Power/Performance Bits: Nov. 2

GaN CMOS ICs
Researchers from the Hong Kong University of Science and Technology (HKUST) are working to increase the functionality available to wide bandgap gallium nitride (GaN) electronics.

GaN is frequently used in power electronics, such as power converters and supplies. However, GaN CMOS technology has been hampered by the difficulties in implementing p-channel transistors and integrating them with n-channel ones.

The team used a GaN power device technology platform in an attempt to tackle a problem associated with the gate-dielectric/channel interface. They engineered a “buried channel” structure enabled by an oxygen plasma treatment (OPT) technique, which resulted in p-channel GaN transistors with a well-balanced performance matrix of threshold voltage for enhancement-mode operation, high ON/OFF current ratio, and high current driving capability. A monolithic integration process was also developed integrate GaN CMOS ICs with GaN power switching devices.

With the device, the researchers were able to demonstrate a complete set of GaN CMOS-based elementary logic gates including NOT, NAND, NOR gates, and the transmission gate. The team also demonstrated multiple-stage logic circuits that can be operated at megahertz frequencies.

“This is an exciting leap forward. We have first proven that all building blocks are functional, then these building blocks could be put together for more complicated entities. Therefore, any GaN-based complementary logic circuits can be constructed by making combinations of these logic gates,” said Kevin Chen, a professor in the Department of Electronic and Computer Engineering at HKUST.

The team said that the development opens to door to possible GaN devices such as energy-efficient power ICs with advanced control, sensing, protection, and drive functions in addition to the basic power switching functions, as well as computing/control electronics for harsh environments.

Sticking antennas on organs
Researchers at Singapore University of Technology and Design (SUTD) are developing antennas that can be used on biological tissues, such as organs, to wirelessly power implantable medical devices.

The antennas are made using Galinstan, a gallium-based low-toxicity liquid metal, to create stretchable and conductive traces for the coil. In tests, they maintained high wireless powering efficiency even when subjected to extreme deformations such as stretching, bending, and twisting.

“Our liquid metal antenna offers a new capability for the design and fabrication of wireless biodevices, which require conformal tissue-device integration. We believe this technology paves the way towards minimally invasive, imperceptible medical treatments,” said Dr. Kento Yamagishi of SUTD.

To construct the antenna, a fast-curing silicone sealant was pneumatically extruded onto a 7-µm thick elastomeric substrate (an Ecoflex microsheet) to pattern the outline of the microchannel. Direct ink writing 3D printing enabled the team to control the width, space, and height of the antenna. After embedding LEDs and jumper wiring, the outline was sealed with a free-standing Ecoflex microsheet to form microfluidic channels.

Development of a wirelessly powered light-emitting device based on a liquid metal microfluidic antenna. Direct ink writing (DIW) of silicone microchannels on an Ecoflex microsheet (7 μm thick) was followed by the injection of gallium-based liquid metal — Galinstan. (Credit: Singapore University of Technology and Design)

A sacrificial layer of polyvinyl alcohol (PVA), a water-soluble polymer, was used to provide mechanical support and enabled the liquid metal to flow in the thin-film microchannel to form the stretchable coil. The fluidic antenna operates at a frequency close to 13.56 MHz, the standard near field communication (NFC) frequency. Additionally, the liquid metal antenna showed a high quality (Q)-factor (>20), demonstrating the efficiency of wireless powering.

To make the antenna stick to moist, soft tissues, a mussel-inspired bioadhesive called polydopamine was used. The researchers found that the Galinstan antenna could experience up to 200% tensile strain, match a 3 mm radius of curvature, and withstand a 180 degree twisting angle while maintaining a high Q factor. Repetitive tensile strain tests showed no degradation in the Q factor or meaningful shift in the operating frequency.

“While we demonstrated the direct fabrication of microchannels on ultrathin films in this work, direct 3D printing of microchannels enables the creation of microchannels and other fluidic components on different types of surfaces, including biological surfaces. We believe that such capabilities will bring new opportunities for biological sensing, communication, and therapeutics,” said Michinao Hashimoto, an associate professor at SUTD.

CMOS-compatible infrared sensor
Researchers at Forschungszentrum Jülich, Polytechnic University of Milan, Roma Tre University, and Leibniz Institute for High Performance Microelectronics developed a cost-effective infrared detector that can be integrated into camera chips and smartphones.

Infrared detectors could be useful in applications such as automotive cameras, as those wavelengths of light are not distorted by rain, fog, or haze.

“There are already other cameras that are used for these purposes. However, their very high cost prohibits their use in daily-life application” said Dr. Dan Buca from Forschungszentrum Jülich. “Our detector bridges a gap, since it covers a range of the spectrum for which there have been no cost-effective sensors to date. The smart combination of elements and alloys that are well compatible with silicon now enables us to use a straightforward manufacturing process with standard industry tools. Therefore, we are now able to construct very inexpensive camera chips that can be integrated in any smartphone just as in visible cameras currently in use.

The device is based on a thin layer of silicon with layers of germanium and germanium-tin deposited on top.

“The germanium–tin semiconductors were developed at Jülich,” said Prof. Giovanni Isella from Polytechnic University of Milan. “It took almost 10 years to optimize all the material parameters and device designs for this. But now these semiconductor layers can be built in any chip factory using the established technology.”

The SWIR detector developed at Jülich makes hidden letters in the NIR and SWIR range visible (VIS = visible light/camera) (Credit: Forschungszentrum Jülich / Politecnico di Milano)

The detector covers two ranges of infrared radiation, shortwave infrared (SWIR) and near infrared (NIR). To switch between them, the bias voltage applied to the detector is reversed. “In doing so, we’re expanding the sensor’s area of application,” said Isella.

Beyond seeing through fog, other applications include peering underneath layers of paint in paintings, checking security features in banknotes, or distinguishing between substances based on their different absorption properties in the NIR and SWIR range.