What are semiconductors?
A semiconductor is a material (commonly a solid chemical element or compound) with distinct electrical properties: in some cases it will conduct electricity, but not in others. Thus, control of electrical current is enabled. A semiconductor’s conductance varies depending on the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays.
Semiconductors can be found at the heart of modern electronics. Some examples include microprocessor chips and transistors, and virtually any device that is computerized or uses radio waves relies on semiconductors. Nowadays, the most commercially important semiconductor is silicon, although many others are also in use.
What is graphene?
Graphene is a one-atom-thick layer of carbon atoms arranged in a hexagonal lattice. It is the building-block of Graphite (which is used, among others things, in pencil tips), but graphene is a remarkable substance on its own - with a multitude of astonishing properties which repeatedly earn it the title "wonder material".
Graphene is the thinnest material known to man at one atom thick, and also incredibly strong - about 200 times stronger than steel. On top of that, graphene is an excellent conductor of heat and electricity and has interesting light absorption abilities. Graphene has potential to revolutionize many applications, among these are solar cells, batteries, sensors and more.
Graphene as a semiconductor
Semiconductors are defined by their band gap: the energy required to excite an electron stuck in the valence band, where it cannot conduct electricity, to the conduction band, where it can. The band gap needs to be large enough so that there is a clear contrast between a transistor’s on and off states, and so that it can process information without generating errors.
Among graphene's superlative properties is exceptional electrical conductivity. This property makes the material attractive for many applications, but it is problematic for use as a semiconductor. For that, graphene would need a bandgap (which it normally lacks), or in other words to behave not just as a conductor but to also have an insulator mode.
Scientists have found various methods to introduce a bandgap to graphene. By fabricating graphene in specific shapes (like ribbons), by using certain growth methods paired with specific materials, by using graphene's morphological structure (namely wrinkles), by doping the material and more. Other 2D materials can be used instead or together with graphene, that have an inherent bandgap. These materials may prove to be an easier path towards next-gen semiconductor based devices.
The latest graphene semiconductor news:
The European Commission (EC) has announced a €20 million investment in the next generation of electronics and semiconductors. The 2D Experimental Pilot Line (2D-EPL) was officially launched as the first graphene foundry to integrate graphene and layered materials into semiconductor platforms. The new project aims to keep Europe at the forefront of this technological revolution.
Born within the EU-funded project, the Graphene Flagship, the 2D-EPL will cover the entire value chain, from tool producers and chemical and material providers to manufacturing lines. This collaborative project will integrate several Graphene Flagship members to pioneer the fabrication of new prototype electronics, photonic devices and sensors integrating graphene and layered materials.
Scientist from the University of Wisconsin-Madison are working towards making more powerful computers a reality. To that end, they have devised a method to grow tiny ribbons of graphene directly on top of silicon wafers. Graphene ribbons have a special advantage over graphene sheets - they become excellent semiconductors.
“Compared to current technology, this could enable faster, low power devices,” says Vivek Saraswat, a PhD student in materials science and engineering at UW-Madison. “It could help you pack in more transistors onto chips and continue Moore’s law into the future”. The advance could enable graphene-based integrated circuits, with much improved performance over today’s silicon chips.
Researchers from Göttingen and Pasadena (USA) have produced an "atomic scale movie" showing how hydrogen atoms chemically bind to graphene in one of the fastest reactions ever studied. The team found that by adhering hydrogen atoms to graphene, a bandgap can be formed.
The research team bombarded graphene with hydrogen atoms. "The hydrogen atom behaved quite differently than we expected," says Alec Wodtke, head of the Department of Dynamics at Surfaces at the Max Planck Institute (MPI) for Biophysical Chemistry and professor at the Institute of Physical Chemistry at the University of Göttingen. "Instead of immediately flying away, the hydrogen atoms 'stick' briefly to the carbon atoms and then bounce off the surface. They form a transient chemical bond," Wodtke exclaims. Something else also surprised the scientists: The hydrogen atoms have a lot of energy before they hit the graphene, but not much left when they fly away. It seems that hydrogen atoms lose most of their energy on collision, but where it goes remained to be examined.
Researchers at Columbia Engineering, working with colleagues from Princeton and Purdue Universities and Istituto Italiano di Tecnologia, have engineered "artificial graphene" by recreating, for the first time, the electronic structure of graphene in a semiconductor device.
Graphene comes in one atomic arrangement: the positions of the atoms in the graphene lattice are fixed, and so all experiments on graphene must adapt to those constraints. On the other hand, in artificial graphene the lattice can be engineered over a wide range of spacings and configurations, making it convenient for condensed researchers because it will have more versatile properties than the natural material.
Researchers at MIT have developed a technique that uses graphene as a kind of “copy machine”, to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.
As a great deal of money is spent in the semiconductor industry on wafers that serve as the substrates for microelectronics components, which can be turned into transistors, light-emitting diodes etc., this method may help reduce the cost of wafer technology and enable devices made from more exotic, higher-performing semiconductor materials than conventional silicon.