InitRech 2015/2016, sujet 22
Summary
This article is talking about Graphene transistors. His autor is Frank Schiwierz from University of Technology of Ilmenau in Germany and he publish it in 2010.
As an introduction of the subject, the autor explain that the growing interest for the Graphene by the electron devices community. He also explain there are two kind of semiconductor electronics. The first one is digitals devices for which is not a priority to include new materials in its technologie because it's focusing on one sort of devices the MOSFET or metal-oxide-semiconductor FET and it is a very profitable buisness. The second one is radiofrequency devices which more open the previous one for new devices technology. For example, this technology built-in high-electron-mobility transistors (HEMTs) based on semiconductors such as GaAs and InP, silicon n-channel MOSFETs, and different types of bipolar transistor. So, the author end his introduction by saying that Graphene is more destinated to radiofrequency devices rather than digitals devices due to its high conductivity and over proprieties.
In a first section, he demonstrate that Graphene is a real opportunity for FET technology because of its thickness .It is abel to considerably reduce the short channel effects for FETs with short gates and fast carrier in the channel which are used in high-speed applications. Moreover, he makes comparaison of Graphene's thickness and over devices channel's tickness, generally based on silicon. He also explain why digitals devices need a switch-off and radiofrequency applications not.
In a second section, he try to explain Graphene properties relevant to transistors. The two important aspects of graphene are the bandgap and the electric charge transport at room temperature. Grafene has a bangap of zero, it never be switched off so it is not suitable for logic applications. But he also show three possibilties to open a bandgap in graphene. First one was by constraining large-area graphene in one dimension to form graphene nanoribbons that have a bandgap, the second one was by biasing bilayer graphene and the third one was by applying strain to graphene. However, The biggest advantage of the graphene is its high carrier mobility. The autor show differents mobilities of electric charge transport in function of the support where the graphene grown and it is impressive (between 10 000 and 1 000 000 cm² V-1 s-1). But he notice that all of those mobilities were for large-area graphene, that mean gapless, so it is not semi-conductive. For nanoribbons (fine area of graphene) and nanotubes (fine pipe of graphene) with same bandgap as silicon, performances are considerably reduces and worst than silicon channel of MOS device. In this section, he also talk about high-field carrier velocity of the graphene whose he explain ,to his point of view with some references, that high-field mobilty of the graphene is not yet estimated with precision and it will be underestimated.
The third section make a state of art of graphene transistors. The autor compare perfomances of reported graphene transistors with over devices based on silicon. In some case, graphene transistors are better than the overs for mobility by example or cut-off frequencies . Again, He also came at the conclusion that graphene transistors are most promised for radiofrequency applications than logics application due to its gapless property, so it does not be able to be switched off.
And on a final section, the author speak about graphene tunnel FET and bilayer pseudospin FET which have both good properties but still in embryonics stage.
Main contribution
This article are contributing for make an over-view of what applications have being tried for the graphene and their results. It also relativises on graphene properties for transistor application especially for digitals devices. It came at the conclusion that the graphene transistor are, from now, not an extraordinary opportunity to improve speed and size of transistor compared to silicon based transistor. But it make a point saying that graphene technology is young and have already make a big and quick move.
