The APD operates in Geiger mode, that is, the reverse bias voltage applied to both ends of the APD is slightly higher than the avalanche voltage of the APD.
When a single photon incident on the surface of APD, a photogenerated carrier will be excited in the absorption layer of APD, and then the self-sustaining avalanche amplification process of APD will be caused. The avalanche signal is extracted, amplified and shaped by the peripheral electronic circuit into the counter, thus completing a single photon detection. APD is different from the PIN photodiode in that a thin P-type layer is inserted between the I layer and the layer of the PIN absorption region to become the structure.