Researchers in the US have developed a new way of making transistors from graphene – a sheet of carbon just one atom thick. The technique overcomes a major obstacle facing those who want graphene to replace silicon as the material of choice in future electronic devices. It has also been used to make the highest-speed graphene transistors ever.
The semiconductor graphene is seen by many as an ideal material for electronic devices because it is extremely thin yet has high electrical and thermal conductivity and great physical strength. Unfortunately, however, the processing techniques currently used by the semiconductor industry cannot be applied to graphene because they introduce defects into the material, which ultimately deteriorate device performance.
Now, Xiangfeng Duan and colleagues the University of California at Los Angeles have developed a new fabrication technique that involves employing alumina-coated nanowires as the gate electrode in a graphene transistor. The device’s source and drain electrodes are then made using a self-aligning process using the nanowires as “masks” – a process that also minimizes resistance in the transistor, so improving its performance even further.
Graphene consists of a single, flat sheet of carbon arranged in a honeycombed lattice. Since the material was first created in 2004, its unique electronic and mechanical properties have amazed researchers, who have been eyeing it up for a host of device applications. In particular, it could be used to make ultrafast transistors because the electrons in graphene behave like relativistic particles with no rest mass. This means that they whiz through the material at extremely high speeds.
There are still lots of challenges to be overcome, however, before the dream of all-graphene electronics becomes reality. One of these is to develop a fabrication technique that produces nearly defect-free devices, something never achieved until now.
Conventional processing methods to make state-of-the-art silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) involve using a self-aligned gate structure to ensure that the edges of the source, drain and gate electrodes are precisely positioned. This avoids any overlapping between the electrodes, thus minimizing resistance in a device. (High resistance is a bane for nanoscale devices since it slows them down). The same technique does not work for graphene though because the technique unavoidably introduces defects into the material’s lattice.
Duan and colleagues have instead used a cobalt-silicide-alumina core-shell nanowire as the top gate in their graphene transistor. This dielectric nanostructure is made in a separate step and then simply placed on top of a monolayer of graphene afterwards. Such an approach does not introduce any appreciable defects into the material, says Duan.
Nanowire mask
The researchers then place a thin layer of platinum on top of the graphene – across the nanowire such that the wire separates the thin film of graphene into two isolated regions. These two separate areas then form self-aligned source and drain electrodes next to the nanowire gate. In this work, the nanowire mask also defines the gate length of the device, in this case about 140 nm.
The finished devices have the highest transconductance value ever reported for such devices, of 1.27 mSµm–1. The transconductance of a transistor determines how well it performs. Microwave measurements on the transistors also show they have a record-breaking intrinsic cut-off frequency in the range of 100–300 GHz, which is about twice as fast as the very best silicon MOSFETs of a similar size. Finally, the mobility of the devices (which determines how fast electrons move through them) is about 20,000 cm2/Vs – a value that is around two orders of magnitude better than that of similarly sized commercial silicon transistors.
“Demonstrating graphene transistors with a cut-off frequency comparable to the very best transistors out there marks an extremely important step in graphene research,” Duan said. “This clearly demonstrates the exciting potential of graphene-based electronics for future high-frequency circuits.”
The team now plans to fabricate transistors with smaller gate lengths to push the cut-off frequency even higher – perhaps towards 1 THz. “We also hope to scale up the approach to fabricate big arrays of high-speed graphene transistors on large-area substrates, including flexible substrates,” revealed Duan.
The work was published in Nature.
