Graphene Enables Broad Spectrum Sensor Development

Team successfully marries a CMOS IC with graphene, resulting in a camera able to image visible and infrared light simultaneously.

Graphene Enables Broad Spectrum Sensor Development

By Wisse Hettinga

Researchers at ICFO—the Institute of Photonic Sciences, located in Catalonia, Spain—have developed a broad-spectrum sensor by depositing graphene with colloidal quantum dots onto a standard, off-the-shelf read-out integrated circuit. It is the first-time scientists and engineers were able to integrate a CMOS circuit with graphene to create a camera capable of imaging visible and infrared light at the same time. Circuit Cellar visited ICFO

Stijn Goossens is a Research Engineer at ICFO- the Institute of Photonic Sciences.

Stijn Goossens is a Research Engineer at ICFO- the Institute of Photonic Sciences.

and talked with Stijn Goossens, one of the lead researchers of the study.


GOOSSENS: ICFO is a research institute devoted to the science and technologies of light. We carry out frontier research in fundamental science in optics and photonics as well as applied research with the objective of developing products that can be brought to market. The institute is based in Castelldefels, in the metropolitan area of Barcelona (Catalonia region of Spain).

HETTINGA: Over the last 3 to 4 years, you did research on how to combine graphene and CMOS. What is the outcome?

GOOSSENS: We’ve been able to create a sensor that is capable of imaging both visible and infrared light at the same time. A sensor like this can be very useful for many applications—automotive solutions and food inspection, to name a few. Moreover, being able to image infrared light can enable night vision features in a smartphone.

HETTINGA: For your research, you are using a standard off-the-shelf CMOS read-out circuit correct?

GOOSSENS: Indeed. We’re using a standard CMOS circuit. These circuits have all the electronics available to read the charges induced in the graphene, the rows and columns selects and the drivers to make the signal available for further processing by a computer or smartphone. For us, it’s a very easy platform to work on as a starting point. We can deposit the graphene and quantum dot layer on top of the CMOS sensor (Photo 1).

PHOTO 1 The CMOS image sensor serves as the base for the graphene layer.

The CMOS image sensor serves as the base for the graphene layer.

HETTINGA: What is the shortcoming of normal sensors that can be overcome by using graphene?

GOOSSENS: Normal CMOS imaging sensors only work with visible light. Our solution can image visible and infrared light. We use the CMOS circuit for reading the signal from the graphene and quantum dot sensors. Tt acts more like an ‘infrastructure’ solution. Graphene is a 2D material with very special specifications: it is strong, flexible, almost 100 percent transparent and is a very good conductor.

HETTINGA: How does the graphene sensor work?

GOOSSENS: There are different layers (Figure 1). There’s a layer of colloidal quantum dots. A quantum dot is a nano-sized semiconductor. Due to its small size, the optical and electronic properties differ from larger size particles. The quantum dots turn the photons they receive into an electric charge. This electric charge is then transferred to the graphene layer that acts like a highly sensitive charge sensor. With the CMOS circuit, we then read the change in resistance of the graphene and multiplex the signal from the different pixels on one output line.

FIGURE 1 The graphene sensor is comprised of a layer of colloidal quantum dots, a graphene layer and a CMOS circuitry layer.

The graphene sensor is comprised of a layer of colloidal quantum dots, a graphene layer and a CMOS circuitry layer.

HETTINGA: What hurdles did you have to overcome in the development?

GOOSSENS: You always encounter difficulties during the course of a research study and sometimes you’re close to giving up. However, we knew it would work. And with the right team, the right technologies and the lab at ICFO we have shown it is indeed possible. The biggest problem was the mismatch we faced between the graphene layer and the CMOS layer. When there’s a mismatch, that means there’s a lack of an efficient resistance read-out of the graphene—but we were able to solve that problem.

HETTINGA: What is the next step in the research?

GOOSSENS: Together with the European Graphene Flagship project, we are developing a production machine that will allow us to start a more automated production process for these graphene sensors.

HETTINGA: Where will we see graphene-based cameras?

GOOSSENS: One of the most interesting applications will be related to self-driving cars. A self-driving car needs a clear vision to function efficiently. If you want to be able to drive a car through a foggy night or under extreme weather conditions, you’ll definitely need an infrared camera to see what’s ahead of you. Today’s infrared cameras are expensive. With our newly-developed image sensor, you will have a very effective, low-cost solution. Another application will be in the food inspection area. When fruit ripens, the infrared light absorption changes. With our camera, you can measure this change in absorption, which will allow you to identify which fruits to buy in the supermarket. We expect this technology to be integrated in smartphone cameras in the near future.


This article appeared in the September 326 issue of Circuit Cellar

Graphene Revolution

The Wonderful Material That Will Change
the World of Electronics

The amazing properties of graphene have researchers, students, and inventors dreaming about exciting new applications, from unbreakable touchscreens to fast-charging batteries.

By Wisse Hettinga

Prosthetic hand with graphene electrodes

Prosthetic hand with graphene electrodes

Graphene gained popularity because of the way it is produced—the “Scotch tape method.” In fact, two scientists, Andre Geim and Kostya Novoselov, received a Nobel Prize in 2004 for their work with the material. Their approach is straightforward. Using Scotch tape, they repeatedly removed small layers of graphite (indeed, the black stuff found in pencils) until there was only one 2-D layer of atoms left—graphene. Up to that point, many scientists understood the promise of this wonderful material, but no one had been able to get obtain a single layer of atoms. After the breakthrough, many universities started looking for graphene-related applications.

Innovative graphene-related research is underway all over the world. Today, many European institutes and universities work together under the Graphene Flagship initiative (, which was launched by the European Union in 2009. The initiative’s aim is to exchange knowledge and collaborate on research projects.

Graphene was a hot topic at the 2017 Mobile World Congress (MWC) in Barcelona, Spain. This article covers a select number of applications talked about at the show. But for the complete coverage, check out the video here:


The Istituto Italiano di Tecnologia (IIT) in Genova, Italy, recently developed a sensor from a cellulose and graphene composite. The sensor can be made in the form of a bracelet that fits around the arm in order to pick up the small signals associated with muscle movement. The signals are processed and used to drive a robotic prosthetic hand. Once the comfortable bracelet is placed on the wrist, it transduces the movement of the hand into electrical signals that are used to move the artificial hand in a spectacular way. More information:


The Scotch tape method used by the Nobel Prize winners inspired a lot of companies around the world to start producing graphene. Today, a wide variety of methods can be used depending on the actual application of the material. Graphenea (San Sebastian, Spain) is using different processes for the production of graphene products. One of them is Chemical Vapor Deposition. With this method, it is possible to create graphene on thin foil, silicon based or in form of oxide. They source many universities and research institutes that do R&D for new components such as supercapacitors, solar, batteries, and many more applications. The big challenge is to develop an industrial process that will combine graphene material with the conventional CMOS technology. In this way, the characteristics of graphene can enhance today’s components to make them useful for new applications. A good example is optical datatransfer. More information:

Transfer graphene on top of a silicon device to add more functionality

Transfer graphene on top of a silicon device to add more functionality


High-speed data communication comes in all sizes and infrastructures. But on the small scale, there are many challenges. Graphene enables new optical communication on the chip level. A consortium of CNIT, Ericsson, Nokia, and IMEC have developed graphene photonics integration for high-speed transmission systems. At MWC, they showcased a packaged graphene-based modulator operating over several optical telecommunications bands. I saw the first package transmitters with optical modulators based on graphene. The modulator is only one-tenth of a millimeter. The transfer capacity is 10 Gbps, but the aim is to bring that to 100 Gbps in a year’s time. The applications will be able to play a key role in the development of 5G technology. More information:

Optical modulator based on graphene technology

Optical modulator based on graphene technology


FGV Cambridge Nanosystems recently developed a novel “spray-on” graphene heating system that provides uniform, large-area heating. The material can be applied to paintings or walls and turned into a ‘heating’ area that can be wirelessly controlled via a mobile app. The same methodology can also double as a temperature sensor, where you can control light intensity by sensing body temperature. More information:

Graphene-based heater

Graphene-based heater


Atheletes can benefit from light, strong, sensor-based shoes that that can monitor their status. To make this happen, the University of Cambridge developed a 3-D printed shoe with embedded graphene foam sensors that can monitor the pressure applied. They combine complicated structural design with accurate sensing function. The graphene foam sensor can be used for measuring the number of steps and the weight of the person. More information:

Graphene pressure sensors embedded in shoes

Graphene pressure sensors embedded in shoes


More wireless fidelity can be expected when graphene-based receivers come into play. The receivers based on graphene are small and flexible and can be used for integration into clothes and other textile applications. AMO GmbH and RWTH Aachen University are developing the first flexible Wi-Fi receiver. The underlying graphene MMIC process enables the fabrication of the Wi-Fi receiver on both flexible and rigid substrates. This flexible Wi-Fi receiver is the first graphene-based front-end receiver for any type of modulated signal. The research shows that this technology can be used up to 90 GHz, which opens it up to new applications in IoT and mobile phones. More information:

Using graphene in flexible Wi-Fi receiver

Using graphene in flexible Wi-Fi receiver


Santiago Cartamil-Bueno, a PhD student at TU Delft, was the first to observe a change in colors of small graphene “balloons.” These balloons appear when pressure is applied in a double layer of graphene. When this graphene is placed over silicon with small indents, the balloons can move in and out the silicon dents. If the graphene layer is closer to the silicon, they turn blue. If it is farther away from the silicon, they will turn red. Santiago observed this effect first and is researching the possibilities to turn this effect into high-resolution display. It uses the light from the environment and turns it into a very low-power consumption process. The resolution is very high; a typical 5″ display would be able to show images with 8K to 12K resolution. More information:

High-Performance Analog Technology A30 for IoT Applications and More

ams AG recently announced the availability of its High Performance Analog Low Noise CMOS process (“A30”). The new A30 technology features performance optimized, isolated 3.3-V devices (NMOSI and PMOSI), isolated 3.3-V low Vt devices (NMOSIL and PMOSIL), an isolated high-voltage device with thin gate oxide (NMOSI20T), vertical bipolar transistors (VERTN1 and VERTPH), and an isolated 3.3-V super-low-noise transistor (NMOSISLN). It enables flicker noise reduction by at least a factor of 4 to 10 for high drain currents compared to H35 process. Passive devices such as various capacitors (poly, sandwich, and MOS varactor) and resistors (diffusion, well based, poly, high resistive poly and precision) complete the device offering.

The A30 process is well suited for ultra-low noise sensing applications and analog read-out ICs that require noise optimized input stages or high signal-to-noise ratios. It allows the development of innovative solutions for consumer electronics, automotive, medical and IoT devices. The A30 process is fully qualified and manufactured in ams’ state of the art 200-mm fabrication facility ensuring very low defect densities and highest yield. All 0.30-µm elements are drawn and verified as 0.35µm devices. The optical shrink (factor of 0.9) is done in the mask shop on the completed GDSII data and results in smaller die sizes respectively more dies per wafer.

The A30 process is supported by the well-known hitkit, ams’s industry benchmark process design kit. Based on Virtuoso Custom IC technology 6.1.6 from Cadence, the new hitkit helps design teams to significantly reduce time-to-market for products in the analog-intensive, mixed-signal arena. The hitkit provides a comprehensive design environment and a proven route to silicon. The new hitkit v4.15 for A30 process is now available on ams’s foundry support server.

Source: ams

Isolated FET Driver for Industrial Relay Replacement Applications

Silicon Labs recently introduced a new CMOS-based isolated field effect transistor (FET) driver family for industrial and automotive applications. The family enables you to use your preferred application-specific, high-volume FETs to replace old electromechanical relays (EMRs) and optocoupler-based, solid-state relays (SSRs).Si875x Silicon Labs

The new Si875x family features the industry’s first isolated FET drivers designed to transfer power across an integrated CMOS isolation barrier. When paired with a discrete FET, the Si875x drivers provide a superb EMR/SSR replacement solution for motor and valve controllers, HVAC relays, battery monitoring, and a variety of other applications.

The Si875x isolated FET driver family’s features and specs:

  • Industry’s first CMOS isolation-based SSR solution, supporting application-specific FETs
  • Best-in-class noise immunity, high reliability and 2.5 kVRMS isolation rating
  • Long lifetimes under high-voltage conditions (100 years at 1000 V)
  • Efficient switching: 10.3 V at the gate with only 1 mA of input current
  • Wide input voltage of 2.25 to 5.5 V enables power savings
  • Unique pin feature optimizes power consumption/switching time trade-off
  • Miller clamping prevents unintended turn on of external FET
  • Small SOIC-8 package integrates isolation and power capacitors for low-power applications
  • AEC-Q100-qualified automotive-grade device options

The Si875x devices come in a small SOIC-8 package. They are available in both industrial (–40°C to 105°C) or automotive (–40°C to 125°C) ambient temperature operating range options. Pricing in 10,000-unit quantities begins at $0.96 for industrial versions and $1.20 for automotive temperature options.

Evaluation kits are available. The Si8751-KIT (digital input) and Si8752-KIT (LED emulator input) evaluation kits cost $39.99 each.

Source: Silicon Labs

New High-Speed CMOS DDR2 Synchronous DRAMs

Alliance Memory recently broadened its line of high-speed CMOS double data rate 2 synchronous DRAMs (DDR2 SDRAM). Its new device featuring high 2-Gb density in a 84-ball 8-mm × 12.5-mm × 1.2-mm FBGA package. The AS4C128M16D2 is available (from a limited number of suppliers) in commercial (0°C to +85°C) and industrial (–40°C to +95°C) temperature ranges.AllianceMemory-DDR2 SDRAM

The AS4C128M16D2 provides a drop-in, pin-for-pin-compatible replacement for a number of similar solutions in industrial, auto, consumer, networking, and medical products that require high memory bandwidth. It is internally configured as eight banks of 16M × 16 bits. The RoHS-compliant DDR2 SDRAM includes a synchronous interface and operates from a single 1.8-V (±0.1 V) power supply. In addition, it features a fast clock rate of 400 MHz and a data rate of 800 Mbps/pin. The DDR2 SDRAM provides programmable read or write burst lengths of 4 or 8. An auto precharge function provides a self-timed row precharge initiated at the end of the burst sequence. Easy-to-use refresh functions include auto- or self-refresh while a programmable mode register allows the system to choose the most suitable modes to maximize performance.

With the AS4C128M16D2, Alliance Memory now offers a variety of DDR2 SDRAMs with densities of 512 Mb, 1 Gb, and 2 Gb. Samples of the AS4C128M16D2 are available now, with lead times of six to eight weeks for production quantities. Pricing for US delivery starts at $6.50 per unit.

Source: Alliance Memory

New High-Speed CMOS DDR SDRAMs

Alliance Memory recently announced new high-speed CMOS double data rate synchronous DRAMs (DDR SDRAM) with densities of 256 Mb (AS4C32M8D1), 512 Mb (AS4C64M8D1), and 1 Gb (AS4C64M16D1) in the 60-ball 8-mm × 13-mm × 1.2 mm TFBGA package and the 66-pin TSOP II package with a 0.65-mm pin pitch. The devices provide reliable drop-in, pin-for-pin-compatible replacements for a number of similar solutions in industrial, medical, communications, and telecommunications products requiring high memory bandwidth. They are particularly well-suited to high performance in PC applications.Alliance Memory DDR

The AS4C32M8D1, AS4C64M8D1, and AS4C64M16D1 are internally configured as four banks of 32M word × 8 bits, 64M word × 8 bits, and 64M word × 16 bits, respectively. The DDR SDRAMs offer a synchronous interface. They operate from a single +2.5-V (±0.2 V) power supply, and they are lead (Pb)- and halogen-free.

The AS4C32M8D1, AS4C64M8D1, and AS4C64M16D1 feature fast clock rates of 200 MHz and 166 MHz. They are offered in commercial (0°C to 70°C) and industrial (–40°C to 85°C) temperature ranges. The DDR SDRAMs provide programmable read or write burst lengths of 2, 4, or 8. An automatic pre-charge function provides a self-timed row pre-charge initiated at the end of the burst sequence. Easy-to-use refresh functions include auto- or self-refresh, while a programmable mode register allows the system to choose the most suitable modes to maximize performance.

With the addition of the AS4C32M8D1, AS4C64M8D1, and AS4C64M16D1 to its portfolio, Alliance Memory now offers the most extensive lineup of DDR SDRAMs in the industry, featuring densities of 64 Mb, 128 Mb, 256 Mb, 512 Mb, and 1 Gb. For Alliance Memory’s customers, the devices eliminate costly redesigns by providing long-term support for end-of-life (EOL) components. In addition, the company doesn’t perform die shrinks, which frees up engineering resources.

Samples and production quantities are available with lead times of six to eight weeks for large orders. Pricing for US delivery starts at $1 per piece.

Source: Alliance Memory

New Ultra-Low Power Precision Op-Amps

ON Semiconductor recently unveiled a new family of ultra-low power precision operational amplifiers. The precision NCS325 and NCS333 CMOS op-amps deliver zero drift operation and quiescent current for front-end amplifier circuits and power management designs.NCS325-333-Hires

The op-amp devices enhance the accuracy of motor control feedback and power supply control loops, thereby contributing to higher overall system efficiency. These devices are complemented by the new NCV333 automotive-qualified (AEC-Q100 grade 1) op-amp offering similar functional performance for power train, braking, electronic power steering, valve controls, and fuel pump and fuel injection system applications.

Features include:

  • High DC precision parameters, such as the 10 µV maximum input offset voltage at ambient temperature and the 30 nV/°C of offset temperature drift
  • Minimal voltage variations over temperature along with close to zero offset
  • Rail-to-rail input and output performance and are optimized for low voltage operation of 1.8 volt (V) to 5.5 V, with a best in class quiescent currents of 21 µA and 17 µA respectively at 3.3 V.
  • Operate with a gain bandwidth of 350 kHz with ultra-low peak-to-peak noise down to 1.1 µV from 0.1 Hz to 10 Hz.

The NCS325 is available in a 3 mm × 1.5 mm five-pin TSOP package. It costs $0.35 per unit in 3,000-unit quantities.

The NCS333 comes in a 1.5 mm × 3 mm SOT23-5 package or in a 2 mm × 1.25 mm SC70-5. It costs $0.5 per unit in 3,000-unit quantities.

Liquid Flow Sensor Wins Innovation Prize

Sensirion recently won the DeviceMed OEM-Components innovation prize at the Compamed 2014 exhibition. The disposable liquid flow sensor LD20-2000T for medical devices features an integrated thermal sensor element in a microchip. The pinhead-sized device is based on Sensirion’s CMOSens technology.sensirionliquidflowsensor

The LD20-2000T disposable liquid flow sensor provides liquid flow measurement capability from inside medical tubing (e.g., a catheter) in a low-cost sensor, suitable for disposable applications. As a result, you can measure drug delivery from an infusion set, an infusion pump, or other medical device in real time.

A microchip inside the disposable sensor measures the flow inside a fluidic channel. Accurate (~5%) flow rates from 0 to 420 ml/h and beyond can be measured. Inert medical-grade wetted materials ensure sterile operation with no contamination of the fluid. The straight, open flow channel with no moving parts provides high reliability. Using Sensirion’s CMOSens technology, the fully calibrated signal is processed and linearized on the 7.4 mm2 chip.

Source: Sensirion


AllianceMemoryThe AS4C4M16D1-5TIN, the AS4C8M16D1-5TIN, the AS4C16M16D1-5TIN, and the AS4C32M16D1-5TIN are high-speed CMOS double data rate synchronous DRAMs (DDR SDRAMs). The devices feature densities of 64 MB (AS4C4M16D1-5TIN), 128 MB (AS4C8M16D1-5TIN), 256 MB (AS4C16M16D1-5TIN), and 512 MB (AS4C32M16D1-5TIN) with a –40°C to 85°C industrial temperature range.

The DDR SDRAMs provide reliable drop-in, pin-for-pin-compatible replacements for industrial, medical, communications, and telecommunications products requiring high memory bandwidth. The devices are well-suited for high performance in PC applications. Internally configured as four banks of 1M, 2M, 4M, or 8M word × 16 bits with a synchronous interface, the DDR SDRAMs operate from a single 2.5-V (± 0.2 V) power supply and are lead- and halogen-free.

The AS4C4M16D1-5TIN, the AS4C8M16D1-5TIN, the AS4C16M16D1-5TIN, and the AS4C32M16D1-5TIN feature a 200-MHz clock rate and are available in a 66-pin TSOP II package with a 0.65-mm pin pitch. The 128-, 256-, and 512-MB devices are also available in a TFBGA package.

The DDR SDRAMs provide programmable read or write burst lengths of 2, 4, or 8. An auto pre-charge function provides a self-timed row pre-charge initiated at the end of the burst sequence. Easy-to-use refresh functions include auto- or self-refresh. A programmable mode register enables the system to choose a suitable mode for maximum performance.
Pricing for the AS4C4M16D1-5TIN, the AS4C8M16D1-5TIN, the AS4C16M16D1-5TIN, and the AS4C32M16D1-5TIN starts at $0.90 per piece.

Alliance Memory, Inc.

The Future of Very Large-Scale Integration (VLSI) Technology

The historical growth of IC computing power has profoundly changed the way we create, process, communicate, and store information. The engine of this phenomenal growth is the ability to shrink transistor dimensions every few years. This trend, known as Moore’s law, has continued for the past 50 years. The predicted demise of Moore’s law has been repeatedly proven wrong thanks to technological breakthroughs (e.g., optical resolution enhancement techniques, high-k metal gates, multi-gate transistors, fully depleted ultra-thin body technology, and 3-D wafer stacking). However, it is projected that in one or two decades, transistor dimensions will reach a point where it will become uneconomical to shrink them any further, which will eventually result in the end of the CMOS scaling roadmap. This essay discusses the potential and limitations of several post-CMOS candidates currently being pursued by the device community.

Steep transistors: The ability to scale a transistor’s supply voltage is determined by the minimum voltage required to switch the device between an on- and an off-state. The sub-threshold slope (SS) is the measure used to indicate this property. For instance, a smaller SS means the transistor can be turned on using a smaller supply voltage while meeting the same off current. For MOSFETs, the SS has to be greater than ln(10) × kT/q where k is the Boltzmann constant, T is the absolute temperature, and q is the electron charge. This fundamental constraint arises from the thermionic nature of the MOSFET conduction mechanism and leads to a fundamental power/performance tradeoff, which could be overcome if SS values significantly lower than the theoretical 60-mV/decade limit could be achieved. Many device types have been proposed that could produce steep SS values, including tunneling field-effect transistors (TFETs), nanoelectromechanical system (NEMS) devices, ferroelectric-gate FETs, and impact ionization MOSFETs. Several recent papers have reported experimental observation of SS values in TFETs as low as 40 mV/decade at room temperature. These so-called “steep” devices’ main limitations are their low mobility, asymmetric drive current, bias dependent SS, and larger statistical variations in comparison to traditional MOSFETs.

Spin devices: Spintronics is a technology that utilizes nano magnets’ spin direction as the state variable. Spintronics has unique properties over CMOS, including nonvolatility, lower device count, and the potential for non-Boolean computing architectures. Spintronics devices’ nonvolatility enables instant processor wake-up and power-down that could dramatically reduce the static power consumption. Furthermore, it can enable novel processor-in-memory or logic-in-memory architectures that are not possible with silicon technology. Although in its infancy, research in spintronics has been gaining momentum over the past decade, as these devices could potentially overcome the power bottleneck of CMOS scaling by offering a completely new computing paradigm. In recent years, progress has been made toward demonstration of various post-CMOS spintronic devices including all-spin logic, spin wave devices, domain wall magnets for logic applications, and spin transfer torque magnetoresistive RAM (STT-MRAM) and spin-Hall torque (SHT) MRAM for memory applications. However, for spintronics technology to become a viable post-CMOS device platform, researchers must find ways to eliminate the transistors required to drive the clock and power supply signals. Otherwise, the performance will always be limited by CMOS technology. Other remaining challenges for spintronics devices include their relatively high active power, short interconnect distance, and complex fabrication process.

Flexible electronics: Distributed large area (cm2-to-m2) electronic systems based on flexible thin-film-transistor (TFT) technology are drawing much attention due to unique properties such as mechanical conformability, low temperature processability, large area coverage, and low fabrication costs. Various forms of flexible TFTs can either enable applications that were not achievable using traditional silicon based technology, or surpass them in terms of cost per area. Flexible electronics cannot match the performance of silicon-based ICs due to the low carrier mobility. Instead, this technology is meant to complement them by enabling distributed sensor systems over a large area with moderate performance (less than 1 MHz). Development of inkjet or roll-to-roll printing techniques for flexible TFTs is underway for low-cost manufacturing, making product-level implementations feasible. Despite these encouraging new developments, the low mobility and high sensitivity to processing parameters present major fabrication challenges for realizing flexible electronic systems.

CMOS scaling is coming to an end, but no single technology has emerged as a clear successor to silicon. The urgent need for post-CMOS alternatives will continue to drive high-risk, high-payoff research on novel device technologies. Replicating silicon’s success might sound like a pipe dream. But with the world’s best and brightest minds at work, we have reasons to be optimistic.

Author’s Note: I’d like to acknowledge the work of PhD students Ayan Paul and Jongyeon Kim.

Can MoS2 Outperform Silicon?

Saptarshi Das

After decades of relentless progress, the evolutionary path of the silicon CMOS industry is finally approaching an end. Fundamental physical limitations do not enable silicon to scale beyond the 10-nm technology node without severely compromising a device’s performance. To reinforce the accelerating pace, there is an urgent and immediate need for alternative materials. Low-dimensional materials in general, and 2-D layered material in particular, are extremely interesting in this context. They offer unique electrical, optical, mechanical, and chemical properties. In addition, they feature excellent electrostatic integrity and inherent scalability, which makes them attractive from a technological standpoint. Graphene, hexagonal boron nitride (h-BN), and more recently the rich family of transition metal dichalcogenides—comprising Molybdenum disulfide (MoS2), WS2, WSe2, and many more—have received a lot of scientific attention as the future of nanoelectronics. The most widely studied material, grapheme, had reported intrinsic field effect mobility value as high as 10,000 cm2/Vs. However, the absence of an energy gap in the electronic band structure of grapheme, along with the challenges associated with making a stable interface with the gate dielectric, raises a lot of concern for grapheme-based nanoelectronics for logic applications. Hence, it paves the way for semiconducting 2-D materials such as MoS2 and others.

MoS2 is a stack of single layers held together by weak van der Waals interlayer interaction, and, therefore, enables micromechanical exfoliation of one or a few layers—similar to the fabrication of graphene from graphite. It is a semiconductor with an indirect bandgap of 1.2 eV. Single- and multilayer MoS2 field-effect transistors (FETs) with high on/off-current ratios (108) and excellent subthreshold swing (74 mV/decade) close to the ideal limit have been demonstrated. Basic integrated circuits (e.g., inverters and ring oscillators) have been reported. And initial studies also indicate that MoS2 has great potential in future nanoelectronics, sensing, and energy harvesting.

While there is a growing interest in MoS2-based nanoelectronics devices, the practice of evaluating their potential usefulness for electronic applications is still in its infancy since we don’t have a complete picture of their performance potential and scaling limits. My research addresses the major issues about the realization of high-performance logic devices based on ultra-thin MoS2 flakes. One of the major challenges in the realization of high-performance nano devices arises from the fact that these nanostructures need to be connected to the “outside” world to capitalize on their ultimate potential. Any interface between a low-dimensional nanostructure and a 3-D metal contact will inevitably affect the total system’s performance, which will strongly depend on the said contact’s quality. We have demonstrated that through a proper understanding and design of source/drain contacts and the right choice of the number of MoS2 layers to use, the excellent intrinsic properties of this 2-D material can be realized. Using scandium contacts on 10-nm-thick exfoliated MoS2 flakes that are covered by a 15-nm Al2O3 film, record high mobilities of 700 cm2/Vs are achieved at room temperature. This breakthrough is largely attributed to the fact that we succeeded in eliminating contact resistance effects that limited the device performance in the past unrecognized. We have also investigated the ultimate scaling potential of multilayer MoS2 field effect transistors (FETs) with channel lengths ranging from 1 µm down to 50 nm. Our results indicate that the multilayer MoS2 FETs are extremely resilient to short channel effects. We have demonstrated record high drive current density of 2.5 mA/µm and record high transconductance of 500 µs/µm for a 50-nm-long MoS2 transistor, which are comparable to state-of-the-art silicon technology.

In short, MoS2 preserves all the important properties of silicon with the added advantage of an ultra-thin layer structure, which allows for aggressive channel length scaling down to 2 nm and, therefore, has the potential to outperform silicon beyond the 10-nm technology node. Properly nourishing the development of MoS2 can be a real game changer for the future of the micro- and nanoelectronics industry.—by Saptarshi Das, Circuit Cellar 270, January 2012

DIY Solar-Powered, Gas-Detecting Mobile Robot

German engineer Jens Altenburg’s solar-powered hidden observing vehicle system (SOPHECLES) is an innovative gas-detecting mobile robot. When the Texas Instruments MSP430-based mobile robot detects noxious gas, it transmits a notification alert to a PC, Altenburg explains in his article, “SOPHOCLES: A Solar-Powered MSP430 Robot.”  The MCU controls an on-board CMOS camera and can wirelessly transmit images to the “Robot Control Center” user interface.

Take a look at the complete SOPHOCLES design. The CMOS camera is located on top of the robot. Radio modem is hidden behind the camera so only the antenna is visible. A flexible cable connects the camera with the MSP430 microcontroller.

Altenburg writes:

The MSP430 microcontroller controls SOPHOCLES. Why did I need an MSP430? There are lots of other micros, some of which have more power than the MSP430, but the word “power” shows you the right way. SOPHOCLES is the first robot (with the exception of space robots like Sojourner and Lunakhod) that I know of that’s powered by a single lithium battery and a solar cell for long missions.

The SOPHOCLES includes a transceiver, sensors, power supply, motor
drivers, and an MSP430. Some block functions (i.e., the motor driver or radio modems) are represented by software modules.

How is this possible? The magic mantra is, “Save power, save power, save power.” In this case, the most important feature of the MSP430 is its low power consumption. It needs less than 1 mA in Operating mode and even less in Sleep mode because the main function of the robot is sleeping (my main function, too). From time to time the robot wakes up, checks the sensor, takes pictures of its surroundings, and then falls back to sleep. Nice job, not only for robots, I think.

The power for the active time comes from the solar cell. High-efficiency cells provide electric energy for a minimum of approximately two minutes of active time per hour. Good lighting conditions (e.g., direct sunlight or a light beam from a lamp) activate the robot permanently. The robot needs only about 25 mA for actions such as driving its wheel, communicating via radio, or takes pictures with its built in camera. Isn’t that impossible? No! …

The robot has two power sources. One source is a 3-V lithium battery with a 600-mAh capacity. The battery supplies the CPU in Sleep mode, during which all other loads are turned off. The other source of power comes from a solar cell. The solar cell charges a special 2.2-F capacitor. A step-up converter changes the unregulated input voltage into 5-V main power. The LTC3401 changes the voltage with an efficiency of about 96% …

Because of the changing light conditions, a step-up voltage converter is needed for generating stabilized VCC voltage. The LTC3401 is a high-efficiency converter that starts up from an input voltage as low as 1 V.

If the input voltage increases to about 3.5 V (at the capacitor), the robot will wake up, changing into Standby mode. Now the robot can work.

The approximate lifetime with a full-charged capacitor depends on its tasks. With maximum activity, the charging is used after one or two minutes and then the robot goes into Sleep mode. Under poor conditions (e.g., low light for a long time), the robot has an Emergency mode, during which the robot charges the capacitor from its lithium cell. Therefore, the robot has a chance to leave the bad area or contact the PC…

The control software runs on a normal PC, and all you need is a small radio box to get the signals from the robot.

The Robot Control Center serves as an interface to control the robot. Its main feature is to display the transmitted pictures and measurement values of the sensors.

Various buttons and throttles give you full control of the robot when power is available or sunlight hits the solar cells. In addition, it’s easy to make short slide shows from the pictures captured by the robot. Each session can be saved on a disk and played in the Robot Control Center…

The entire article appears in Circuit Cellar 147 2002. Type “solarrobot”  to access the password-protected article.