property of a dielectric material is its permittivity.
a measure of the ability of a material to be polarized by an electric
It is, however, easier to
grasp the concept of permittivity by first discussing a
closely related property,
capacitance (C). Capacitance is a measure of the ability of a material
to hold charge if a voltage is applied across it, and is best modeled by
a dielectric layer that's sandwiched between two parallel conductive
If a voltage
V is applied across a capacitor of capacitance C, then the charge Q that
it can hold is directly proportional to the applied voltage V, with the
as the proportionality constant.
Q = CV,
or C = Q/V. The unit of measurement for capacitance is the farad
(coulomb per volt).
capacitance of a capacitor depends on the permittivity
dielectric layer, as well as the area A of the capacitor and the
separation distance d between the two conductive plates.
Permittivity and capacitance are mathematically related as follows:
dielectric used is
then the capacitance Co
is the permittivity of vacuum
(8.85 x 10-12 F/m
of a material
is the ratio of its permittivity
to the permittivity of vacuum
so k =
The dielectric constant is
therefore also known as the relative permittivity of the material. Since
the dielectric constant is just a ratio of two similar quantities, it is
definition, the dielectric constant of vacuum is 1. A
material is able to polarize more than vacuum, so the k of a material is
always > 1. Note that the dielectric constant is also a function of
frequency in some materials, e.g., polymers, primarily because
polarization is affected by frequency.
is a dielectric that has a low permittivity, or low ability to polarize
and hold charge. Low-k dielectrics are very good insulators for
isolating signal-carrying conductors from each other. Thus, low-k
dielectrics are a necessity in very dense multi-layered IC's, wherein
coupling between very close metal lines need to be suppressed to prevent
a degradation in device performance.
on the other hand, has a high permittivity. Because high-k
dielectrics are good at holding charge, they are the preferred
dielectric for capacitors. High-k dielectrics are also used in
memory cells that store digital data in the form of charge.