The working principle of an APD detector is based on two core mechanisms: the photoelectric effect and the avalanche multiplication effect, and the entire process can be divided into two key stages.

 

The core of an APD detector is a PN junction made of semiconductor materials. When the target optical signal irradiates the photosensitive region of the PN junction, the energy of photons is absorbed by electrons in the semiconductor. If the photon energy is greater than the band gap width of the semiconductor, electrons will jump from the valence band to the conduction band, leaving holes in the valence band at the same time and forming pairs of "photogenerated carriers" (electron-hole pairs). This step completes the preliminary conversion of optical signals into electrical signals, which is consistent with the working principle of ordinary photodiodes.

 

This is the key that distinguishes APD detectors from ordinary devices. A reverse bias voltage much higher than its breakdown voltage is applied across the PN junction of the device, creating an extremely strong electric field inside the PN junction. The photogenerated carriers generated in the first stage will be accelerated under the action of the strong electric field to gain extremely high kinetic energy. High-speed moving carriers will collide with atoms in the semiconductor lattice, knocking out electrons from the lattice atoms and forming new electron-hole pairs. These newly generated carriers are also accelerated by the strong electric field and continue to collide with other atoms, generating more carriers—this process is like an avalanche, causing the number of carriers to increase sharply, thereby amplifying the initial weak photogenerated current by thousands or even tens of thousands of times.

 

After avalanche multiplication, the electrical signal can be accurately detected and read through subsequent circuit processing. It is this unique mechanism of "photoelectric conversion + avalanche amplification" that allows APD detectors to demonstrate incomparable advantages over ordinary photoelectric detection devices in weak optical signal detection scenarios.