Graphene may find itself upstaged as the electronic chip material of the future by the readily available mineral molybdenite, MoS2, judging by recent results emerging from the EU’s ongoing FLATRONICS project. Electronic devices made with molybdenite – as with graphene – would be much thinner and consume less energy than conventional silicon-based electronics. But unlike graphene, molybdenite is a direct band gap semiconductor in its native state, which means that electronic devices made from it can be readily switched on and off.
FLATRONICS’ researchers have reported producing a transistor with MoS2 – the first time a two dimensional material other than graphene has been used to make such a device (Nature Nanotechnology, doi: 10.1038/ NNano.2010.279). ‘Single layer MoS2 is 0.65nm thick – the thinnest semiconducting sheet – and is similar to graphene, except that it is a direct gap semiconductor,’ with a band gap of 1.8eV, which is ideal for electronic switching, explains author Andras Kis at the École Polytechnique Federale de Lausanne (EPFL) in Switzerland. Because graphene in its native state is not a semiconductor with a band gap, this has to be engineered into the structure, making device fabrication more difficult.
‘The advantage of using MoS2 or similar material when compared with silicon,’ Kis elaborates, ‘would be that its small thickness would allow us either to fabricate transistors that have the same dimensions and consume less energy or consume the same amount of energy but with smaller dimensions.’
‘In addition to low leakage current, our transistors also show mobility comparable to 2nm thin silicon films,’ Kis adds, pointing out that higher mobility leads to better device performance as it means that charges can move more quickly through the material. Leakage current is the current that passes through the device when it is supposed to be switched off, hence transistors based on molybdenite could potentially consume much less energy in their standby state than conventional silicon transistors.
‘The paper shows an example of better transistors with extreme on/off ratio and low power dissipation,’ commented Jiming Bao, an electrical engineering expert at the University of Houston, US. ‘This is just the beginning, MoS2, like graphene, will be exploited in the near future for other novel applications.’
‘The other significance of the work is that they have shown a way to improve the electronic properties of MoS2 by using hafnium oxide as a gating material,’ Bao adds. ‘This indicates that MoS2 can be easily modified and integrated with other materials, this opens a lot of new opportunities for MoS2.’