Avalanche Photodiodes: Working, Types and Applications

Introduction

Avalanche Photodiodes (APDs) are highly sensitive semiconductor light detectors that provide internal current gain through the Avalanche multiplication method. This makes them ideal for detecting weak light signals for applications such as LiDAR and fiber-optic communication systems. 

The APDs have many benefits in addition to being highly sensitive. They also offer fast response time, have a better signal-to-noise ratio, and are compact and durable. 

Working Principle of Avalanche Photodiodes

The working principle of APDs depends on generating charge carriers via photon absorption and then multiplying the carriers through impact ionization in a high-electric-field region. This is an important process that boosts the sensitivity to weak light signals. 

Photocarrier Generation 

The process begins when incident photons strike the semiconductor material in the absorption region. 

If a photon has sufficient energy, it is then absorbed and excites an electron from the valence band to the conduction band. At the same time, this creates a free electron and a hole. This initial process is called the inner photoelectric effect. 

Impact Ionization and Avalanche Multiplication 

An APD is usually operated under a high reverse bias voltage. This is close to the breakdown voltage that creates a strong internal electric field. 

• Acceleration: The intense electric field accelerates the photogenerated electrons and holes. This makes them gain kinetic energy as they drift through the device. 
• Impact ionization: The energetic carriers collide with atoms in the crystal lattice, which means more additional bound electrons are knocked out from their positions. This generates new secondary electron-hole pairs. 
• Avalanche: The newly created secondary carriers are still accelerated by the field, causing further impact ionization events. This leads to a chain reaction. This cascade of carrier generation is now known as the Avalanche effect, which rapidly multiplies the initial photocurrent. 

Structure of an APD

The Avalanche photodiode construction features three main regions, including the absorption, multiplication, and depletion regions. 

Absorption region: This is a wider, low electric field region. It is mostly made of an intrinsic or lightly doped layer. It is where most incident photons are absorbed, helping generate the primary electron-hole pairs. 

Multiplication region: It is much thinner compared to the absorption region. It is a region with a very high electric field. It is specifically designed for impact ionization, enabling it to occur more efficiently and with minimal noise. 

Depletion region: Under high reverse bias, the depletion region encompasses both the multiplication and absorption regions to ensure efficient carrier sweep-out and high-speed operation. 

Types of Avalanche Photodiodes

Now that you know what is Avalanche photodiode, let us look at the various types available. The categorization can be based on several factors, including semiconductor and structural variants. 

Semiconductor Material Variants 

Choosing a semiconductor material determines the APD’s spectral response and its inherent noise characteristics. Here are typical examples:

Silicon (Si) APDs

These are highly sensitive to light in the UV, visible, and near-infrared spectra (250nm to 1100nm). Silicon is preferred because of its mature processing technology and its very low impact ionization coefficient ratio. These features result in low excess noise and high usable gain. 

Such APDs are used in laser rangefinders, medical imaging, and scientific instruments. 

Germanium (Ge) APDs

Germanium detects longer infrared wavelengths than silicon. This is typically from 800nm to 1600nm. However, germanium tends to have a high K-ratio, resulting in higher noise than silicon APDs. You may find modern Ge APFs with a silicon substrate as a hybrid structure to combine the infrared absorption of Ge and the low-noise multiplication of Si. 

InGaAs and InP APDs

Indium Gallium Arsenic (InGaAs) APDs are best known for their performance in high-speed fiber-optic communication in the infrared spectrum (1310nm to 1550nm). These APDs are used in the absorption layer, while the Indium Phosphide (InP) is in the multiplication layer. This combination leads to excellent noise performance, high bandwidth, and high quantum efficiency. 

Structural Variants 

Specific internal designs can be engineered to achieve specific results, such as controlling the electric-field distribution, enhancing performance, and managing noise. Here are examples of structural variants of APDs. 

Reach-through APDs

This is the most common design, especially in the silicon APDs. It features a wide, lightly dope absorption region and a narrow, high-field multiplication region. The depletion region reaches through the entire intrinsic layer at the operating voltage. This ensures all photogenerated carriers are rapidly swept into the multiplication region via the electric field. This leads to faster response times and high quantum efficiency. 

Separate Absorption and Multiplication (SAM) APDs

This structure separates the material optimized for light absorption from the material optimized for avalanche multiplication. Having this separation is vital for independent optimization of each process. This reduces the noise and enhances overall efficiency. 

Separate Absorption, Grading, Charge, and Multiplication (SAGCM) APDs

You can look at this as an advanced version of SAM APD. Its performance makes it usable in InGaAS APDs. The APD comes with an additional grading layer to smooth the bandgap differences between the multiplication and absorption layers. It also has a charge layer to precisely control the electric field distribution. Such prevents carrier accumulation at interfaces and further optimizes noise and speed characteristics. 

Geiger-mode APDs (Gm-APDs)

These detectors are operated at a reverse bias voltage above their breakdown voltage. In this case, a single absorbed photon triggers a self-sustaining, macroscopic avalanche current pulse. This output is a digital pulse, enabling single-photon counting. 

Single-Photon Avalanche Diodes (SPADs)

These are Gm-APDs that are designed and optimized for single-photon detection. They come with integrated or external quenching circuits to stop avalanche pulses after detection and then reset the device for the next photon event. These avalanche photodiodes are highly sensitive, making them well-suited for LiDAR, time-resolved imaging, and quantum communication applications. 

Key Performance Parameters of APDs

APDs can be complex devices whose performance is measured by key parameters. These include their sensitivity, speed, efficiency, stability, and signal quality. 

Quantum Efficiency 

This measures the effectiveness of the APD’s absorption region. It is the ratio of the number of electron-hole pairs generated to the number of incident photons. 

A high quantum efficiency ranges from 60 to 95%. It ensures that a maximum number of incoming photons are converted into the initial charge carriers before the multiplication stage. 

Responsivity 

This is a practical measure of the device’s output current relative to the input optical power. It is expressed as Amperes per Watt. 

Avalanche Gain 

The avalanche gain or the multiplication factor is the central performance metric of an APD. It is the average number of secondary carriers produced for each primary carrier injected into the multiplication region. 

The gain varies from a few tens to hundreds or thousands, depending on the type of avalanche photodiode. The gain mechanism is non-linear with respect to the applied voltage. 

Breakdown Voltage Stability 

The breakdown voltage is the critical reverse bias at which the avalanche process is self-sustaining. The stability of the breakdown voltage is vital as the gain mechanism is susceptible to voltage fluctuations. 

A variation of only a fraction of a volt can have drastic changes in the gain factor. 

The breakdown voltage stability increases with temperature. This means there is a need for active temperature-compensation circuits or highly regulated power supplies to maintain a stable gain. 

Noise Figure and Excess Noise Factor 

All photodiodes can generate noise, but the avalanche process introduces additional statistical noise or excess noise. 

The excess noise factor is a multiplier that describes how much noisier an APD is compared to an ideal noiseless amplifier having the same gain. 

The excess noise factor depends on the ratio of the impact-ionization coefficients for electrons and holes. 

Materials with a low K-ratio, such as silicon, come with a low excess noise factor and superior noise performance. 

Dark Current and Thermal Effects

Dark current is the small leakage current that flows even if there is no light incident on the APD. This is due to the thermal generation of carriers. 

A high dark current reduces the overall sensitivity of the APD as it generates spurious noise that can mask weak light signals. 

Response Time and Bandwidth 

These are important parameters that determine how quickly the APD can respond to changes in light intensity. 

Response time is limited by carrier transit time across the depletion regions and the device’s resistance and capacitance time constant. 

Modern APDs are able to achieve bandwidths in tens of Gigahertz, which makes them suitable for high-speed fiber-optic data transmission. The gain-bandwidth product is a key figure of merit, indicating a trade-off between maximizing bandwidth and maximizing gain. 

Wave Sensitivity 

Wavelength sensitivity describes the range of wavelengths the APD can efficiently detect. The semiconductor bandgap energy determines this. Different materials can be used for specific spectral windows. 

Applications of Avalanche Photodiodes

Avalanche photodiodes are commonly used in numerous fields where reliable detection of weak, high-frequency, or low-intensity light signals is required. Here is where you can expect to find Avalanche photodiodes. 

Telecommunications 

APDs are essential in fiber-optic networks to achieve better distance and data rate performance. 

APDs work as sensitive detectors in the receiver module. Their internal gain allows the receiver to detect much weaker optical signals being transmitted over long distances before amplification is necessary. 

Imaging and Photon Counting 

APD’s ability to detect very few photons makes them invaluable in sensitive imaging and counting applications. 

You will come across APDs in Positron Emission Tomography scanners to detect faint flashes of light produced when gamma rays interact with the detector crystals. APDs offer a fast response time and a compact size, making them suitable for this application. 

LiDAR and Ranging 

APDs have revolutionized distance measurement systems by providing rapid, sensitive detection of reflected laser pulses. 

APDs and SPADs are the detectors of choice for applications such as automotive LiDAR systems. These are used in Advanced Driver Assistance Systems and autonomous driving. This is because they offer the necessary sensitivity needed to detect weak reflections from distant objects. 

Scientific and Industrial Applications 

APDs can be integrated into a wide variety of industrial tools and scientific research equipment. 

The optical time-domain reflectometry instruments are examples of applications where APDs are crucial. In this case, the APDs are crucial for detecting the extremely weak backscatter signals. 

Choosing the Right Avalanche Photodiode

1. Wavelength Sensitivity and Material 

Always make sure that the APD’s spectral response to the wavelength of the light you need to detect is paramount. 

The Silicon APDs are best for the UV to near-infrared (300 nm to 1100 nm) range. Expect them to be widely used with 905 nm lasers in consumer and industrial LiDAR applications. 

The InGaAS APDs are ideal for the infrared range of 900 nm to 1700 nm. This is essential for fiber optic communication systems. 

Germanium APDs are an alternative for the infrared range (800 nm to 1600 nm). They are often used in cost-sensitive applications, but can be noisier than InGaAs. 

2. Sensitivity and Signal-to-Noise Ratio

APDs are usually chosen because of their ability to detect low light levels, their gain, and noise characteristics. 

The Avalanche gain determines how much the signal is amplified. For example, Silicon APDs can reach higher gains of up to 1000+ than the InGaAs APDs, which are only around 100. 

The excess noise factor is worth considering, as the avalanche process adds noise. Silicon has a lower excess noise factor than InGaAs or Germanium. This leads to having a better SNR overall. 

3. Speed and Bandwidth 

The required detection speed dictates the physical design of the APD. A trade-off exists between the detector’s active area and its bandwidth or speed. Having large active areas can increase the junction capacitance. This reduces the bandwidth and slows the response time. 

High-speed fiber optic communication requires APDs with bandwidths in the GHz range. Low-frequency applications can use larger-area, higher-capacitance APDs. 

4. Operating Mode and System Integration 

You should determine whether you need an analog output or a digital photon counting. 

The linear mode operates below the breakdown voltage for analog measurement of light intensity. This mode requires a stable high voltage bias and temperature compensation circuitry. 

Geiger mode operates above the breakdown voltage for single-photon counting applications. It requires specific quenching circuits to reset the device after each photon detection.

Consider the mechanical requirements as well. This helps with choosing the APD depending on packaging. 

Conclusion

The Avalanche Photodiodes are critical components in photonics. They offer exceptional sensitivity via impact ionization for signal multiplication. This allows for the detection of weak light signals in challenging, noise-limited environments. Their selection involves balancing the material properties and structural designs while also requiring complex circuitry. APDs provide significant performance advantages that are important across different industries.