Optical sensors can be classified into three different categories based on their operation as shown in the
picture above. For high photon levels applications, a simple P-I-N design photodiodes are used where
one photon creates one electron and the photo gain is one. For low-level photons detection
applications, avalanche photodiodes (APDs) are used. APDs produce multiple electrons for every photon
through avalanche process and the photo gain depends upon bias voltage.
There are two modes of APDs operation: linear and Geiger modes of operation. In the linear mode
operation, the photo detector is operated at high bias voltages and incident photons create multiple
electrons through avalanche process and the photo gain depends upon with the bias voltage. Typically,
linear mode APDs are operated with gain varying from 10 to 1000.
In Geiger-mode operation the APDs are biased above the breakdown voltage, for a short period of time
during which the sensor is active. During this period, a single photon creates a huge burst of electrons,
hence these typed of APDs are called single photon avalanche detectors (SPAD).