15-12-2020 | | By Robin Mitchell
Recently, researchers have demonstrated atom-thin molybdenum disulphide transistors that use half the voltage of typical transistors. What is molybdenum disulphide, what have the researchers achieved, and why is reducing the working voltage of a transistor important?
Molybdenum disulphide (often shortened to Moly) is a compound that consists of molybdenum and sulphur with the chemical formula MoS2. The compound appears as a silvery black solid that is found naturally, and when mined, is processed to remove impurities such as carbon. While the applications for Moly generally include lubricants and as a catalyst, its electrical properties change when used in a monolayer and becomes a direct bandgap semiconductor.
As a result, scientists have experimented with Moly to create transistors that have features only 1nm in size, and a thickness of just three atoms. What makes such nano-scale materials of interest is that when electrons travel through monolayers (such as graphene and Moly), the method in which electrons move from one side to the other is through ballistic collision. When this happens, electrons do not scatter amongst the material, and since this scattering is the cause of resistivity, such materials exhibit a fixed resistance regardless of length.
The work on Moly transistors has shown some interesting results, with a team of researchers who were able to develop a 1-bit microprocessor slice using just 117 moly transistors in 2017, as well as the development of 2D memristors and memtransistors.
Recently, researchers from the University of Buffalo have been able to create a 2D transistor utilising Moly as the semiconductor which is only one atom thick. While the structure of the transistor itself is much larger than one atom, the moly layer is a pure monolayer. The new transistor utilises a single atom layer of graphene stacked on top of a single layer of moly which creates a Field Effect Transistor.
Unlike typical transistors which require around 60mV to create a decade change in current conduction, the new transistor developed by the research team achieves the same effect with a voltage of just 29mV. The ability to switch at such a low voltage comes from the properties of graphene which confine the electrons to a single layer as well as the Dirac-source injection effect when electrons transfer from the graphene to the moly layer.
The new transistor developed by the research team not only allows for a lower operating voltage, but the use of graphene allows for a greater current density. The ability to operate at lower voltages provides a design with two advantages; greater frequency of operation and increased energy efficiency.
Lower voltage transistors generally allow for higher-frequency of operation as the time needed for signals to change between a logical 0 and a logical 1 (i.e. the slew rate), is smaller. However, the reduction in operating voltage makes such transistors vulnerable to noise as the differentiation between a 0 and 1 is smaller (therefore making it easier for noise to interfere).
By lowering the voltage, a transistor also reduces thermal dissipation, and the use of graphene in the moly transistor also further reduces the generation of heat. Therefore, more transistors can be integrated into a single chip while retaining the same heat dissipation methods previously used on older designs. At the same time, reducing the overall thermal generation of a transistor increases its energy efficiency as less energy is wasted into generating heat.
While the transistors demonstrated by the research team show how graphene and moly can be used in place of standard silicon devices, it is not clear if these will become the dominant technology of future devices. As the size of silicon transistors continues to decrease, it becomes exponentially harder to continue the shrinking trend due to silicon transistors approaching the atomic scale. However, the use of monolayers such as graphene will most likely play a key role thanks to their quantum behaviour of fixed resistance regardless of size, ability to keep electrons “cold”, and their superior conductive qualities.