Effect

The core characteristic of avalanche diodes is the avalanche breakdown effect. When the reverse voltage increases to a specific value (i.e., the breakdown voltage), the current in the diode suddenly increases significantly. This process is called avalanche breakdown. Avalanche breakdown occurs because a strong electric field accelerates charge carriers and causes them to collide with lattice atoms, generating new electron-hole pairs. These newly generated charge carriers are further accelerated and continue to collide, producing more charge carriers in a snowball-like multiplication effect, leading to a sharp increase in current.

• High reverse voltage capability

Avalanche diodes have a high reverse voltage withstand capability, which is the foundation for their stable operation in high-voltage environments. By precisely controlling the doping concentration and geometric structure of the semiconductor material, the breakdown voltage of an avalanche diode can be adjusted to meet the requirements of different applications.

• Stability

The breakdown voltage of avalanche diodes is relatively insensitive to temperature, meaning that their performance remains relatively stable under different temperature conditions. This characteristic enables avalanche diodes to maintain good performance even in environments with significant temperature fluctuations.

• Noise source

During avalanche breakdown, avalanche diodes generate random noise. Although this noise may interfere with specific applications, it can be utilized as a noise generator in communication and signal processing fields to provide the necessary random input for systems.

• High current gain

In the avalanche breakdown state, the current gain of an avalanche diode significantly increases, enabling it to output larger currents. This characteristic gives avalanche diodes an advantage in applications requiring high current output.

The Function of Avalanche Diodes

• Overvoltage Protection

One of the primary functions of a snow avalanche diode in a circuit is to provide overvoltage protection. When the voltage in the circuit exceeds a safe threshold, the snow avalanche diode rapidly enters a breakdown state. It conducts, thereby clamping the overvoltage to a lower voltage level and protecting other electronic components from overvoltage damage. This protective mechanism is fundamental in applications such as power supply circuits, communication equipment, and automotive electronics.

• Voltage Regulation

Since avalanche diodes can maintain a stable current output in the breakdown state, they can also be used as voltage regulation components. By designing the circuit appropriately, avalanche diodes can be combined with resistors, capacitors, and other components to form voltage regulation circuits, enabling precise control of the output voltage. Such voltage regulation circuits have widespread applications in power supply systems, electronic measurement instruments, and other fields.

• High-frequency oscillation

Avalanche diodes can also utilize the avalanche breakdown effect to generate high-frequency oscillations. When the reverse voltage increases to a specific value, the current in the avalanche diode suddenly increases and produces a time delay effect (i.e., the current lags behind the voltage). By appropriately controlling this time delay effect, high-frequency oscillation signals can be generated. These high-frequency oscillation signals have significant application value in fields such as wireless communication and radar systems.

• Noise Generation

As mentioned earlier, avalanche diodes generate random noise during breakdown. While this noise may interfere with specific applications, it can be utilized as a noise generator in communication and signal processing fields. By adjusting the operating conditions and circuit parameters of the avalanche diode, precise control over noise characteristics can be achieved to meet the requirements of different applications.

• Other Applications

In addition to the primary functions mentioned above, avalanche diodes can be used in various other applications. For example, they can serve as high-speed switching elements in pulse generators, frequency multipliers, and other circuits; as detection elements for measuring the amplitude and frequency of high-speed signals or pulses; and by combining multiple avalanche diodes into an array structure, they can be used to achieve complex functions such as high-power microwave radiation sources.

The structure of an avalanche diode

The structural characteristics of an avalanche diode are summarized as follows:

• Structural composition

 An avalanche diode consists of two P-type and N-type semiconductors arranged in an alternating pattern, forming a PN junction. This structure enables the avalanche diode to exhibit unique performance under specific voltage conditions.

• Carrier acceleration

When a reverse voltage is applied to the PN junction, the carriers (electrons and holes) are accelerated by the built-in electric field. This acceleration process causes electrons and holes to collide within the PN junction, triggering the avalanche effect.

• Avalanche effect

During the collision of electrons and holes, more electron-hole pairs are generated. This process is known as the avalanche effect, which causes the PN junction to break down, resulting in a sudden increase in current.

The symbol for an avalanche diode

The circuit symbol for an avalanche diode is identical to that of a Zener diode.

Its standard representation is:

• Triangular arrow (anode): Represents the P-type semiconductor.
• Vertical cathode line: Represents the N-type semiconductor.
• Key feature - small hook/bend: At the end of the vertical cathode line, there is a small hook pointing outward (sometimes drawn as a slight bend or the starting point of a "Z" shape). This small hook is the distinguishing feature of breakdown diodes (including Zener diodes and avalanche diodes) from ordinary rectifier diodes.

Standard symbol diagram:

Why do they have the same symbol as a Zener diode?

• Both are reverse breakdown diodes: Zener diodes and avalanche diodes are devices that operate based on the breakdown characteristics of a semiconductor PN junction under reverse bias voltage. Their design objectives are to begin conducting at a specific reverse voltage (breakdown voltage) and maintain a relatively stable voltage.
• Different physical mechanisms, symbols not distinguished: Although the physical mechanisms of Zener breakdown (occurring at lower voltages, < approximately 5.6V, primarily due to the direct destruction of covalent bonds by a strong electric field) and avalanche breakdown (occurring at higher voltages, > approximately 5.6V, primarily due to the chain reaction of carrier collision ionization) are different, international standards have not designed different symbols for these two types of diodes based on different breakdown mechanisms at the circuit symbol level. They share the same "breakdown diode" symbol.
• Dependent on textual annotations: In actual circuit diagrams, distinguishing whether a breakdown diode symbol represents a Zener diode or an avalanche diode primarily relies on device model annotations (e.g., BZX85C5V6 may be a Zener diode, SADxxxx may be an avalanche diode) or textual notes added next to the symbol (e.g., "Zener" or "Avalanche"). Sometimes the breakdown voltage value is also labeled next to the symbol (e.g., "5.6V" or "200V"), and the voltage value typically indicates the operating mechanism (lower voltage is more likely a Zener, higher voltage is more likely an avalanche).

How does an avalanche diode work?

The working principle of an avalanche diode is based on a PN junction with a low dopant concentration. When the reverse voltage across the PN junction increases, the electric field in the space charge region becomes stronger.

As a result, electrons and holes in the space charge region gain energy under the influence of the electric field. These electrons and holes, moving through the crystal, continuously collide with crystal atoms. When the energy of electrons and holes becomes sufficiently high, such collisions can excite electrons in covalent bonds to form free electron–hole pairs. The newly generated electrons and holes move in opposite directions, reacquire energy, and can again produce electron-hole pairs through collisions. This is the carrier multiplication effect. When the reverse voltage increases to a specific value, the multiplication of carriers occurs like an avalanche on a steep snow-covered slope, with carriers increasing rapidly and significantly. As a result, the reverse current surges, and the PN junction undergoes avalanche breakdown. This characteristic can be utilized to fabricate high-voltage diodes.

The figure below illustrates the avalanche breakdown process.

An avalanche diode is a negative resistance device characterized by high output power but also high noise levels. The primary noise originates from avalanche noise, which is caused by the generation of electrons and holes during the avalanche multiplication process and their irregular behavior, similar to shotgun noise. Avalanche noise is the main reason why the noise level of an avalanche diode oscillator is significantly higher than that of other oscillators.

It is a transistor that can generate high-frequency oscillations under the influence of an external voltage. The principle of generating high-frequency oscillations is as follows: avalanche breakdown is used to inject carriers into the crystal. Since carriers require a certain amount of time to cross the crystal, the current lags behind the voltage, resulting in a delay time. If the crossing time is appropriately controlled, an adverse resistance effect will appear in the current-voltage relationship, thereby generating high-frequency oscillations. It is commonly used in oscillation circuits in the microwave field.

The PN junction has unidirectional conductivity, with low forward resistance and high reverse resistance.

When the reverse voltage increases to a specific value, the reverse current suddenly increases. This is called reverse breakdown. It is divided into avalanche breakdown and Zener breakdown (tunnel breakdown).

Avalanche breakdown occurs when the reverse voltage across the PN junction increases to a specific value, causing the number of charge carriers to multiply rapidly, similar to an avalanche.

A diode made using this characteristic is called an avalanche diode. Under the influence of an electric field, the energy of the charge carriers increases, causing them to collide with crystal atoms and excite electrons in the covalent bonds, forming free electron-hole pairs.

The newly generated carriers collide to produce more free electron-hole pairs, creating a multiplication effect. One becomes two, two become four, and so on, increasing the number of carriers like an avalanche.

Zener breakdown is entirely different. At high reverse voltages, a strong electric field exists in the PN junction, which can directly break covalent bonds to separate bound electrons and form electron-hole pairs, resulting in a large reverse current. Zener breakdown requires a very high electric field strength and is only achievable in PN junctions with extremely high impurity concentrations.

Ordinary diodes do not have such high impurity concentrations, and their breakdown is avalanche breakdown. Zener breakdown is primarily observed in special diodes, namely Zener diodes.

Applications of Avalanche Diodes

An avalanche diode is a special type of semiconductor device that operates under high reverse bias and converts light signals into electrical signals through the avalanche effect. The following are some typical applications of avalanche diodes:

• Fiber Optic Communications

Avalanche diodes play a crucial role in fiber optic communication systems, converting received optical signals into electrical signals. They offer high gain and fast response times, making them suitable for long-distance and high-speed communication systems.

• LiDAR (Light Detection and Ranging)

 In autonomous driving and robot navigation, LiDAR systems use avalanche diodes as detectors to achieve precise distance measurement and environmental sensing.

• Biomedical Imaging

Avalanche diodes are used in fluorescence microscopy and optical imaging to detect weak bioluminescent signals, enhancing imaging sensitivity and resolution.

• Single-photon detection

In quantum computing and quantum communication, single-photon avalanche diodes (SPADs) are used to detect individual photons, which is crucial for achieving quantum key distribution and quantum imaging.

• Security Systems

In security monitoring and intrusion detection systems, avalanche diodes can serve as photodetectors to detect minute changes in light signals, thereby enhancing system sensitivity.

• Environmental Monitoring

In atmospheric and water pollution monitoring, avalanche diodes can detect light signals of specific wavelengths to analyze pollutant concentrations.

• Scientific Research

In fields such as physics, chemistry, and materials science, avalanche diodes are used to measure the emission and absorption characteristics of photons, aiding in the understanding of materials' optoelectronic properties.

The high gain and fast response characteristics of avalanche diodes make them highly versatile in applications requiring high Sensitivity and rapid signal processing. As technology advances, its application scope may further expand.

FAQs

• What is the difference between a zener diode and an avalanche diode?

Among the many types of diodes, avalanche diodes and Zener diodes are like two brilliant pearls, each emitting a unique glow.

Zener diodes, based on the unique principle of the P-N junction rectification effect, play important roles in various circuits, such as voltage regulation and signal detection. Their proper use is crucial for the stable operation of circuits. Like Zener diodes, avalanche diodes also have unique structures and functions, playing key roles in different electronic applications.

The avalanche diode, also known as the negative resistance diode or avalanche breakdown diode, operates based on the fascinating avalanche breakdown effect. Structurally, it consists of P-type and N-type semiconductor materials, with a special doped region between them, referred to as the avalanche region. This region has a very high doping concentration, resembling a bustling "marketplace" for electrons, where electrons and holes frequently collide, resulting in frequent avalanche breakdown phenomena. When the reverse voltage exceeds a specific threshold, electrons and holes collide violently in the avalanche region, causing the current to increase rapidly. This characteristic gives avalanche diodes significant advantages in voltage regulation and overvoltage protection. It is worth noting that it is also a negative resistance device with high output power. However, it also has relatively high noise, primarily due to the irregular movement of electrons and holes during the avalanche multiplication process, resulting in noise similar to shotgun noise. For example, in microwave oscillation circuits requiring high-power output, avalanche diodes play an indispensable role.

Moving on to the Zener diode, also known as a voltage-regulating diode or Zener breakdown diode. Its structure also includes P-type and N-type semiconductor materials, but unlike the avalanche diode, it does not have a dedicated avalanche region. Instead, it relies on the critical P-N junction to achieve its unique functionality. The P-N junction acts as a one-way "gate" for electrons, allowing current to flow in only one direction. When a forward voltage is applied to the P-type end and a reverse voltage to the N-type end, current flows smoothly; conversely, current is almost completely blocked. Based on this rectifying effect of the P-N junction, Zener diodes excel in rectifier circuits and signal detection applications.

Additionally, Zener diodes are heavily doped, resulting in a skinny depletion layer. This characteristic enables them to conduct current under both forward and reverse bias conditions, thereby achieving rectification and voltage regulation functions. In various circuits requiring a stable voltage, they serve as a reliable "helper."

Now, let us compare the differences between avalanche diodes and Zener diodes in detail. Structurally, the avalanche region is the distinctive feature of the avalanche diode, while the Zener diode is centered around the P-N junction. In terms of operating principles, the avalanche diode relies on the avalanche breakdown effect, whereas the Zener diode is based on the rectification effect of the P-N junction. This is akin to two different "engines" driving them to perform distinct functions in circuits. In terms of doping levels, avalanche diodes have lower doping levels, while Zener diodes are heavily doped. This difference further leads to distinct conductive behaviors: Zener diodes, with their high doping and thin depletion layer, can conduct current in both forward and reverse bias, whereas avalanche diodes primarily rely on avalanche breakdown to increase current in reverse bias. Finally, in terms of application scenarios, avalanche diodes, with their negative resistance characteristics and high output power, are crucial in microwave oscillation circuits, voltage regulation, and overvoltage protection applications; Zener diodes, with their rectification and voltage regulation functions, are widely used in rectification circuits and signal detection applications.

Although avalanche diodes and Zener diodes belong to the same family of diodes, they exhibit significant differences in structure, operating principles, doping levels, conductivity, and application scenarios. It is precisely these differences that have enabled them to find their respective niches in the field of electronic engineering, each with unique application values and development prospects. As electronic technology continues to advance at a rapid pace, these two types of diodes are expected to emerge in more fields, be studied more deeply, and be applied more widely, contributing even more to the development of the electronic world.

• What is the difference between an avalanche diode and a regular diode?

Avalanche diodes and normal diodes differ significantly in several aspects, primarily in their operating principles, characteristics, application fields, and internal structures.

(1) Operating principle

A regular diode (typically referred to as a rectifier diode or switching diode) operates based on the unidirectional conductivity of the PN junction. When the diode is forward-biased (i.e., the P-type region is connected to a positive voltage and the N-type region to a negative voltage), the built-in electric field at the PN junction is weakened, allowing the majority carriers (holes in the P-type region and electrons in the N-type region) to diffuse toward the opposite region under the influence of the electric field, thereby forming a forward current. When the diode is reverse-biased (i.e., the P-type region is connected to a negative voltage and the N-type region is connected to a positive voltage), the built-in electric field of the PN junction is strengthened, blocking current flow, with only a weak reverse saturation current present.

An avalanche diode (Avalanche Diode, abbreviated as AVD) or avalanche photodiode (Avalanche Photodiode, abbreviated as APD) exhibits a unique avalanche effect. When the avalanche diode is reverse-biased and the bias voltage approaches or exceeds its breakdown voltage, the electric field strength within the PN junction becomes excessively high, enabling minority carriers (i.e., reverse-injected electrons or holes) to gain sufficient energy in the electric field to collide with lattice atoms and excite new electron-hole pairs. These newly generated carriers continue to collide, producing more electron-hole pairs and forming a chain reaction, known as the avalanche multiplication effect. This effect causes the reverse current to increase sharply, potentially reaching hundreds or even thousands of times the forward current.

(2) Characteristic Differences

• Breakdown Voltage and Reverse Current

Ordinary diodes: The reverse breakdown voltage is one of their important electrical parameters, but it should be avoided during regular operation to prevent diode damage. The reverse saturation current is tiny and can be neglected. Avalanche diodes: Their design is intended to allow reverse breakdown under certain conditions and utilize the avalanche effect to amplify current. Therefore, the reverse breakdown voltage is a crucial parameter for avalanche diodes, and the reverse current can increase significantly after breakdown.

• Sensitivity and gain

Ordinary diodes: Due to the limitations of their working principle, they have weak detection capabilities for weak signals and lack an internal gain mechanism.Avalanche photodiodes: By utilizing the avalanche effect to achieve internal gain, they can significantly enhance Sensitivity to weak light signals. The magnitude of gain depends on the reverse bias voltage and the structural parameters of the avalanche region.

• Noise Performance

Ordinary diodes: Due to their relatively low reverse saturation current, they exhibit good noise performance but are not suitable for applications requiring extremely high Sensitivity.Avalanche photodiode: Although the avalanche effect introduces some noise, it can be controlled within a low range through optimized design and appropriate circuit techniques, thereby meeting the requirements for high-sensitivity detection.

• Response speed

Ordinary diodes: They have a fast response speed and are suitable for fast switching and rectification requirements in most electronic circuits.Avalanche photodiodes: Due to their internal gain mechanism, they perform well in high-speed optical signal detection. In particular, the fast response characteristics of avalanche photodiodes are fundamental in fiber optic communications and spectral analysis.

(3) Application fields

Ordinary diodes are widely used in various electronic circuits, such as rectifier circuits, voltage regulator circuits, and switching circuits. Their main functions are to convert, control, and protect electrical energy.

Additionally, in the power supply sections of electronic devices, signal processing circuits, and power Avalanche diodes (particularly avalanche photodiodes) are primarily used in applications requiring high-sensitivity detection of weak light signals. For example, in fiber optic communication systems, avalanche photodiodes serve as a critical component of optical receivers, converting received light signals into electrical signals for amplification and processing.

Furthermore, avalanche photodiodes also play an important role in fields such as spectral analysis, optical electron microscopy, and laser radar.

• What is the difference between a pin diode and an avalanche diode?

PIN: When the light-sensitive junction receives light of the corresponding wavelength, a photocurrent is generated.

Avalanche diode: In addition to the same components as a PIN diode, it has an avalanche gain region where the photoelectric current is amplified. The amplification factor is called the avalanche gain coefficient. However, noise current is also generated.

PIN photodiodes and avalanche photodiodes are both semiconductor photodetectors, using the same materials and having the same spectral response range. The advantages of PIN photodiodes include high responsivity, fast response speed, wide bandwidth, low operating voltage, simple bias circuit, and the ability to withstand high reverse voltage under reverse bias, resulting in a wide linear output range. The drawback is that the resistance of the I layer is very high, resulting in low output current, typically ranging from a few microamperes to several microamperes. Therefore, PIN photodiodes are typically connected to a preamplifier.

Avalanche photodiodes are internal gain photodetectors. Although the avalanche gain is much smaller than that of photomultiplier tubes (PMTs), it still makes the Sensitivity of APDs much higher than that of PIN photodiodes, addressing the low Sensitivity issue in PIN photodiodes. This advantage becomes more evident in high-speed modulation and weak signal detection. However, due to its gain effect, noise in the signal is also amplified, and its gain coefficient is affected by temperature, necessitating temperature compensation measures when required. Compared to APDPIN, photodiodes are less sensitive to temperature and have fewer application restrictions, so most systems use PIN photodiodes. However, in conditions where signal loss is excessive, the light signal is too weak, or long-distance transmission is required, APDs are highly necessary.

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