Silicon solutions for transistors

Materials World magazine
1 Aug 2008

Researchers at the Advanced Technology Institute (ATI) at the University of Surrey, UK, have developed two thin film silicon transistors that they claim are easier and cheaper to produce than current structures, and deliver improved performance with lower operating voltages. This could lead to devices with increased electronic functions.

In thin film transistors of disordered nanocrystalline silicon used on glass or plastic substrates, the switching performance of the transistors is often limited by defects in the bandgap resulting from disorder in the material. This can cause low mobility and current leakages. The Surrey scientists have proposed a way to overcome this by reducing the thickness of the conduction channel in the transistor to just two nanometers.

‘When the channel becomes thinner, OFF-state leakage current decreases much more significantly than the ON-state current attributed to the stronger quantum confinement along the channel thickness direction,’ explains Professor Ravi Silva of the ATI. ‘The subthreshold swing also becomes steeper because the effect of the gate voltage on the channel surface potential increases.’

Controlling the chemical vapour deposition process to a low rate achieves channel thicknesses of below five nanometers, he adds.

At the gate

The Surrey team has also created a source-gated transistor (SGT) that operates on less power with larger gains. In traditional field-effect transistors, the gate modulates the conductance of a channel and the current saturates when the drain end is depleted of carriers. However, in the SGT, which can be produced from amorphous silicon or an organic semiconductor, the gate controls the carrier supply, and the current saturates when the source is depleted.

This allows the transistor to operate with short source-drain separations, even with a thick gate insulator layer. ‘It offers improved analogue and high frequency performance, and the methodology is more suitable for producing short channel devices by simple fabrication techniques,’ explains Silva.

Professor Jean-Pierre Colinge, Head of the Micro-Nanoelectronics Centre at the Tyndall National Institute in Cork, Ireland, comments, ‘Often the problem with thin films is you have a high source/drain resistance, [but this device] does not have this problem. It seems to be a more efficient transistor’.

Silva notes both transistor projects use readily available materials and process technologies, and would therefore be easy to mass manufacture. It is also cheaper to produce transistors from amorphous silicon than grow a polysilicon layer on heat-sensitive materials, as is done with current high-performance monolithic transistors.

His group is working on prototyping for large area applications.