Electronic devices need a component to store electricity for their working. This is typically a battery or a capacitor. But these take up space, costs something and there is energy loss as electricity is transmitted from the battery to the chips, where the processing work is done.

What if the energy could be stored right on the chip? The answer to this question opens up a field of technology, called on-chip storage. On-chip storage uses micro-capacitors. (Capacitors are storage devices into which you can dump large amounts of energy — they dump the energy back when you ask them to, unlike batteries which charge or discharge slowly.)

Unlike batteries, which store energy through electrochemical reactions, capacitors store energy in an electric field established between two metallic plates separated by a dielectric material (a type of insulator). Also, capacitors do not degrade with repeated charge-discharge cycles, leading to much longer lifespans than batteries.

However, capacitors generally have much lower energy densities than batteries — they can store less energy per unit volume or weight. The problem only gets worse when you try to shrink them down to micro capacitor size, for on-chip energy storage.

So, scientists have been toiling for a long time to come out with better micro-capacitors. In this, a group of eleven scientists (including three of Indian origin and one from Bangladesh) at the Lawrence Berkeley University, California, have recently reported groundbreaking success. They have achieved record-high energy densities in their micro-capacitors made with engineered thin films of hafnium oxide and zirconium oxide. The findings, published in the journal Nature, pave the way for advanced on-chip energy storage and power delivery in next-generation electronics.

Engineered thin films

“We’ve shown that it’s possible to store a lot of energy in micro-capacitors made from engineered thin films, much more than what is possible with ordinary dielectrics,” said Sayeef Salahuddin, senior scientist and UC Berkeley professor who led the project, in a press release. “We’re doing this with a material that can be processed directly on top of microprocessors.”

Effectively, the reduced size of micro-capacitors limits their capacity (or, ‘capacitance’) to store electricity. To understand how Berkeley Lab countered this, it is essential to know the concept of ‘negative capacitance’ materials. Typically, when the applied voltage increases, capacitance should also increase, but in certain materials, it decreases.

Normally if you layer one dielectric material over another, the overall capacitance falls. Berkeley Lab scientists figured out that if one of the layers is of a negative-capacitance material, then the overall capacitance increases. They engineered thin films (of HfO2-ZrO2) to achieve negative-capacitance effect. This hybrid dielectric material raised the overall capacitance of the micro capacitor. This is a groundbreaking development in the field of electronics.

These high capacitance micro-capacitors will find applications in edge computing systems, AI processors and IoT sensors.

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