The newly developed graphene transistors could ultimately replace silicon in computer chips. Furthermore, the transistors could be made at the nanoscale, which would allow chip manufacturers to incorporate more transistors into their chips, thus making them more powerful.
‘The main benefit of using graphene to make transistors is that you can aggressively scale down the size — probably down to 10–15 atoms across,’ said Dr Leonid Ponomarenko who led the experimental effort at Manchester University.
Silicon, unlike graphene, is very good at alternating between different states of electrical conductivity, which is an important characteristic for effective transistors.
The difference between electrical states is called the on/off ratio and in the case of graphene the ratio isn’t very high. This means that graphene continues to conduct a lot of electronics in its ‘off’ state.
Ponomarenko explained that the transistor’s on/off ratio needs to be as large as possible, otherwise electrical current will leak through and waste energy.
‘If this happens, the whole processor, which contains millions of transistors, will become very hot,’ he said.
He added that scientists have been attempting to increase graphene’s on/off ratio over the last year.
‘If you use simple graphene [to make transistors] then the on/off ratio is something like 1:15–1:20,’ said Ponomarenko. ‘People are ultimately trying to improve it and make it at least 1:1,000, which is the lower limit at which we can start using them in computers.’
A range of different methods have been employed in an effort to increase this on/off ratio, including: making graphene nanoribbons; making graphene quantum dots; using bilayer graphene; and making chemical derivatives of graphene, such as hydrogenated graphene.
Ponomarenko said: ‘All these methods increase the on/off ratio but decrease mobility and other valuable properties of graphene.
‘Our transistors are multi-layered structures in which layers of graphene are combined with atomically thin insulators such as boron nitride [BN] or molybdenum disulfide [MoS2].
‘The on/off ratio for transistors with BN is rather modest at 50. In case of MoS2 this number is larger than 1:1,000.’
The next step for the team is to use different materials in combination with graphene to create different structures and further increase the on/off ratio.