Power/Performance Bits: Dec. 23

Detecting early damage in power electronics
Researchers at Osaka University to detect early damage in power electronics. The team used acoustic emission analysis to monitor in real time the propagation of cracks in a silicon carbide Schottsky diode during power cycling tests.

During the power cycling test, the researchers mimicked repeatedly turning the device on and off, to monitor the resulting damage to the diode over time. Increasing acoustic emission corresponds to progressive damage to aluminum ribbons affixed to the silicon carbide Schottsky diode. The researchers correlated the monitored acoustic emission signals to specific stages of device damage that eventually led to failure.

“A transducer converts acoustic emission signals during power cycling tests to an electrical output that can be measured,” said ChanYang Choe, a PhD candidate at Osaka University. “We observed burst-type waveforms, which are consistent with fatigue cracking in the device.”

Monitoring anomalous increases in the forward voltage during power cycling tests is a typical way of checking whether a power device is damaged. Using this method, the researchers saw an abrupt increase in the forward voltage, but only when the device was near complete failure. In contrast, acoustic emission counts were much more sensitive. Instead of an all-or-none response, there were clear trends in the acoustic emission counts during power cycling tests.

“Unlike forward voltage plots, acoustic emission plots indicate all three stages of crack development,” said Chuantong Chen, an associate professor at Osaka University. “We detected crack initiation, crack propagation, and device failure, and confirmed our interpretations by microscopic imaging.”

The researchers hope the early-warning method can help determine why silicon carbide devices fail and improve future designs in common and advanced technologies.

Multimodal thin-film transistor
Researchers at University of Surrey and Université de Rennes developed a new device, called a Multimodal Transistor (MMT), that is immune to parasitic effects.

In the MMT, on/off switching is controlled independently from the amount of current passing through the structure. The researchers say that this allows the MMT to operate at a higher speed than comparable devices and to have a linear dependence between input and output for ultra-compact digital-to-analog conversion and simplified circuits.

“Our Multimodal Transistor is a paradigm shift in transistor design. It could change how we create future electronic circuits. Despite its elegantly simple footprint, it truly punches above its weight and could be the key enabler for future wearables and gadgets beyond the current Internet of Things,” said Dr Radu Sporea, Senior Lecturer in Semiconductor Devices at the University of Surrey.

“It has been an incredible journey since approaching Dr Sporea during my BEng with the idea to create a device based on neural function. When we started in 2017, we could not imagine all the benefits that would result from a relatively simple device design,” said Eva Bestelink, a student at University of Surrey.

According to the researchers, the MMT shows improved functionality from separately controlling charge injection from charge transport, which “results in: low distortion for single‐stage amplifiers; up to 90% faster response than other contact‐controlled transistors; immunity to gain loss when used in analog floating gate (FG) applications; and substantial tolerance to geometrical registration errors.”

The test device was based on microcrystalline silicon processed at low temperature technology (<200 °C).

Compressible on-skin sensor
Researchers at the National University of Singapore (NUS) came up with a new sensor material for flexible and skin-contact applications that improves performance reliability.

When used as compressive sensors, soft materials tend to have hysteresis, where material properties can change in between repeated touches and affects the reliability of the data.

The Tactile Resistive Annularly Cracked E-Skin, or TRACE, is five times better than conventional soft materials with almost hysteresis-free performance, according to the team. They developed a process to crack metal thin films into desirable ring-shaped patterns on a flexible material called polydimethylsiloxane (PDMS).

The team integrated this metal/PDMS film with electrodes and substrates for a piezoresistive sensor and characterised its performance. They conducted repeated mechanical testing, and verified that their design innovation improved sensor performance.

Flexible TRACE sensor patches can be placed on the skin to measure blood flow in superficial arteries. (Credit: National University of Singapore)

“With our unique design, we were able to achieve significantly improved accuracy and reliability. The TRACE sensor could potentially could be used in robotics to perceive surface texture or in wearable health technology devices, for example to measure blood flow in superficial arteries for health monitoring applications” said Benjamin Tee, an assistant professor from the Institute for Health Innovation & Technology and the Department of Materials Science and Engineering at NUS.

Next, the researchers plant to further improve the conformability of their material for different wearable applications, and to develop AI applications based on the sensors.

“Our long-term goal is to predict cardiovascular health in the form of a tiny smart patch that is placed on human skin. This TRACE sensor is a step forward towards that reality because the data it can capture for pulse velocities is more accurate, and can also be equipped with machine learning algorithms to predict surface textures more accurately,” said Tee. Other applications include use in prosthetics.