SAMFET secrets revealed

An international team of researchers from the Netherlands, Russia and Austria has found that monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors (SAMFETs).

This opens the door to extremely sensitive chemical sensors that can be produced cost effectively. The research was carried out at Philips Research Eindhoven and Eindhoven University of Technology.

The SAMFET is a recent example of the development of ‘plastic micro-electronics’ - ie, electronics based on organic materials. Last year, Philips Research built the transistor by immersing a silicon substrate into a solution containing liquid crystalline molecules that self-assemble onto this substrate, resulting in a semi-conductive layer, a single molecule thick.

The monolayer of the SAMFET consists of molecules that are standing upright. Conduction takes place by charges jumping from one molecule to the other.

In previous attempts to make a SAMFET, it was found that, as the length of the SAMFET increased, its level of conductivity decreased exponentially.

In a joint project, Philips Research, the Eindhoven University of Technology (TU/e), the University of Groningen, the Holst Centre, the Enikolopov Institute for Synthetic Polymer Materials in Moscow and the Technical University in Graz, Austria, discovered that this decrease is determined by the monolayer coverage, which could be explained with a widely applicable two-dimensional percolation model.

The researchers compared the effect to crossing a river by jumping from rock to rock. The closer the rocks are to each other, the quicker one can jump or even walk to the other river bank. So if the monolayer displays more voids, the conductivity decreases.

Until now, this behaviour was an unchartered area and inhibited the use of SAMFETs in applications such as sensors and plastic electronics.

The SAMFET's sensitivity could open doors to the development of the ultimate chemical sensor, the research team points out.

“If we go back to the river again, another benefit of a SAMFET becomes clear,” Martijn Kemerink, assistant professor at the TU/e, said.

“Imagine that there are just enough rocks to cross that river. When you remove one rock the effect is significant, for it is impossible to make it to the other side of the river. The SAMFET could be used to make sensors that give a large signal that is triggered by a small change,” he said.

At present, SAMFETs are not widely used, for there are alternatives of which the production process is well established. However, the production process of SAMFETs is extremely simple and material efficient.

The transistor requires only a single layer of molecules that is applied by immersion into a chemical solution.

The same solution can be used for many substrates, for the substrate only takes the necessary (small) amount of molecules. This makes future large-scale production of monolayer electronics efficient, simple and cost effective.