APDs versus SPADs
Both APDs and SPADs are reverse biased semiconductor p-n junctions. However, APDs are biased close to, but below the breakdown voltage of the semiconductor. This high electric field provides an internal multiplication gain only on the order of few hundreds, since the avalanche process is not diverging as in SPADs. The resulting avalanche current intensity is linearly related to the optical signal intensity. A SPAD however operates with a bias voltage above the breakdown voltage. Because the device is operating in this unstable above-breakdown regime, a single photon (or a single dark current electron) can set off a significant avalanche of electrons. Practically, this means that in an APD a single photon produces only tens or few hundreds of electrons, but in a SPAD a single photon triggers a current in the mA region (billions of billions of electrons per second) that can be easily "counted".
Therefore, while the APD is a linear amplifier for the input optical signal with limited gain, the SPAD is a trigger device so the gain concept is meaningless.
Read more about this topic: Single-photon Avalanche Diode