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