We present SPICE simulations of a CMOS APS pixel element consisting of the optical sensor and the circuit. The photodiode is simulated at the physics level by means of the so-called generalized devices, without any predefined compact model. Conversely, regular compact models are used for MOSFETs. Modeling with Generalized Devices takes into account in SPICE simulations both the layout and the physics of the photodiode, that is drift-diffusion transport, optical generation and recombination of excess carriers, capacitive effects and surface recombination. This approach is supported by T CAD simulations and opens the way to full SPICE simulation of Active Pixel Sensor down to the semiconductor level.