This article features a model that allows the design and simulation of perimeter-gated single photon avalanche diodes. The model enables both Geiger mode and DC mode simulations. The key parameters of the model were extracted from measured characteristics of a perimeter-gated single-photon avalanche diode fabricated in a 3-metal, 2-poly, single well CMOS process. The article also features a survey of state-of-the-art SPAD models. And, lastly, the article describes the design and measured characteristics of a pixel that includes a perimeter-gated single-photon avalanche diode and a mixed-signal readout circuit.