Transistors designed for semiconductors of the future

Transistors that can change properties are important elements in the development of semiconductors. With standard transistors approaching the limit for how small they can be, having more functions on the same number of units is becoming more important for the development of small, energy-efficient circuits. Researchers at Lund University in Sweden have now created configurable transistors and shown how to exert control on a more precise level. In view of the increasing need for more powerful and efficient circuits, there is greater interest in reconfigurable transistors. The advantage of these is that, in contrast to standard semiconductors, it is possible to change the transistor’s properties after they have been manufactured.

Historically, computers’ computational power and efficiency have been enhanced by scaling down the silicon transistor’s size (known as Moore’s Law). However, a stage has been reached where the costs for continuing development along those lines have become higher and quantum mechanics problems have arisen that have slowed development. Instead, researchers are searching for new materials, components and circuits. Lund University is a leader in III-V materials, which are an alternative to silicon. These materials have potential in the development of high-frequency technology (such as parts for future 6G and 7G networks), optical applications and increasingly energy-efficient electronic components.

Ferroelectric materials are used to realise this potential; these materials can change their inner polarisation when exposed to an electric field. It can be compared to an ordinary magnet, but instead of a magnetic North and South Pole, electric poles are formed with a positive and a negative charge on each side of the material. By changing the polarisation, it is possible to control the transistor. Another advantage is that the material ‘remembers’ its polarisation, even if the current is turned off. Through a new combination of materials, the researchers have created ferroelectric ‘grains’ that control a tunnel junction — an electrical bridging effect — in the transistor. This is on a small scale, as the grain is 10 nanometres in size. By measuring fluctuations in the voltage or current, it has been possible to identify when polarisation changes in the individual grains and thus understand how this affects the transistor’s behaviour.

The research has examined new ferroelectric memory in the form of transistors with tunnel barriers in order to create new circuit architectures. Anton Eriksson, a researcher who recently completed his doctoral degree in nanoelectronics, said the researchers aim to create neuromorphic circuits that are adapted for artificial intelligence in that their structure is similar to the human brain with its synapses and neurons.

The new results indicate that it is possible to create tunnel junctions using ferroelectric grains that are located directly adjacent to the junction. These nanograins can then be controlled on an individual level, when previously it was only possible to keep track of entire groups of grains, known as ensembles. In this way, it is possible to identify and control separate parts of the material. Lars-Erik Wernersson, professor of nanoelectronics, said in order to create advanced applications, it is vital to understand the dynamics in individual grains down to the atomic level, as well as the defects that exist.

“The increased understanding of the material can be used to optimise the functions. By controlling these ferroelectric grains, you can then create new semiconductors in which you can alter properties. By changing the voltage, you can thus produce different functions in one and the same component,” Wernersson said.

The researchers have also examined how this knowledge can be used to create different reconfigurable applications by manipulating in various ways the signal that goes through the transistor. It could, for example, be used for new memory cells or more energy-efficient transistors. This new type of transistor is called ferro-TFET and can be used in both digital and analog circuits. Another advantage of these transistors is that they can function at low voltage. This makes them energy-efficient, which could be required in wireless communication, Internet of Things and quantum computers.

“I consider this to be leading-edge research of international standing. It’s good that in Lund and Sweden we are at the forefront regarding semiconductors, especially in view of the EU’s recently enacted Chips Act, which aims to strengthen Europe’s position regarding semiconductors,” Wernersson said.

Image caption: The millimetre-sized chip on which the transistors are located. Image credit: Anton Persson.