Organic field-effect transistors (FETs) are usually thin-film devices. However, structural imperfections in the thin films can adversely affect the performance of the transistor. Organic FETs can also be fabricated on the surface of a single crystal, but this requires a processing method that will not destroy the fragile crystal. Instead the Delft team grew high-quality single crystals of tetracene and relied on electrostatic forces to make them adhere to a silica surface onto which gold source and drain electrodes had already been deposited. Crystals that are very thin - less than a micron - strongly adhere to the substrate without the need for any further processing steps and this minimizes any damage to the crystals.

Klapwijk and co-workers measured the current flowing from source to drain as they varied the drain voltage for different gate voltages. They found that the current had a very weak dependence on the gate voltage, which confirmed the high quality of the devices.

The room temperature mobility of the holes carrying the current was 0.4 centimetres squared per volt per second, which is higher than that of the best tetracene thin-film transistors. The mobility also shows temperature-dependent behaviour typically only seen in high purity organic crystals. Moreover, the mobilities at the surface and in the bulk of the device are similar – further proof that the quality of the crystals remains intact during processing.

Klapwijk and co-workers now hope to optimize their devices by improving the quality of the source and drain contacts, and by further reducing the level of impurities and imperfections in the crystals. They also plan to investigate organic materials other than tetracene.

Research into organic semiconductor devices suffered a setback last year when it emerged that a string of seemingly impressive results reported by Jan Hendrik Schön of Bell Labs - including several based on tetracene crystals - had been fabricated. “It has been very unfortunate that progress in this field appeared to be extremely rapid due to the flurry of results claimed by Schön and co-workers and then, all of a sudden, it dropped to a normal pace,” says Klapwijk. “However, we are now back on the right track, and we are learning step-by-step how to determine the intrinsic properties of these already used but poorly understood ‘messy’ materials.”