In-depth analysis of the subtle differences between SPAD and APD

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Exploring the Differences Between Single-Photon Avalanche Diodes and Avalanche Photodiodes

Single-photon avalanche diodes (SPADs) and avalanche photodiodes (APDs) are both photodetectors capable of converting light signals into electrical signals, but there are still some subtle differences between them:

Operating Principle: SPADs are designed to detect individual photons; in contrast, APDs aim

to detect and measure light signals across a wide intensity range.

Sensitivity: SPADs have higher sensitivity than APDs, allowing precise detection of individual photons; APDs have lower sensitivity for detecting and measuring light signals of varying intensities.
Linearity: The linear relationship between output electrical signal and input light intensity is usually not as ideal for SPADs compared to APDs.
Applications: SPADs are often used where high sensitivity and single photon detection are needed, such as quantum key distribution, lidar and sensing; APDs are often used where detection and measurement of a wide range of light intensities is required, such as in optical communications, medical imaging and military applications. In summary, the main differences between SPADs and APDs lie in sensitivity and types of detected light signals. SPADs have high sensitivity for detecting individual photons, while APDs have lower sensitivity for detecting a wide range of light intensities.

Why Do APDs Require Higher Bias Voltages?

Avalanche photodiodes (APDs) require relatively high bias voltages to operate properly. The high bias is necessary to generate a sufficiently strong electric field within the APD to drive carrier (electron and hole) avalanche multiplication. When an APD absorbs an incident photon, an electron-hole pair is generated. The strong electric field causes electrons and holes to move in opposite directions and collide with other carriers. These collisions trigger a chain reaction known as avalanche multiplication, producing more carriers. The increase in carrier numbers boosts the APD’s electrical current, allowing it to detect and measure different light intensities.

Several factors determine the bias voltage needed for an APD, including material composition, doping levels, size and geometry. Generally, larger APDs, higher doping levels require higher biases to generate the electric fields needed for avalanche multiplication. In summary, a high bias voltage is critical for normal APD operation, as it produces the electric field to drive carrier avalanche multiplication so the APD can detect and measure a wide range of light intensities.

What is the Avalanche Effect in APDs?

The avalanche effect in avalanche photodiodes (APDs) occurs when an incident photon is absorbed in the APD, creating an electron-hole pair. The avalanche effect is key to the normal operation of APDs, allowing them to detect and measure different light intensities. The avalanche effect relies on the formation of a sufficiently strong internal electric field within the APD to drive carrier avalanche multiplication. Within the strong electric field, electrons and holes collide with each other. These collisions trigger chain reactions leading to the generation of more carriers through avalanche multiplication. The increase in carrier numbers enhances the electrical current in the APD, enabling detection and measurement of varying light intensities. The magnitude of the avalanche effect is proportional to the strength of the internal electric field and doping levels in the APD material. Stronger electric fields and higher doping lead to greater carrier multiplication and higher APD gain. Overall, the avalanche effect is an important operating mechanism for APDs that relies on electric field-driven carrier avalanche multiplication to enable APDs’ ability to detect a wide range of light intensities.