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Contacts to blame for noise in Graphene Transistors, shows research

Photo: Siddharth Kankaria/Research Matters

A new study by researchers at the Indian Institute of Science, Bangalore, has shown that metal contacts in graphene transistors generate unwanted electrical noise - undesired disturbance in the functioning of devices. The team, led by Prof. Arindam Ghosh, has attributed this noise to the reaction between graphene in the transistor and metals fused to form contacts for the flow of electricity. This reaction spontaneously leads to chemical modification and introduction of defects at the contacts.

Graphene, nicknamed a ‘wonder’ material, is single layer of carbon atoms arranged in a honeycomb-shaped structure. It is a better electrical conductor than copper, is 200 times stronger than steel but six times lighter, and is almost transparent, absorbing only 2% of light that falls on it. Its discovery is regarded as a breakthrough in the field of material science and the discoverers, Andre Geim and Konstantin Novoselov, were awarded the Nobel Prize in Physics in 2010.

Studies have shown that mobility of electrons, a measure of how quickly an electron moves through semiconductor, is 200 times larger in graphene than in silicon. Hence, graphene is increasingly finding applications in the semiconductor industry. A report by the International Technology Roadmap for Semiconductors (ITRS) estimates that by 2021, graphene will replace Silicon and Germanium from the semiconductor market. Now, the current study is set to have a major implication on the applicability of graphene in future electronic devices.

The effects of contacts in conventional devices have been well studied. However, until now, it was unknown what was the dominant source of electrical noise in graphene devices - was it the contacts, or graphene itself. The researchers of this study have found that when transistors made of graphene are used in an electronic circuit, it leads to the generation of noise.

“Contacts affect graphene more severely compared to conventional semiconductors due to its single atomic thickness. Here, the metal atoms chemically modify graphene and leave it vulnerable to potential fluctuations nearby causing large noise. This research shows that as the electrical quality of graphene improves, the relative effect of the contacts becomes stronger, presenting a challenge in creating high quality devices”, explains Mr. Paritosh Karnatak, the lead author of the study.

The researchers performed multiple experiments to measure the resistance of contacts by using graphene of different structure and electron mobility. They found that this noise due to contacts has microscopic origin and is linked to fluctuating electrostatic environment of metal-graphene interface. Graphene with high electron mobility was found to be most severely affected by contact noise.

Graphene, being as thin as an atom, promises to miniaturize many electronic devices and hence, there is a great deal of excitement today in the semiconductor industry about using graphene in futuristic technologies. This study is an important step towards understanding the properties of graphene and implications of its usage. “The results of this work will not only lead to new design guidelines for low-noise graphene electronics, but also inspire material scientists to investigate the electronic structure at the metal-graphene junctions,” Prof. Ghosh signs off.