Two Dimensional Semiconductors for Transistors

A new type of transistor has been built from a novel material that can be as thin as one or two atomic layers, making it an essentially two-dimensional device. Because of this extreme thinness, it dissipates less heat and requires less charge to operate than any conventional semiconductor device

Allotropic Elements

It’s well known that both diamonds and the graphite that makes up the common pencil are both composed of the element carbon. The difference between these two examples lies in the way in which carbon atoms link up with one another. These contrasting molecular organizations are called allotropes.

The world is rapidly becoming familiar with graphene, an allotrope of carbon that can exist in layers of only one atom’s depth, and is consequently called a 2-D (two dimensional) material. Graphene is renowned as being the strongest material known to science, and is also known for its astounding ability in conducting electricity.

What is Black Phosphorus?

Black arsenic phosphorus, or phosphorene, is another material that can be structured into sheets only one atomic layer thick. But the difference here is that by doping it with arsenic, it can be made to function as a semiconductor.

The doping has the effect of altering the band gap, which is the energy it takes for electrons to jump from what is known as the valence band to the conduction band. When electrons make this move, the material switches from being an insulator to a conductor of electricity.

Phosphorene in Transistors

This is the essence of a semiconductor. The band gap can be adjusted by varying the concentration of arsenic in the material. A new technique for creating this material was developed by Marianne Koepf at the Technical University of Munich and at the University of Regensburg. This technique allows for the building of phosphorene without the expensive, difficult requirement for high atmospheric pressure. Professor Zhou and Dr. Liu, faculty members at the University of Southern California’s electrical engineering department, accomplished the next step, which was to build an actual field effect transistor out of the new material. 

Phosphorene has the added property of being mechanically flexible, which is a much-desired property in semiconductor devices for the new generation of wearable devices now being developed. By varying the band gap, phosphorene will also be very useful in the development of electronic sensors.

Limits on 2D Semiconductors

While graphene has been proven to be very delicate in its thinnest embodiment, when a more robust version that is a few atomic layers thick is built, electrical charge can be devised so as to travel in only one atomic layer. Thus, while only a bit of space is sacrificed, no additional current is needed, because, electrically speaking, it is still only one atomic layer thick.

This novel semiconductor is still only a laboratory device, and a myriad of manufacturing challenges will need to be surmounted before a commercial device can be manufactured. However, it does seem that a limit to how small the transistor can shrink is now being approached, because nothing can be thinner than one layer of atoms.


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