Graphene in High-Frequency Electronics
This two-dimensional form of carbon has properties not seen in any other substance
Graphene in Transistors
The property of very high electron mobility has excited the electronics community and led to countless predictions. Our apparently insatiable need for speedier computers and greater data throughput in wireless devices has resulted in a universal acceptance that electronic devices and circuits must be made to operate ever faster. The traditional way of achieving this result has been by reducing the size of transistors.
Transistors are made with the class of materials called semiconductors, which conduct electricity under some conditions but prevent its flow in others. That property lets the transistor act as an on/off switch for the binary signals used in computers. The quantum mechanical energy bands that form when atoms are bound together in a crystalline solid make it possible for currents in semiconductors to be carried by both positively charged and negatively charged particles. The negatively charged particles are electrons and the positively charged particles are called holes, implying a place where an electron is missing. Materials that conduct electrons are called n-type (for negative-type) and ones that conduct holes are called p-type (for positive-type). Both n-type and p-type materials can be used in transistors.
The field-effect transistor (FET) is the most common type used in modern computers. A gate electrode at the center of the device is separated from the semiconductor surface by a thin insulating layer. A current of electrons or holes passing between the FET’s two terminals, called the source and the drain, is controlled by the amount of voltage on the gate. In an n-type FET, for instance, a positive voltage on the gate electrode forms a layer of electrons in the channel region, creating a conducting path in the semiconductor through which current flows.
To speed up a FET, the concept is simple: shorten the distance—the channel length—between the source and drain, and signals will pass between the terminals more quickly. This reduction of dimensions was dubbed scaling by IBM researcher Robert Dennard in the 1970s. (An important side benefit to scaling is that it leads to cramming more transistors into a given area, creating the potential of more functionality on a similarly sized semiconductor.)
However, the difficulty of continuing to reduce the size of conventional silicon-based transistors has caused many to think that the era of scaling is near an end. How can we then make transistors pass signals more quickly? The solution lies in the material of the transistor. Between the source and drain terminals of a transistor is the semiconductor. Although reducing the distance between source and drain increases speed, another way to achieve the same goal is to use a material that conducts electrical signals more rapidly. This aim is why graphene’s mobility is important.
Mobility is a measure of the speed with which electrical signals travel through a material when a voltage is applied. It is easy to see that a material with higher mobility transmits signals faster than one of lower mobility if the dimensions are the same. Compared to other semiconducting materials, silicon actually has a relatively low mobility, but its use is widespread because it has lots of other advantages, such as great mechanical strength and relative ease of manufacture. However, transistors are made from materials that have higher mobility than silicon, such as gallium arsenide (GaAs) and indium phosphide (InP), when they are destined for certain special applications, such as high-frequency wireless transmitters and receivers used in cell phones, or in specialized electronics such as military communications equipment. Graphene transistors may be expected to play a role in these applications because the highest mobility measured for graphene is greater than for the other compounds.
Many scientists and engineers around the world have become excited about the possibility of replacing silicon with graphene to make faster transistors and circuits. Even before graphene’s properties were fully measured or understood, my colleagues and I at IBM, as well as other groups worldwide, believed we should jump right in and try to build transistors and circuits in a way that might lead to technology that could eventually be manufactured just as silicon is used today. The U.S. Defense Advanced Research Projects Agency also believed in this goal and supported this work. Not surprisingly, we encountered many difficulties, but the progress has been very rapid.