We present SPICE simulations of a CMOS APS pixel element consisting of the optical sensor with the sensing 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 present in the circuit. Modeling with Generalized Devices takes into account in SPICE simulations both the layout and the physics of the photodiode, i.e., drift-diffusion transport, optical generation and recombination of excess carriers, capacitive effects and surface recombination. Moreover, electrical coupling between two adjacent pixels, i.e., the electrical crosstalk, is well predicted by the network of Generalized Devices. This approach is supported by TCAD Sentaurus simulations and opens the way to full SPICE simulation of Active Pixel Sensor from the circuit down to the semiconductor level within the same circuit simulation tool.