This paper describes a metal-insulator-semiconductor (MIS) capacitor with flat capacitance voltage characteristics and a small quadratic voltage capacitance coefficient. The device characteristics resemble a metal-insulator-metal diode except that here the capacitance depends on illumination and exhibits a strong frequency dispersion. The device incorporates Fe nanoparticles (NPs), mixed with SrF₂, which are embedded in an insulator stack of SiO₂ and HfO₂. Positively charged Fe ions induce dipole type traps with an electronic polarization that is enhanced by photogenerated carriers injected from the substrate and/or by inter nanoparticle exchange of carriers. The obtained characteristics are compared with those of five other MIS structures: two based on Fe NPs, one with and the other without SrF₂ sublayers. Additionally, devices contain Co NPs embedded in SrF₂ sublayers, and finally, two structures have no NPs, with one based on a stack of SiO₂ and HfO₂ and the other which also includes SrF₂. Only structures containing Fe NPs, which are incorporated into SrF₂, yield a voltage independent capacitance, the level of which can be changed by illumination. These properties are essential in radio frequency/analog mixed signal applications.