Carbon nanotube-based integrated power converters

A substrate having trenches containing carbon nanotubes has elements to provide a power converter. One set of trenches is configured to form an inductor and another set is configured to form a capacitor. Trenches in the substrate are also configured to receive semiconductor material and carbon nanotubes to form a power field effect transistor. All the elements are coupled together using carbon nanotubes placed in connection trenches of the substrate.

BACKGROUND

Power converters are indispensable devices in electrical platforms and systems such as computing platforms, communication and mobile systems, medical systems, electric vehicles, military systems, renewable energy systems, aerospace systems, and almost all peripherals and devices. These systems and related applications impact people's daily life. New technology for power electronics and power converters is critical for achieving higher energy efficiency and significant cost reduction and size.

Power converters convert voltage or current from one level to another and/or from one form to another in order to supply energy to a specific load. Such power converters are of several types, such as DC-DC power converters, AC-DC power converters, DC-AC power inverters, and AC-AC power inverters.

Switching DC-DC power converters have the advantage of much higher energy efficiency as compared with converters using linear regulators. However, the switching DC-DC power converter is generally larger than the linear regulator converter primarily because it typically requires power inductors, power transformers, more switching power devices and control circuits. Nonetheless, the switching DC-DC power converters are widely used especially when the energy efficiency is crucial.

In general, the integration and size reduction of other technologies, such as Integrated Circuits (ICs) for microprocessors and other general purpose processors (e.g., graphic ICs and communications ICs), are advancing at a faster pace than switching power converter technologies. Integrating power converters “on chip” yields several advantages such as smaller size, lighter weight, reduced distribution, reduced distribution losses, and potentially reduced EMI (Electromagnetic Interference).

When the dimensions of solid materials are reduced to nanometer size, the materials often exhibit new and interesting behavior which can constitute the basis for a new generation of electronic devices. Hence, nanotechnology helps to achieve reliable nanometer-scale power devices with small footprints and reduced power consumptions. Consequently, there is a need to develop new nanotechnology-based power devices that can result in transformative advances.

A DC-DC switching power converter typically comprises switching power devices such as MOSFETs (Metal Oxide Field Effect Transistor), analog and/or digital control circuits filter capacitors, power inductors, and sometimes power transformers. Switching power devices and control circuits can be easily integrated on a single chip, but the power inductors, transformers and capacitors are often bulky and difficult to integrate with other components. In most cases, such off-chip components are an obstacle for reducing the size of switching power converters.

For many electronic systems, it is desirable to develop integrated DC-DC power converters for System on Chip (SoC) applications. The most common approaches to integrate power converters include: Power System in Package (PSiP) and Power System on Chip (PSoC). PSiP uses off-the-shelf components in order to supply large power requirements, but such components often limit size reduction. PSoC often integrates components, such as power inductors and capacitors, directly on IC (Integrated Circuit) chips. Such an approach can minimize size by taking advantages of known micro-fabrication technologies. Carbon nanotubes (CNTs) at the nanometer scale have sometimes been used to form electrical components. However, there are interfacial resistance problems associated with conventional connectors, such as copper traces on a substrate, that are often used to couple CNT components of a converter.

Therefore, if the connection problem can be diminished or removed, it appears to be advantageous to replace conventional technology with nanotechnology to achieve reliable nanometer-scale power devices with smaller footprints and less power consumption. Scaling the components to nanometer size helps to reduce converter size. Nanotechnolgy power components would provide for high-density on-chip integration of the power converters resulting in optimum power management, elimination of discrete component, smaller footprint, lower distribution losses, and lower parasitic delays.

DETAILED DESCRIPTION

A circuit for a DC-DC switching power converter10is depicted inFIG. 1in a single-phase topology. The input to the converter10is a DC voltage source Vin. Switches, Suand Sl, of the converter10are controlled by signals from a controller. A power inductor, Lo, and a capacitor, Co, are energy storage components of the converter10. Switching converter10is often used for supplying power in hand-held and other small devices. It is therefore desirable for the converter10to be as small as possible. Inductors, such as Lo, and capacitors, such as Co, typically account for a significant portion of the board space on a printed circuit board or substrate where components of the converter are mounted. Hence, one way to significantly reduce the size of a converter is to reduce the size of such components. Exemplary embodiments of power inductors and transformers are described in commonly-assigned U.S. patent application Ser. No. 13/464,783, entitled “Carbon Nanotube-Based Integrated Power Inductor for On-Chip Switching Power Converters” and filed on May 4, 2012, which is incorporated herein by reference. Embodiments of other carbon nanotube (CNT) components for other converter components, formed by novel CNT structures, are described herein. Such embodiments of components are based on trenches having a variety of structures that are formed in a substrate. In addition connections between the components are provided by CNT couplings that eliminate converter efficiency loss caused by interfacial resistance. The CNTs in these devices (e.g., power interconnects, Lo, Co, Suand Sl) may be fabricated by variety of methods, such as inside trenches in a substrate or directly on the substrate surface. There are a variety of substrate types that could be used. Moreover, several of each of these devices (e.g., power interconnects, Lo, Co, Suand Sl) could be connected in parallel and in series.

An exemplary converter100having single-phase power converter configuration and comprising carbon nanotube (CNT) components and connections is depicted inFIG. 2. An input voltage, Vin, is coupled to the converter100via a pair of metallic terminals160, such as a copper terminal, a mixture of conductive, non-CNT material (e.g., copper) and CNT, or a mixture of other nano-structures/materials and/or other type of structures/materials, that is in electrical communication with CNT conductors136. As an example, a power supply (not shown) may have metallic (e.g., copper) leads that are respectively coupled to the terminals160in order to apply Vinto the terminals160so that current flows through the circuit shown byFIG. 2.

Each CNT conductor136is composed of CNTs that may be formed in a trench of or deposited on a surface of one or more substrates forming a conductive trace for carrying current. As an example, CNT-based components, such as CNT-based FETs110, a CNT-based inductor120, and a CNT-based capacitor130may be formed on a substrate, and CNT conductors136may be formed in trenches of the substrate and used to interconnect the CNT-based components.

In particular, one of the CNT conductors136couples one terminal160of the input voltage to an input terminal161of a first CNT-based power field effect transistor (FET), Su110, and the other CNT conductor136couples the other terminal160of the input voltage to ground. In one exemplary embodiment, each terminal160is composed of a mixture of CNTs and a conductive material, such as copper, but other configurations of the terminals160are possible. The output side of Su110is coupled, via another CNT conductor136, to a first terminal162of a second CNT-based power FET, Sl110. A second terminal163of Sl110is connected to ground via a CNT conductor136. The FETs, Suand Sl, are controlled by controller (having driver and control circuits)155. In one embodiment, the driver and control circuits155are provided by conventional technology circuits. In another embodiment, the driver and control circuits155are fabricated using CNT components and may be combined with the other CNT components of the converter100.

The CNT-based FETs110, Suand SlofFIG. 2are configured to respond to signals from controller155to provide a time-varying voltage to a terminal164of CNT-based inductor120. The other terminal165of CNT-based inductor120is connected to a terminal166of a CNT-based capacitor130via a CNT conductor136. Another terminal167of the CNT-based capacitor130is connected to ground via a CNT conductor136. The CNT-based inductor120and CNT-based capacitor130are configured to exchange energy in order to maintain output voltage, Vo. The output voltage of converter100has an output value that remains within a specified range of voltages, such as, e.g., 5 volts, plus or minus one percent. The output voltage, Vo, is available to supply energy to a load via output terminals160. Details of an exemplary CNT-based FET110are depicted inFIG. 8, and details of an exemplary a CNT-based capacitor120are depicted inFIG. 9.

In one exemplary embodiment, a CNT conductor136is provided when one or more CNTs, such as Bundled Single-Walled CNTs are placed, via a fabrication process, in trenches or on surface of a substrate. Further, the substrate can have trenches with a variety of shapes that contain CNTs, coated CNTs, and other materials to provide all or most of the components for converter100. The CNT-based power devices could also be fabricated in a three dimensional stacked layers manner (vertically) and connected by CNT power conductors through vias (holes between the different layers).

FIG. 3depicts a typical schematic for a conventional half-bridge power converter20with a current-doubler secondary side. The converter20comprising an AC generator22, a transformer24, and output module26converts an input voltage to an output voltage. Input voltage, Vin, is applied to FETs S1and S2. The FETs, S1and S2, are configured to cooperate with input capacitors CS1and CS2to generate an alternating voltage. The FETs S1and S2receive control signals from a controller (not shown) that causes such cooperative action. The voltage across terminal A and terminal B of AC generator22is applied to a primary winding of transformer24. The output of transformer24is applied across the series arrangement of inductors L1and L2. An output capacitor, Co, has one terminal coupled between the junction of L1and L2and another terminal coupled to electrical ground. The voltage across Cois the output voltage, Vo. The FETs, Saand Sb, of the output module26provide for energy transfers between the inductors L1, L2, and capacitor, Co. Those skilled in the art would generally understand the operation of converter20.

FIG. 4depicts an exemplary embodiment of a converter100providing the function as described for current-doubler converter20ofFIG. 3. Converter100comprising AC generator122, transformer170, and output module126provides output voltage, Vo, in response to input voltage, Vin. The input voltage, Vin, is applied to terminals160and that input voltage is coupled via CNT conductors136to CNT-based capacitors130, CS1and CS2and to CNT-based FETs110. The capacitors130and FETs110operate cooperatively (based on a control signal) to provide an alternating voltage. The alternating voltage is carried by CNT conductors136that are coupled to the CNT-based transformer170. Detailed descriptions of CNT-based inductors and CNT-based transformers are described in commonly-assigned U.S. patent application Ser. No. 13/464,783, entitled “Carbon Nanotube-Based Integrated Power Inductor for On-Chip Switching Power Converters” and filed on May 4, 2012, which is incorporated herein by reference. The voltage across the output terminals of transformer170is applied to output module126via CNT conductors136. Output module126has CNT-based FETs110coupled to CNT-based inductors120. The output voltage, Vo, is available across output capacitor Co.

FIGS. 5, 6 and 7depict inductive devices, inductors and a transformer that are described in further detail U.S. application Ser. No. 13/464,783. The figures are provided to illustrate the structure of exemplary embodiments of inductive devices.FIG. 5depicts a CNT-based inductor120comprising a substrate70having a trench80with a square-spiral shape. Formed within the trench80is a conductive CNT coil132. Ends of the CNT coil132serve as terminals that may be coupled to CNT conductors136for connecting to other components of a CNT-based converter or other device.FIG. 6Adepicts an inductor120having a round-spiral shape and is formed in way similar to that ofFIG. 5. In one exemplary embodiment, an end134of the CNT coil132of the inductor120shown byFIG. 6Ais coupled to terminal164shown byFIG. 2, and an opposite end135of the CNT coil132(around which the coil132spirals) is coupled to the terminal165shown byFIG. 2.

FIG. 6Bdepicts a transformer170that is provided by placing two CNT coils132and133in two spirally-shaped trenches, respectively (e.g., one trench for each coil) or within a single spirally-shaped trench large enough to accommodate both coils132and133. The two CNT coils132and133are electrically separated from each other. Such electrical separation may be provided by coating one of the coils with an insulating material. Energy is coupled between the coils of CNT132of the transformer170via transformer action (electromagnetic coupling). The performance of inductors and transformers may be enhanced by placing magnetic material on each side of their respective substrates as depicted inFIG. 7.

FIGS. 8A and 8Bdepict an exemplary embodiment of a CNT-based FET110in accordance with the present disclosure.FIG. 8Ashows a partially fabricated CNT-based FET110. The partially fabricated CNT-based FET110comprising a substrate70with trenches72defined by surfaces of the substrate are configured to receive semiconductor material for providing a drain84and source82for FET110. In addition there are trenches74for a plurality of CNT structures80composed of CNTs, each CNT structure80having a generally cylindrical shape with its longitudinal axis extending in the y direction. Another plurality of CNT structures86composed of CNT is fabricated so that each CNT structure86is generally cylindrical with its longitudinal axis extending in the x direction. Note that other shapes of the structures80and86are possible.

As depicted inFIGS. 8B and 8C, the CNT structures86are positioned above the CNT structures80, and an insulator88covers the structures86. The source82and drain84for the CNT-based FET110are provided by inserting N++ material in trenches in the substrate70. The CNT structures86are p-type semiconductors that form a gate for physically connecting the source82and drain84. The upper CNT structures80contact the lower CNT structures86to form a crossover type connection. That is, lower CNT structures86form a conductor for carrying a gate signal and applying the gate signal to the upper CNT structures86for controlling a conductivity of the upper CNT structures86, thereby controlling the amount of current that flows via the upper CNT structures86between the source82and drain84.

FIG. 9depicts an embodiment of a CNT-based capacitor120in accordance with the present disclosure. The CNT-based capacitor120comprising a first CNT conductor90having an L-shape in a trench of a substrate and a second CNT conductor94having an L-shape in another trench of the substrate are configured for connecting to other CNT components of converter100. The first CNT conductor90has fingers92of CNT material in trenches that are perpendicular to the long arm of the L-shape. The second CNT conductor94has fingers96of CNT material that are perpendicular to the long arm of the L-shape. The fingers92,96are configured so that they are interlocked, but the fingers of the first and second conductors do not touch. That is, the fingers92,96are formed such that a finger92of conductor90is positioned between fingers96of conductor94and vice versa. The interlocked fingers92,96are dimensioned such that their edges are close to each other in order to provide capacitance between the first CNT conductor90and the second CNT conductor94. In one embodiment the fingers92,96of the CNT conductors90,94are coated with a dielectric material. Other geometries are possible to provide a CNT-based capacitor that serves as an element of converter100.

Exemplary embodiments of converters100have been described inFIG. 2andFIG. 4, other types of switched converters, inverters and devices may benefit from the innovations described above. For example, a variety of small form factor devices, such as modulators and amplifiers, may be fabricated using all or some of the CNT-based components described herein above. The improved connections between CNT-based components of a device using CNT-based conductors136increases the device energy efficiency, thereby reducing power requirements of a device. The CNT-based devices may use a variety of types of CNTs.