Patent Application
20120038429
This disclosure relates generally to the field of oscillator circuit configuration, and more specifically to use of a graphene field effect transistor (FET) in an oscillator circuit.
Graphene refers to a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a truly planar graphene structure does not exist in a three-dimensional space, being unstable with respect to formation of curved structures such as soot, fullerenes, nanotubes or buckled two dimensional structures. However, a two-dimensional graphene structure may be stable when supported on a substrate, for example, on the surface of a silicon carbide (SiC) crystal. Free standing graphene films have also been produced, but they may not have the idealized flat geometry.
Structurally, graphene has hybrid orbitals formed by sp2 hybridization. In the sp2 hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp2 orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene is only one atomic layer thin.
Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions with a zero effective mass and are governed by Dirac's equation. Graphene sheets may have a large carrier mobility of greater than 200,000 cm2/V-sec at 4K. Even at 300K, the carrier mobility can be higher than 15,000 cm2N-sec.
Graphene layers may be grown by solid-state graphitization, i.e., by sublimating silicon atoms from a surface of a silicon carbide crystal, such as the (0001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer. Graphene layers on another material are also known in the art. For example, single or several layers of graphene may be formed on a metal surface, such as copper and nickel, by chemical deposition of carbon atoms from a carbon-rich precursor.
Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers.
While single-layer graphene sheet has a zero band-gap with linear energy-momentum relation for carriers, two-layer graphene, i.e. bi-layer graphene, exhibits drastically different electronic properties, in which a band gap may be created under special conditions. In a bi-layer graphene, two graphene sheets are stacked on each other with a normal stacking distance of roughly 3.35 angstrom, and the second layer is rotated with respect to the first layer by 60 degree. This stacking structure is the so-called A-B Bernel stacking, and is also the graphene structure found in natural graphite. Similar to single-layer graphene, bi-layer graphene has zero-band gap in its natural state. However, by subjecting the bi-layer graphene to an electric field, a charge imbalance can be induced between the two layers, and this will lead to a different band structure with a band gap proportional to the charge imbalance.
Field effect transistors (FETs) based on graphene have shown high mobility, with cut-off frequencies beyond 100 gigahertz (GHz), outperforming traditional semiconductor devices such as silicon MOSFETs. Graphene FETs may also have relatively low noise. Therefore, graphene FETs are promising components for use in radio-frequency (RF) electronics.
In one aspect, an oscillator circuit includes a field effect transistor (FET), the FET comprising a channel, source, drain, and gate, wherein at least the channel comprises graphene; an LC component connected to the FET, the LC component comprising at least one inductor and at least one capacitor; and a feedback loop connecting the FET source to the FET drain via the LC component.
In one aspect, a method for providing an oscillation in an oscillator circuit includes connecting a source of a field effect transistor (FET) to a drain of the FET via an LC component, wherein the FET comprises a channel, source, drain, and gate, wherein at least the channel comprises graphene, and wherein the LC component comprises at least one inductor and at least one capacitor.
Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings.
Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:
Embodiments of an oscillator including a graphene FET are provided, with exemplary embodiments being discussed below in detail. High-frequency oscillator circuits, with an output frequency in the range of GHz or higher, are basic components in many electronic systems, such as RF transmitters and receivers. Oscillators based on silicon (Si) or gallium arsenide (GaAs) FETs may operate at frequencies in the range of GHz, but suffer from high noise, significant nonlinearity, poor reliability, and relatively low cutoff frequency limits. However, use of a graphene FET (i.e., a FET in which at least the FET channel is made of graphene) in an oscillator circuit may provide an oscillator with an output frequency beyond tens of GHz (beyond 100 GHz in some embodiments) with good reliability, as graphene FETs exhibit high mobility, good linearity, and relatively low noise. A graphene FET also has a higher cut-off frequency than a silicon-based FET. The oscillation may be provided by operating the graphene FET in saturation mode, and through use of a feedback loop connecting the FET drain to the FET source via an LC component.
f=1/(2π√{square root over ((L(C1*C2)/(C1+C2)))}{square root over ((L(C1*C2)/(C1+C2)))}). EQ. 1
f=1/(2π√{square root over ((L(C1*C2)/(C1+C2)))}{square root over ((L(C1*C2)/(C1+C2)))}). EQ. 2
f=1/(2π√{square root over ((LC))}). EQ. 3
The technical effects and benefits of exemplary embodiments include a reliable oscillator circuit that provides an oscillation having a relatively high frequency for use in an electronic system.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
