General decoupling applications require capacitors with a combination of high capacitance and low reactance (i.e., low equivalent series resistance [ESR] and equivalent series inductance [ESL]) in the circuit. The trend in digital applications for greater data throughput at higher speed is being met by lower voltage logic operating at increasing switching speeds.
Going back to the standard capacitance equation C = KA/d, in which C = capacitance, k = dielectric constant, A = surface area of the capacitor plates, and d = distance between the capacitor plates, we can see that you can achieve higher capacitance by increasing the dielectric constant and/or surface area and decreasing the distance between the plates, the latter of which also results in lower breakdown voltage, which can be very useful in low voltage applications.
Once a voltage is applied in an ideal capacitor (i.e., one with nothing but free space between the plates), opposite charges will build up on the plates, creating an electric field between the two. The amount of charge stored is controlled by the geometry of the plates (i.e., the area and distance between them) multiplied by the dielectric constant of the free space. If the voltage is varied (e.g., if an AC signal is applied), the charges on the plates will oscillate and the AC signal will pass, but the DC signal will be blocked.
Since the dielectric constant of the free space has a very small value, insulating materials capable of increasing the electric field (dielectrics) can be sandwiched between the plates. Many of these materials are classified as para-electric, and may be amorphous or have some degree of crystallinity, but all have the ability to polarize, which is when the internal dipoles of the material align with and increase the electric field. These dipoles may arise from microscopic internal molecular alignment with the field direction (ionic polarization), a smaller effect from the electronic charge distribution in the material (electronic polarization), or an even smaller effect resulting from the internal atomic alignment (atomic polarization).
What dielectrics all have in common is that, although the plate area and separation remain constant, the relative dielectric constant of the material exhibits non-linear variability with temperature and signal frequency. So, apart from the intrinsic capacitance range of a given technology, each dielectric is best suited for different application temperatures and frequencies. Typically, ionic polarization prevails in the 100kHz – 1GHz range, which is useful for digital to general high frequency applications, and electronic polarization prevails in higher frequency RF to optical range: 1GHz – 40GHz.