Paraelectric and superparaelectric varactors

Nano-scale tunable capacitors with extremely small leakage currents


The varactor with nanometer-scale pillars (105) made from a paraelectric or superparaelectric material. Located between each of the nanopillars is a linear dielectric medium (104) that can be an amorphous solid material or an ambient medium such as air, a vacuum, hermetically sealed gas, and the like.

Varactor diodes are semiconductor devices that are widely used in the electronics industry in applications where a voltage-controlled variable capacitance is required, such as in the radio frequency (RF) design arena. They provide a method of varying the capacitance within a circuit by the application of a control voltage.

Various materials and dopants have been used to enhance performance properties of varactors like tunability or to reduce undesired performance properties like microwave power loss. In addition, temperature stability has been improved by grading thin-film dopant concentrations or growing dopant-stratified heterostructures. However, the nano-structuring of such devices has been limited to the thickness of the thin films and further improvements in tunability or reduced power losses would be difficult without changes in the material structure. To address this point, Army researchers have developed varactors with two-dimensional (2-D) arrays of nanopillars made from material with a paraelectric or superparaelectric phase and separated by a dielectric medium. The 2D array is electrically connected and forms a micron-sized varactor.  The research team has fabricated these materials via E-beam lithography and inductively coupled plasma dry etch.

System applications that directly benefit from these nano-engineered paraelectric and/or superparaelectric varactors include tunable filters, tunable resonators, tunable phase shifters, tunable delay lines, tunable impedance matching networks, and voltage-controlled oscillators. Such components are essential to agile communication networks using software-defined reconfigurable radios, phased array radars, beam steering antennas, and related devices.

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