lanar Integrated Circuits In a CMOS integrated circuit, electrons move into very small wires and experiment several quantum effects due to their associated wave nature, like a tunnel effect. Consider an electron moving inside the channel of a CMOS transistor at a nonrelativistic energy of 2eV and with an effective conduction mass of 1.08m, Me being the electron rest mass. In these conditions, which are realistic in a standard CMOS working, the electron wavelength is around 6 . With this small wavelength it never happens that the electron behaves like a wavelength into the CMOS circuits and the entire design can be performed considering particle properties. On the contrary, the wavelengths used in optical integrated circuits are generally on the same order or even smaller than the characteristic dimensions of optical integrated circuits. Thus the photon should always be considered mainly like a wave. This difference deeply influences the way in which integrated optical and electronic circuits are realized. The shape of the waveguide core that is used in optical integrated circuits cannot be scaled below a certain dimension, not to risk losing any guiding property. On the contrary, scaling is always possible with present-day technology in the case of electronics. 5.5.3.2 Digital and Analog Planar Integrated Circuits Another element that distinguishes planar integration in optics and electronics is the fact that using CMOS circuits it is always possible to realize the required functionalities with a digital circuit, even if they are analogical in nature. In the optical case, only analogical functionalities exist that have to be integrated together. It is well known that analog functionalities are much more difficult to insulate in an inte- grated circuit, especially in a waveguide-based circuit, where the behavior of the photons reflects oscillatory field characteristics. Just to give an example, it is enough to consider the evanescence-wave directional coupler that is based on the principle that a field can complet