The dielectric constant, k, is a parameter defining ability of material to store charge.

In Si technology the reference is a value of k of silicon dioxide, $SiO_2$, which is 3.9.

Dielectrics featuring k>3.9 are referred to as “high”-k dielectric while dielectric featuring k<3.9 are defined as “low”-k dielectrics.

In cutting edge silicon nano electronics both high- and low-k dielectrics are needed to implement fully functional very high-density integrated circuit, although, for drastically different reasons.

Need for high-k dielectric:

1. Silicon dioxide (SiO2) has been used as a gate oxide material for decades.
2. As transistors have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase the gate capacitance and thereby drive current, raising device performance.
3. As the thickness scales below 2 nm, leakage currents due to tunneling increase drastically, leading to high power consumption and reduced device reliability.
4. Replacing the silicon dioxide gate dielectric with a high-k material allows increased gate capacitance without the associated leakage effects.

Need for low-k dielectric:

1. In digital circuits, insulating dielectrics separate the conducting parts (wire interconnects and transistors) from one another.
2. As components have scaled and transistors have gotten closer together, the insulating dielectrics have thinned to the point where charge build-up and crosstalk adversely affect the performance of the device.
3. Replacing the silicon dioxide with a low-k dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation.