There can be no doubt that the creation of the transistor was one of the most important scientific developments of the 20th century. Recently the possibility of building such devices from graphene has been explored [for example, see Materials Today, March 2010, page 44]. A collaboration between researchers at Rice University and the University of California-Riverside has now demonstrated the first triple-mode graphene amplifier [Yang et al., ACS Nano, (2010), doi:10.1021/nn1021583], which may be beneficial to the communications industry, due to the potential efficiency of such a streamlined device.
The operation mode of standard metal-oxide field-effect-transistors (MOSFETs) is determined by which of the source, drain and gate terminals are used for signal input and output. The three most basic single-transistor amplifier arrangements are labelled by which terminal remains, as common-source, common-drain and common-gate. In addition MOSFETs are also defined by whether their current carriers are electrons (n-type) or holes (p-type). This property is determined during construction, as it is dependant on the dopant introduced into the semiconductor.
This new device takes advantage of the unique ambipolar nature of graphene, where both electrons and holes can act as current carriers. If the voltages between the gate & source, and the drain & source are correctly controlled, then the transistor can be alternated between n-type and p-type. It is therefore possible to operate the amplifier in common-source, common-drain or frequency multiplication mode, without any need to rewire or replace the device.
Several issues remain to be resolved, with the most significant being the low gain. However, this problem with the device is being addressed, as Dr. Alexander Balandin, professor of electrical engineering and chair of materials science and engineering at UC-Riverside reveals, “there is a clear plan on how to improve it”. “The next step will be using a different design of transistors with thinner gate oxide, which would allow us to improve the gain”.
Phase shift keying (PSK) and frequency shift keying (FSK) have both been demonstrated with the device. These are the processes of altering the phase and frequency of an input carrier wave, such that information can be transmitted. It is PSK that lies at the heart of modern wireless technology. The researchers responsible believe their device offers significant advantages over established approaches in terms of power consumption and bandwidth.
Stewart Bland