Tag Archives: letter-wafer-tech

Graphene quantum dots made from coal, introduced in 2013 by the Rice University lab of chemist James Tour, can be engineered for specific semiconducting properties in either of two single-step processes.

Vials hold solutions with graphene quantum dots that fluoresce in different colors depending on the dots' size. Techniques to produce the dots in specific sizes using coal as a source were developed at Rice University. Credit: Tour Group/Rice University

Vials hold solutions with graphene quantum dots that fluoresce in different colors depending on the dots’ size. Techniques to produce the dots in specific sizes using coal as a source were developed at Rice University.
Credit: Tour Group/Rice University

In a new study this week in the American Chemical Society journal Applied Materials & Interfaces, Tour and colleagues demonstrated fine control over the graphene oxide dots’ size-dependent band gap, the property that makes them semiconductors. Quantum dots are semiconducting materials that are small enough to exhibit quantum mechanical properties that only appear at the nanoscale.

Tour’s group found they could produce quantum dots with specific semiconducting properties by sorting them through ultrafiltration, a method commonly used in municipal and industrial water filtration and in food production.

The other single-step process involved direct control of the reaction temperature in the oxidation process that reduced coal to quantum dots. The researchers found hotter temperatures produced smaller dots, which had different semiconducting properties.

Tour said graphene quantum dots may prove highly efficient in applications ranging from medical imaging to additions to fabrics and upholstery for brighter and longer-lasting colors.

“Quantum dots generally cost about $1 million per kilogram and we can now make them in an inexpensive reaction between coal and acid, followed by separation. And the coal is less than $100 per ton.”

The dots in these experiments all come from treatment of anthracite, a kind of coal. The processes produce batches in specific sizes between 4.5 and 70 nanometers in diameter.

Graphene quantum dots are photoluminescent, which means they emit light of a particular wavelength in response to incoming light of a different wavelength. The emitted light ranges from green (smaller dots) to orange-red (larger dots). Because the emitted color also depends on the dots’ size, this property can also be tuned, Tour said. The lab found quantum dots that emit blue light were easiest to produce from bituminous coal.

The researchers suggested their quantum dots may also enhance sensing, electronic and photovoltaic applications. For instance, catalytic reactions could be enhanced by manipulating the reactive edges of quantum dots. Their fluorescence could make them suitable for metal or chemical detection applications by tuning to avoid interference with the target materials’ emissions.

M/A-COM Technology Solutions Inc., a supplier of high-performance analog RF, microwave and optical semiconductor products, today announced the new MAGX-000912-650L00 and MAGX-000912-650L0S, a 650 W gallium nitride (GaN) on silicon carbide (SiC) HEMT pulsed power transistor for L-band pulsed avionics applications. This transistor is available in standard flange or earless flange packaging.

The MAGX-000912-650L00/MAGX-000912-650L0S is a gold metalized, internally matched, GaN on SiC depletion mode RF power transistor. Operating in the 960 to 1215 MHz frequency range, the MAGX-000912-650L0x is a rugged and robust transistor, boasting a mean time to failure (MTTF) of 600 years.

The internally matched MAGX-000912-650L0x features 650 W of peak output power with 20 dB typical gain and 62 percent drain efficiency. The semiconductor structure is designed to achieve a high drain breakdown voltage (BVdss), which enables reliable and stable operation at 50V in extreme mismatched load conditions unparalleled with older semiconductor technologies. Other features include flat gain versus frequency performance and a common-source configuration for broadband class AB operation.

The MAGX-000912-650L0x was developed using wafer fabrication processes, and provides customers with high gain, efficiency, bandwidth and ruggedness to meet today’s demanding application needs. This transistor is optimized for civilian and military pulsed avionics amplifier applications in the 960 to 1215 MHz range, for Mode-S, TCAS, JTIDS, DME and TACAN operation.

“The transistor is a clear leader in high pulsed power GaN technology with 650 W of output power combined with excellent gain, efficiency and rugged performance,” said Gary Lopes, Senior Product Director, MACOM. “The device is an ideal candidate for customers looking to upgrade L-Band avionics systems to the next level of pulsed power performance and experience the solid reliability that is offered by MACOM GaN solutions.”

By Kevin Nguyen, SEMI

450mm notchless wafer standardization has been under discussion for over a year.  Driven by the G450C Consortium, the proposed changes will improve wafer symmetry, but more importantly, increase the utilization of wafer surface area. The International Polished Wafer and International 450mm Wafer Task Forces (TF) led the effort to add a 450mm notchless wafer specification via ballot 5604.  The document was completed late last year and is now published in the latest edition of SEMI M1-0215 Specification for Polished Single Crystal Silicon Wafers.

SEMI T7-0709 – Specification for Back Surface Marking of Double-Side Polished Wafers with a Two-Dimensional Matrix Code Symbol, is an important specification, which provides marking symbology with no intrusion in the fixed quality area of the wafer.  This standard is applicable for various size of wafers, and not limited to 450mm notchless wafers.  In alignment with SEMI M1, the Fiducial Mark Interoperability TF, in cooperation with the International 450mm Wafer TF, balloted and successfully received committee approval for doc. 5752, Revision of SEMI T7 to cover 450mm wafers.  (See figure below for the primary location of the SEMI T7 mark on the polished wafer.) The document was approved at the Traceability Committee meeting during SEMICON Japan in December 2014.  The new version of SEMI T7 is currently being processed for publication and will be available in Spring 2015.  When the revision of the T7 is published, SEMI will have a total of 20 SEMI standards for 450mm manufacturing.

450mm Diameter Wafer with Location of Orientation Fiducial Marks (Source: SEMI M1 -Figure 6)

450mm Diameter Wafer with Location of Orientation Fiducial Marks (Source: SEMI M1 -Figure 6)

The International 450mm Wafer TF is also pursuing doc. 5794, Specification of Developmental 450 mm Diameter Polished Single Crystal Notchless Silicon Wafers With Back Surface Fiducial Marks.  This standard is required for the development and test of wafer alignment systems for notchless wafers with back side fiducial laser scribed marks. The project is anticipated to be completed by SEMICON West, July 2015.

The International Advanced Wafer Geometry TF completed the revision of SEMI M49-1014, Guide for Specifying Geometry Measurement Systems for Silicon Wafers for the 130nm to 16nm Technology Generations, by reducing edge exclusion on 450mm wafer from 2mm to 1.5mm for 16 nm technology generation for measurement system.

Meanwhile doc. 5655 still needs to be completed for increasing the Fixed Quality Area (FQA) from 223mm to 223.5mm for 450mm polished wafers in SEMI M1-0215 Specification for Polished Single Crystal Silicon Wafers. These changes are significant since there may be issues in verifying the quality of wafers close to the wafer edge due to limitations of measurement equipment.  No timetable is set, but doc. 5655 may be issued in cycle 4-2015 ballot for review at SEMICON West after discussion at the NA Standards 2015 Spring meetings.

In addition, other physical interfaces and carriers standardization efforts are underway including doc. 5069B, Specification for 450mm Wafer Shipping System. SEMI has a 300mm wafer shipping system standard (M45), but a 450mm wafer version is not yet available.  The International 450mm Shipping Box Task Force is currently drafting doc. 5069B for the third time after it failed at the previous meeting in 2014.  More progress will be reported at the next NA Physical Interfaces and Carriers TC Chapter meeting.

For complete listing of revisions and new standards efforts, visit (http://www.semi.org/node/42416)

While the timing of a transition to 450mm wafers remains unclear, the industry is continuing to develop the standards that will be essential to high-volume manufacturing on larger substrates. The first standard proposal was submitted in 2007, and SEMI has since published nineteen 450mm-related Standards.  New standards are being developed as the industry identifies new requirements, and revisions to published standards are being made as areas for improvement are recognized.

If your company is not yet involved in these efforts to shape the future, learn more about SEMI Standards by visiting www.semi.org/en/Standards. Note that participation in the SEMI Standards Program is free, but requires registration. To learn more, contact your local SEMI Standards staff or register at: www.semi.org/standardsmembership.

For additional 450 information (PPTs, viewpoints, Standards revisions, etc.), visit 450 Central at www.semi.org/450.

EV Group (EVG), a supplier of wafer bonding and lithography equipment for the MEMS, nanotechnology and semiconductor markets, today introduced two new configurations to its EVG 580 ComBond series of automated high-vacuum covalent wafer bonding systems. Addressing the needs of universities and R&D institutes, and high-volume manufacturing (HVM) requirements, respectively, both system configurations achieve electrically conductive and oxide-free bonds of materials with different lattice constants and coefficients of thermal expansion at room temperature.

Applications that demand room-temperature bonding of substrates with very different material properties and that are supported by the EVG580 ComBond series include advanced engineered substrates, power devices, stacked solar cells and emerging technologies such as silicon photonics.

The new entry-level EVG580 ComBond system for universities and R&D institutes comes with one cassette station or manual load port as well as a single-arm robot, supporting up to three process modules. The EVG580 ComBond HVM system can be configured with two cassette stations or an equipment front-end module with up to four cassettes for continuous mode operation, as well as comes with a dual-arm robot to support up to six process modules for maximum throughput.

Both new ComBond system configurations, as well as the standard system that can accommodate up to five process modules, are built on a modular platform supporting wafers up to 200mm in diameter. In addition to one or more bond chambers, the systems feature a dedicated ComBond Activation Module (CAM), which provides advanced surface preparation by directing energized particles to the substrate surface to achieve a contamination-free and oxide-free bond interface. The systems operate in a high-vacuum-process environment with base pressures in the range of 5×10-8 mbar, which prevents re-oxidation of the treated wafers prior to the bonding step.

“The EVG580 ComBond system with its standard five-module configuration, which was launched last autumn, has already demonstrated its capabilities with multiple R&D partners and customers,” stated Dr. Thomas Glinsner, corporate product management director at EV Group. “With the new three-module system, we will now make this breakthrough technology available to universities and smaller R&D institutes, which often are at the forefront of pioneering advanced electronic materials and device research, such as heterogeneous integration of compound semiconductors for silicon photonics and other leading-edge applications. All ComBond systems can be further customized to address specific application development needs, such as with special metrology modules utilizing free ports of the high-vacuum handling.”

wafer bonding ev group

A frequency comb source is a light source with a spectrum containing thousands of laser lines. The development of these sources has been revolutionary for fundamental science. It has allowed the construction of a link between the optical part of the electromagnetic spectrum and the radio frequency part. As such, it has allowed researchers to determine optical frequencies with an unprecedented precision. Amongst others, frequency comb light sources have been used in optical clocks enabling precise time keeping. The enormous impact of frequency comb light sources on science was highlighted in 2005, when the Nobel Prize for physics was awarded to Prof. T. Haensch and Prof J. Hall for their work on optical frequency metrology using frequency combs.

Lately, frequency combs have been used to target more real life applications. In several experiments, it has been shown that the specific properties of the sources can be used to do fast, high-resolution spectroscopy over a broad spectrum. However, traditional comb sources are not at the right wavelength spectrum for doing spectroscopy.

Ghent University, imec, the Max Planck Institute for Quantum Optics in Garching and the Auckland University in New Zealand have developed mid-infrared frequency combs, working in the mid-infrared molecular fingerprinting region of the electromagnetic spectrum. In this wavelength region, many molecules have specific absorption bands that can be used in spectroscopy to determine the presence and concentration of these molecules in samples. The researchers successfully realized the broad frequency combs, by combining the strong light-matter interaction in silicon with its broad transparency window. By fabricating so-called nanowire silicon photonics waveguides to confine the light in a very small area waveguide, they further enhanced the strong light-matter interaction allowing them to broaden the spectrum of the frequency combs into the mid-infrared. The achievements were possible through the use of a unique pump laser source, previously developed by ICFO, Spain. The results are an important step towards a small-footprint chip scale mid-infrared frequency comb source. Such sources could act as sensitive cheap gas sensors in the mid-infrared. These would be important for example for environmental monitoring for measuring air-pollution or in medical diagnostics as a cheap tool to do breath analysis. It is worth noting that the reported work has been the result of collaboration between three grants of the European Research Council (ERC), i.e. Multicomb, Miracle and InSpectra.

CEA-Leti today announced the launch of its Silicon Impulse IC design competence center, a comprehensive IC technology platform offering IC design, advanced intellectual property, emulator and test services along with industrial multi-project wafer (MPW) shuttles.

Silicon Impulse provides immediate access to Leti’s and CEA-List’s advanced IC technologies and systems expertise. It also leverages Leti’s extensive experience in technology transfer. Collaboration between Leti and List experts and ecosystem partners aims to shorten time to market from idea to production. This process includes prototyping and pre-production runs utilizing Leti’s advanced industrial infrastructure from concept to production hand-off.

Silicon Impulse offers its partners the full range of Leti and List’s expertise in analog, RF, digital and memory design and hardware/software-integrated solutions at the technology node that most cost-effectively meets their needs. The IC competence center combines Leti’s large portfolio of leading-edge technologies and novel low-power design solutions with a unique service for speeding integration of Fully Depleted Silicon-on-Insulator (FD-SOI) and many other more advanced technologies (ReRAM, MEMS, 3DVLSI, Silicon Photonics), enabling heterogeneous low-power co-integration. These services are targeted to enable the rapidly emerging third-generation information-acquisition and processing devices that are key to the Internet of Things (IoT).

“Pervasive wireless networking and groundbreaking low-power technologies are critical to the widespread adoption of the Internet of Things, because they improve the performance of portable devices and their network infrastructure,” said Leti CEO Marie–Noëlle Semeria. “With Silicon Impulse’s one-stop-shop platform, 28nm FD-SOI heterogeneous, low-power design becomes a reality for the IoT community. Silicon Impulse helps Leti’s partners introduce innovative products that deliver optimal performance for these applications, and benefit from the most advanced technologies.”

Silicon Impulse, in cooperation with a rich network of ecosystem partners, is set up to develop, produce and integrate innovative customized or standard systems that exploit advanced technologies for all of Leti’s industrial partners.

Specific features of the offer include:

  • Leti’s pool of expertise in advanced IC design and low-power technologies
  • Leading-edge IC technologies for application-oriented device, circuit and system solutions
  • Regularly scheduled MPW shuttles for silicon prototyping and small-volume runs
  • An established high-tech supply chain to accelerate production ramp-up and hand-off
  • Customized collaboration to fit partner needs

Leti’s experts will collaborate with partners from the feasibility-study stage through device design, prototyping, testing and production ramp up. The wide range of IC technologies available in the platform is further augmented by List’s comprehensive embedded software solutions.

Leti will showcase its new Silicon Impulse platform at major industry events this year: DATE, Silicon Impulse workshop during VLSI-DAT, DAC, LetiDays Grenoble, LetiDay San Francisco during SEMICON West, LetiDay Tokyo, Leti’s Devices Workshop at IEDM and SOI consortium events.

With more than five times the thermal conductivity of copper, diamond is the ultimate heat spreader. But the slow rate of heat flow into diamond from other materials limits its use in practice. In particular, the physical process controlling heat flow between metals and diamond has remained a mystery to scientists for many years.

By applying extreme pressure in a diamond anvil cell to metal films on diamond, researchers at the University of Illinois at Urbana-Champaign have now determined the physical process dominating this unexplained heat flow, which has implications for understanding and improving heat flow between any two materials.

“Overheating has become a major limiting factor in the performance of high-power RF devices,” said David Cahill, a professor and head of the Department of Materials Science and Engineering at Illinois. “Modern RF electronics for wireless devices such as satellites and cellphones generate so much heat in a microscopic area that the packing density and performance of RF devices isn’t limited by Moore’s Law anymore, so much as by how fast we can pull heat away from those devices. For overheating at microscopic length scales, it’s not enough to just swap out silicon for diamond; we need a microscopic understanding of how heat enters materials like diamond.”

Cahill explained that this work lies more on the fundamental side of thermal physics research, although materials like diamond and silicon carbide are being actively developed as alternative substrates for high powered radio-frequency (RF) devices. “Studies of extremes like metals on diamond at high pressure are valuable because they allow us to test our ideas about what is happening in this complex problem. The experiments we designed let us test and falsify a series of hypotheses, ultimately leading to a better understanding of heat flow between dissimilar materials.”

“The simplest way for a phonon to cross an interface is by a two-phonon elastic processes: a phonon comes in, a phonon of the same frequency goes out.” explained Greg Hohensee, first author of the paper appearing in Nature Communications. “But metals on diamond are a special case. The diamond is so stiff that it’s like banging a pot attached to a rope and expecting the rope to dance. The vibrations stay in the pot, because the rope is not stiff enough to carry such high frequency vibrations. Likewise, you can’t make the pot sing by shaking the rope. But somehow, metals on diamond are doing exactly that.”

The thermal conductance of an interface determines the rate of heat flow for a given temperature difference between the materials. Typically, the carriers for heat in crystalline materials like diamond are traveling vibrational waves called phonons. A central challenge of thermal physics–and of interfacial thermal conductance, specifically–is that phonons exist over a wide frequency range, and how phonons interact with interfaces and other phonons depends on their frequencies.

“Stiffer metals seem to have higher thermal conductance on diamond, so our initial hypothesis was that the thermal conductance depended on the metal’s stiffness,” Hohensee added. “We designed an experiment to vary the stiffness in a controlled way. We deposited different metal films on one of the two diamond anvils in a diamond anvil cell, sealed the cell with a gasket between the diamonds, and measured the thermal conductance to pressures as high as 500,000 atmospheres. You’d have to go 1000 km into the Earth’s mantle to find comparable pressures outside of a laboratory.”

“To our surprise, the initial data with a gold-palladium alloy (Au(Pd)) and lead (Pb) showed no such trend,” said co-author Rich Wilson. “In fact, the conductance seemed to saturate at high pressure, as if by some limiting thermal resistance between the metals and diamond. Proving yourself wrong can sometimes seem like a letdown but each falsified hypothesis brings you closer to the correct explanation. To inspire a better hypothesis, sometimes you just have to go back and collect more data.”

“To get the additional comparisons, we measured platinum (Pt) for electronic contrast against Au(Pd), and aluminum (Al) for stiffness contrast against Pb,” Hohensee said. “A pattern emerged: the Pt and Au(Pd) data were similar, but the excess conductance of every metal aside from Al were nearly identical at high pressure.

“We realized that we could explain the data with what we call partial transmission processes, where metal phonons ‘feed’ a much higher frequency diamond phonon at the interface. We had originally guessed that metal phonons could combine to form a higher frequency diamond phonon, but that process would have been sensitive to the metal stiffness and hence pressure. In partial transmission any diamond phonon can eat a metal phonon, even diamond phonons with frequencies far higher than can exist in the metal, so it hardly matters how stiff the metal is.”

“Before this experiment, researchers had been proposing and modeling theories for metal-diamond thermal conductance for some twenty years, based on data as a function of temperature,” Cahill said. “We designed a new experiment with a new independent variable, pressure, in order to test these hypotheses. Our new information ended up falsifying some theories and supporting a new picture for how heat flows between dissimilar materials in general. Now that we know partial transmission processes can be important, researchers can build microscopic models and simulations to explore them in more detail, and engineers can design devices that enhance or take advantage of this aspect of thermal conductance for a variety of materials.”

In the race to miniaturize electronic components, researchers are challenged with a major problem: the smaller or the faster your device, the more challenging it is to cool it down. One solution to improve the cooling is to use materials with very high thermal conductivity, such as graphene, to quickly dissipate heat and thereby cool down the circuits.

At the moment, however, potential applications are facing a fundamental problem: how does heat propagate inside these sheets of materials that are no more than a few atoms thick?

In a study published in Nature Communications, a team of EPFL researchers has shed new light on the mechanisms of thermal conductivity in graphene and other two-dimensional materials. They have demonstrated that heat propagates in the form of a wave, just like sound in air. This was up to now a very obscure phenomenon observed in few cases at temperatures close to the absolute zero.Their simulations provide a valuable tool for researchers studying graphene, whether to cool down circuits at the nanoscale, or to replace silicon in tomorrow’s electronics.

Quasi-Lossless Propagation

If it has been difficult so far to understand the propagation of heat in two-dimensional materials, it is because these sheets behave in unexpected ways compared to their three-dimensional cousins. In fact, they are capable of transferring heat with extremely limited losses, even at room temperature.

Generally, heat propagates in a material through the vibration of atoms. These vibrations are are called “phonons”, and as heat propagates though a three-dimensional material,, these phonons keep colliding with each other, merging together, or splitting. All these processes can limit the conductivity of heat along the way. Only under extreme conditions, when temperature goes close to the absolute zero (-200 0C or lower), it is possible to observe quasi-lossless heat transfer.

A wave of quantum heat

The situation is very different in two dimensional materials, as shown by researchers at EPFL. Their work demonstrates that heat can propagate without significant losses in 2D even at room temperature, thanks to the phenomenon of wave-like diffusion, called “second sound”. In that case, all phonons march together in unison over very long distances. “Our simulations, based on first-principles physics, have shown that atomically thin sheets of materials behave, even at room temperature, in the same way as three-dimensional materials at extremely low temperatures” says Andrea Cepellotti, the first author of the study. “We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet,” explains Cepellotti. “This is an extremely valuable information for engineers, who could exploit the design of future electronic components using some of these novel two-dimensional materials properties.”

Engineers at The University of Texas at Dallas have created semiconductor technology that could make night vision and thermal imaging affordable for everyday use.

Researchers in the Texas Analog Center of Excellence (TxACE) in the University’s Erik Jonsson School of Engineering and Computer Science created an electronic device in affordable technology that detects electromagnetic waves to create images at nearly 10 terahertz, which is the highest frequency for electronic devices. The device could make night vision and heat-based imaging affordable.

Presently, night vision and thermal imagers are costly, in part because they are made with specialty semiconductor devices or need isolation from the environment.

The UT Dallas device is created using Schottky diodes in Complementary Metal-Oxide Semiconductor (CMOS) technology. CMOS is used to make affordable consumer electronic devices such as personal computers, game consoles and high-definition TVs. In addition to being affordable, these devices could be more easily incorporated into smartphones.

“There are no existing electronic detection systems operating in CMOS that can reach above 5 terahertz,” said Zeshan Ahmad, lead author of the work, electrical engineering doctoral candidate and a research assistant in TxACE. “We designed our chip in such a way that it can be mass produced inexpensively, has a smaller pixel and operates at higher frequencies.”

Dr. Kenneth O, professor of electrical engineering in the Jonsson School and director of TxACE, noted the time it took for the field to reach this frequency in CMOS.

“This is a truly remarkable accomplishment,” said Dr. O, holder of the Texas Instruments Distinguished Chair.

“Twenty years ago, we were struggling to build CMOS circuits operating at 1 gigahertz. Now we are building circuits working at frequencies that are 10,000 times higher.”

The device could eventually be used for imaging animals near a road while driving at night; imaging intruders in darkness; providing light for night hiking; and estimating how many people are in a room to better control heating, air conditioning and light. It also could be used for other tasks such as finding pipes covered by concrete or walls.

“This technology could provide a very superior means to use the infrared portion of the spectrum,” said Dr. Robert Doering, research strategy manager at Texas Instruments.” Electronic control of generating infrared directly from CMOS integrated circuits will enable a wide variety of important new applications.”

The next step in the research is to realize CMOS devices that can reach even higher frequencies, up to 40 terahertz

At this year’s Consumer Electronics Show in Las Vegas, the big theme was the “Internet of things” — the idea that everything in the human environment, from kitchen appliances to industrial equipment, could be equipped with sensors and processors that can exchange data, helping with maintenance and the coordination of tasks.

Realizing that vision, however, requires transmitters that are powerful enough to broadcast to devices dozens of yards away but energy-efficient enough to last for months — or even to harvest energy from heat or mechanical vibrations.

“A key challenge is designing these circuits with extremely low standby power, because most of these devices are just sitting idling, waiting for some event to trigger a communication,” explains Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering at MIT. “When it’s on, you want to be as efficient as possible, and when it’s off, you want to really cut off the off-state power, the leakage power.”

This week, at the Institute of Electrical and Electronics Engineers’ International Solid-State Circuits Conference, Chandrakasan’s group will present a new transmitter design that reduces off-state leakage 100-fold. At the same time, it provides adequate power for Bluetooth transmission, or for the even longer-range 802.15.4 wireless-communication protocol.

“The trick is that we borrow techniques that we use to reduce the leakage power in digital circuits,” Chandrakasan explains. The basic element of a digital circuit is a transistor, in which two electrical leads are connected by a semiconducting material, such as silicon. In their native states, semiconductors are not particularly good conductors. But in a transistor, the semiconductor has a second wire sitting on top of it, which runs perpendicularly to the electrical leads. Sending a positive charge through this wire — known as the gate — draws electrons toward it. The concentration of electrons creates a bridge that current can cross between the leads.

But while semiconductors are not naturally very good conductors, neither are they perfect insulators. Even when no charge is applied to the gate, some current still leaks across the transistor. It’s not much, but over time, it can make a big difference in the battery life of a device that spends most of its time sitting idle.

Going negative

Chandrakasan — along with Arun Paidimarri, an MIT graduate student in electrical engineering and computer science and first author on the paper, and Nathan Ickes, a research scientist in Chandrakasan’s lab — reduces the leakage by applying a negative charge to the gate when the transmitter is idle. That drives electrons away from the electrical leads, making the semiconductor a much better insulator.

Of course, that strategy works only if generating the negative charge consumes less energy than the circuit would otherwise lose to leakage. In tests conducted on a prototype chip fabricated through the Taiwan Semiconductor Manufacturing Company’s research program, the MIT researchers found that their circuit spent only 20 picowatts of power to save 10,000 picowatts in leakage.

To generate the negative charge efficiently, the MIT researchers use a circuit known as a charge pump, which is a small network of capacitors — electronic components that can store charge — and switches. When the charge pump is exposed to the voltage that drives the chip, charge builds up in one of the capacitors. Throwing one of the switches connects the positive end of the capacitor to the ground, causing a current to flow out the other end. This process is repeated over and over. The only real power drain comes from throwing the switch, which happens about 15 times a second.

Turned on

To make the transmitter more efficient when it’s active, the researchers adopted techniques that have long been a feature of work in Chandrakasan’s group. Ordinarily, the frequency at which a transmitter can broadcast is a function of its voltage. But the MIT researchers decomposed the problem of generating an electromagnetic signal into discrete steps, only some of which require higher voltages. For those steps, the circuit uses capacitors and inductors to increase voltage locally. That keeps the overall voltage of the circuit down, while still enabling high-frequency transmissions.

What those efficiencies mean for battery life depends on how frequently the transmitter is operational. But if it can get away with broadcasting only every hour or so, the researchers’ circuit can reduce power consumption 100-fold.