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Transport properties

Transport properties

Ultrathin transistor can be turned on with a tiny voltage

08 Oct 2015 Belle Dumé
Tunnel vision: electron-microscope image of the ATLAS-TFET

A new transistor with an atomically thin current-carrying channel that operates at ultralow supply voltages has been unveiled by a team of researchers in the US. The new device, which is made from a 2D semiconducting crystal and a bulk germanium substrate, can be switched on at just 0.1 V. It could be used to create extremely dense and lower-power integrated circuits, and could also form the basis of ultrasensitive sensors of biological molecules.

Field-effect transistors (FETs) are the workhorses of modern-day electronics, and the size of FETs have been decreasing steadily over the last few decades – allowing more and more devices to be packed onto computer chips. However, this relentless downsizing cannot go on forever, and chip designers are running into major difficulties. One challenge on the horizon is that the switching of conventional FETs is limited by a quantity known as “sub-threshold swing” that cannot be lower than 60 mV per decade of drain current at room temperature. This puts a lower limit on the supply voltage required to operate the FET, which makes it difficult to create the low-power devices needed for the denser chips of the future.

Swing low

The tunnel field-effect transistor (TFET) is a new type of device that works by controlling the amount of current that can quantum-mechanically tunnel through a potential barrier. In contrast, a conventional FET involves the current being thermally excited over a potential barrier. This difference means that TFETs can have sub-threshold swing values lower than 60 mV per decade.

Now, Kaustav Banerjee and colleagues at the University of California, Santa Barbara, and Rice University have developed TFETs made from bilayer molybdenum disulphide and bulk germanium that have a sub-threshold swing of 31.1 mV per decade of drain current at room temperature. This makes the device a very good contender for making practical transistors that operate with supply voltages as low as 0.1 V. Such devices would require 90% less power to run compared with conventional FETs.

Dubbed the atomically and layered semiconducting channel TFET (ATLAS-TFET), the new device uses highly doped germanium as its source electrode. The current-carrying channel is an extremely thin (1.3 nm) layer of molybdenum sulphide just two molecules thick. Resembling graphene, molybdenum sulphide is a semiconductor that occurs in sheets one molecule thick. The resulting layered device has a strain-free interface, a low barrier for current-carrying electrons to tunnel through, and a large tunnelling area – which are all desirable properties for TFETs.

On the road map

The International Technology Roadmap for Semiconductors (ITRS) has called on researchers to develop devices with sub-threshold swing values lower than 60 mV per decade, over four decades of current. The only experimental TFET made so far to meet this criterion relies on nanowire structures, which are difficult to produce and manipulate.

“Our ATLAS-TFET is the first TFET with in-planar architecture to satisfy the ITRS prescription and, as such, might be used in the development of next-generation ultralow-power integrated electronics and ultrasensitive sensors,” say Banerjee and colleagues.

Creating a sensor from the ATLAS-TFET involves removing the gate junction and placing receptor molecules on the molybdenum-sulphide channel. When a molecule of interest binds to a receptor, it changes the current flowing through the channel – thus signalling the presence of the molecules, even at tiny concentrations.

The thin-channel TFET is described in Nature.

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