The SPCM-AQ4C uses a unique silicon avalanche photodiode (SLiK) that has a circular active area of 180 μm diameter with a peak photon detection efficiency exceeding 60% at 650 nm. Each photodiode is both thermoelectrically cooled and temperature controlled, ensuring stabilized performance despite changes in the ambient temperature.

The SPCM-AQ4C module uses an improved circuit with a peak count rate >4M c/s for short bursts of time on all four channels and a count rate of 1.5 M c/s for continuous operation. There is a "dead time" of 50 nanoseconds (ns) between pulses. The module requires +2 Volts, +5 Volt, and +30 Volt power supplies. The output of each channel – a TTL pulse that is 4.5 Volts high (into a 50 Ω load) and 25 ns wide – is available at the card edge behind the circuit board. Each TTL pulse corresponds to a detected photon. All input and output signals are available at the card connector.