Patent Publication Number: US-2012038429-A1

Title: Oscillator Circuits Including Graphene FET

Description:
FIELD 
     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. 
     DESCRIPTION OF RELATED ART 
     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 sp 2  hybridization. In the sp 2  hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp 2  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&#39;s equation. Graphene sheets may have a large carrier mobility of greater than 200,000 cm 2 /V-sec at 4K. Even at 300K, the carrier mobility can be higher than 15,000 cm 2 N-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. 
     SUMMARY 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates an embodiment of an oscillator circuit including a graphene FET. 
         FIG. 2  illustrates another embodiment of an oscillator circuit including a graphene FET. 
         FIG. 3  illustrates another embodiment of an oscillator circuit including a graphene FET. 
         FIG. 4  illustrates another embodiment of an oscillator circuit including dual graphene FETs. 
         FIG. 5  illustrates an embodiment of an oscillator circuit including a graphene FET. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  illustrates an embodiment of an oscillator circuit  100  including a graphene FET  103 . Graphene FET  103  has a relatively high cutoff frequency and can operate in the current saturation mode. Line voltage  101  and bias current source  102  are connected to the source of graphene FET  103 . The gate of graphene FET  103  is connected to node  106 , which may be ground or a direct current (DC) voltage source in various embodiments. The drain of graphene FET  103  is connected to LC component  104 . LC component  104  acts as a frequency-selective network, and may include one or more inductors and one or more capacitors; any appropriate arrangement and number of inductors and capacitors may comprise LC component  104  in various embodiments. LC component  104  is connected to ground connection  107 . A feedback loop  105  connects the drain of graphene FET  103  to the source of graphene FET  103  via LC component  104 . The gain provided by feedback loop  105  causes an oscillation in the circuit. Graphene FET  103  as shown in  FIG. 1  is a p-type graphene FET; in other embodiments of an oscillator circuit including a graphene FET, an n-type graphene FET may be substituted for a p-type graphene FET (discussed in further detail below with respect to  FIG. 5 ). 
       FIG. 2  illustrates an embodiment of an oscillator circuit  200  including a graphene FET  203 , in which LC component  104  of  FIG. 1  is embodied as inductor  204 A in parallel with capacitors  204 B-C in series. Graphene FET  203  has a relatively high cutoff frequency and can operate in the current saturation mode. Line voltage  201  and bias current source  202  are connected to the source of graphene FET  203 . The gate of graphene FET  203  is connected to node  206 , which may be ground or a DC voltage source in various embodiments. The drain of graphene FET  203  is connected to inductor  204 A in parallel with series capacitors  204 B and  204 C. Inductor  204 A and capacitor  204 C are connected to ground connection  207 . Feedback loop  205  is connected from between series capacitors  204 A and  204 B to the source of graphene FET  203 . The gain provided by feedback loop  205  causes an oscillation in the circuit. Assuming inductor  204 A has an inductance L, capacitor  204 B has a capacitance C 1 , and capacitor  204 C has a capacitance C 2 , the frequency (f) of the oscillation provided by oscillator circuit  200  is given by: 
         f= 1/(2π√{square root over (( L ( C 1 *C 2)/( C 1 +C 2)))}{square root over (( L ( C 1 *C 2)/( C 1 +C 2)))}).  EQ. 1
 
       FIG. 3  illustrates an embodiment of an oscillator circuit  300  including a graphene FET  303 , in which LC component  104  of  FIG. 1  is embodied as inductor  304 A and capacitors  304 B-C. Graphene FET  303  has a relatively high cutoff frequency and can operate in the current saturation mode. Line voltage  301  and bias current source  302  are connected to the source of graphene FET  303 . The gate of graphene FET  303  is connected to node  306 , which may be ground or a DC voltage source in various embodiments. The drain of graphene FET  303  is connected to inductor  304 A. Inductor  304 A is also connected to ground connection  307 . Capacitor  304 B is connected from line voltage  301  to feedback loop  305 , and capacitor  304 C is connected from the drain of graphene FET  303  to feedback loop  305 . Feedback loop  305  is connected from between capacitors  304 A and  304 B to the source of graphene FET  303 . The gain provided by feedback loop  305  causes an oscillation in the circuit. Assuming inductor  304 A has an inductance L, capacitor  304 B has a capacitance C 1 , and capacitor  304 C has a capacitance C 2 , the frequency (f) of the oscillation provided by oscillator circuit  300  is given by: 
         f= 1/(2π√{square root over (( L ( C 1 *C 2)/( C 1 +C 2)))}{square root over (( L ( C 1 *C 2)/( C 1 +C 2)))}).  EQ. 2
 
       FIG. 4  illustrates an embodiment of an oscillator circuit  400  including dual graphene FETs  403 A-B. Oscillator circuit  400  is a differential circuit. Graphene FETs  403 A-B each have a relatively high cutoff frequency and can operate in the current saturation mode. Line voltage  401  and bias current source  402  are connected to the source of graphene FET  403 A and to the source of graphene FET  403 B. The drain of graphene FET  403 A and the gate of grapheme FET  403 B are connected to capacitor  404 C and inductor  404 A via junction  407 A, and the drain of graphene FET  403 B and the gate of grapheme FET  403 A are connected to capacitor  404 D and inductor  404 B via junction  407 B. Inductors  404 A-B are connected to ground connection  406 . Capacitors  404 C-D are connected to line voltage  401  via feedback loops  405 A-B, respectively. The signal provided by feedback loops  405 A-B to line voltage  401  is fed back into the respective sources of graphene FETs  403 A-B via bias current source  402 . The gain provided by feedback loops  405 A-B causes an oscillation in the circuit. In an embodiment in which the inductance of inductors  404 A-B are each about 0.5 nanohenries (nH), and the capacitance of capacitors  404 C-D are each about 0.5 femtofarads (fF), the frequency f of the oscillation of oscillator circuit  400  may be about 100 GHz. Assuming inductor  404 A and inductor  404 B each have an inductance L, and capacitor  404 C and capacitor  404 D each have a capacitance C, the frequency (f) of the oscillation of oscillator circuit  400  is given by: 
         f= 1/(2π√{square root over (( LC ))}).  EQ. 3
 
       FIG. 5  illustrates an embodiment of an oscillator circuit  500  including a graphene FET  503 . Graphene FET  503  is an n-type FET. Graphene FET  503  has a relatively high cutoff frequency and can operate in the current saturation mode. Line voltage  501  and bias current source  502  are connected to LC component  504 . LC component  504  acts as a frequency-selective network, and may include one or more inductors and one or more capacitors; any appropriate arrangement and number of inductors and capacitors may comprise LC component  504  in various embodiments. LC component  504  is connected the drain of graphene FET  503 . The gate of graphene FET  503  is connected to node  506 , which may be ground or a DC voltage source in various embodiments. The source of graphene FET  503  is connected to ground connection  507 . Feedback loop  505  connects the drain of graphene FET  503  to the source of graphene FET  503  via LC component  504 . The gain provided by feedback loop  505  causes an oscillation in the circuit. 
     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.