New graphene inverters could lead to improved transistors

After its discovery in 2004, graphene — a one-atom-thick layer of carbon that conducts electricity with little resistance or heat generation — was touted as a potential replacement for silicon, possibly leading to devices with simplified circuits that might be less expensive to manufacture.

However, researchers have discovered that it has no band gap — a trait that is needed for switching on and off, which is critical for digital transistors.

‘The fact that graphene is a zero-band-gap material by nature has raised many questions in terms of its usefulness for digital applications,’ said Purdue University doctoral student Hong-Yan Chen.

Electrons in semiconductors such as silicon exist at two energy levels, known as the valence and conduction bands. The energy gap between these two levels is called the band gap. Having the proper band gap enables transistors to turn on and off, which allows digital circuits to store information in binary code, consisting of sequences of ones and zeroes.

Chen has led a team of researchers in creating a new type of graphene inverter. Other researchers have created graphene inverters, but they had to be operated at 77°K.

‘If graphene could be used in digital applications, that would be really important,’ said Chen, who is working with Joerg Appenzeller, a professor of electrical and computer engineering and scientific director of nanoelectronics at Purdue’s Birck Nanotechnology Center.

The Purdue researchers are said to be the first to create graphene inverters that work at room temperature and have a gain larger than one — a basic requirement for digital electronics that enables transistors to amplify signals and control its switching from zero to one.

So far, graphene transistors have been practical only for specialised applications. However, the new inverters are claimed to represent a step forward in learning how to use the material to create graphene transistors for broader digital applications, including computers and consumer electronics.

To create electronic devices, silicon is impregnated with impurities to change its semiconducting properties. Such ‘doping’ is not easily applicable to graphene. However, the researchers have potentially solved this problem by developing ‘electrostatic doping’, which makes it possible for graphene inverters to mimic the characteristics of silicon inverters.

Electrostatic doping is induced through the electric field between metal gates, which are located 40nm away from graphene channels. The doping can reportedly be altered by varying the voltage, enabling researchers to test specific doping levels.