Researchers from the University of California, Riverside and the Rensselaer Polytechnic Institute have collaborated in a study to discover that molybdenum disulfide (MoS2) material may hold promise for producing thin-film transistors for applications in extreme temperature environments.
The team has reported the method to manufacture molybdenum disulfide thin-film transistors and their performance at high temperatures to demonstrate the potential of the material for extreme-temperature electronics.
Electronics that can function reliably at extreme temperatures of above 200°C and harsh environments is desired by many industries. Typical applications include control of turbine engines in aerospace, and sensors and electronics used in oil and gas drilling operations. Conventional cooling systems may help certain electronics to work at high temperatures.
However, in certain cases cooling may not be possible, and in other cases it would be better if the electronics could function at hot conditions, which would help reduce cost or enhance the reliability of the system. Currently, very limited transistors and circuits are available that can function at such high temperatures.
Our study shows that molybdenum disulfide thin-film transistors remain functional to high temperatures of at least 500 Kelvin [220 Celsius].
The transistors also demonstrate stable operation after two months of aging, which suggests new applications for molybdenum disulfide thin-film transistors in extreme-temperature electronics and sensors.
Alexander Balandin - Team Leader / Professor
Department of Electrical and Computer Engineering at UC-Riverside
Molybdenite is a naturally occurring material that is abundantly available. It is a mineral of molybdenum disulfide and is used in lubricants as an additive. Molybdenum disulfide can be synthesized using chemical vapor deposition, and this material has been found to hold promise for producing flexible, thin-film transistors. These transistors have the ability to control electric current and electron movement, in the same way as a water faucet functions.
Balandin states that molybdenum disulfide is a member of the van der Waals family of materials that have a specific layered crystal structure where the atomic layers are weakly bonded to each other. This weak bonding is referred to as van der Waals interactions; the weak connection that exists between the atomic sheets allows exfoliation in a layer by layer manner.
Graphene is obtained by peeling thin sheets off graphite chunks and the exfoliation that is allowed is similar to this process. When chemical vapor deposition is used on an industrial scale very thin layers of high-quality could possibly be produced.
Although devices made of conventional large-band-gap-semiconductors, such as silicon carbide or gallium nitride, hold promise for extended high-temperature operation, they are still not cost-effective for high volume applications.
A single-layer molybdenum disulfide shows a band gap of 1.9 eV, which is larger than that of silicon and gallium arsenide. This is beneficial for the proposed application.
Molybdenum disulfide is being considered for device applications, however, the first team to study the potential of the material for high-temperature electronics is Balandin's team.
In a clean room environment, Balandin's team used standard lithography techniques to build molybdenum disulfide transistors on substrates made of silicon for high-temperature experiments. Some of the transistors were made up of multiple-layers in the range of 15-18 layers, while other transistors were made up of few-layers in the range of 1-3 layers. Balandin commented that at elevated temperatures, the relatively thick films demonstrated better thermal stability.
The researchers performed a direct current measurement, which involves applying a constant current or voltage for a long duration. The functional performance or the current-voltage characteristics of the fabricated transistor were studied at temperatures ranging from 300K-500K. When the temperature increased, the device remained functional but performed differently.
Both mobility and threshold voltage decrease with temperature. Decreasing mobility results in current decrease through the device channel, while decreasing threshold voltage leads to current increase. Therefore, the exact behavior of current with increasing temperature would depend on the interplay of decreasing mobility and threshold voltage.
A characteristic "kink" was observed on the current-voltage graph for temperatures exceeding 450K at 0V(zero voltage). This was an interesting feature and the memory effect was similar to that observed in electron glasses and graphene transistors. Further, this effect suggests the possibility of using this material for high-temperature sensors.
Balandin states that operations exceeding a month are required for practical application of molybdenum disulfide transistors in sensors or control circuits at high temperatures. When the team studied the transistors after two months they found that the devices operated in a stable manner. They were characterized by weaker temperature dependence, lower mobility and a higher threshold voltage.
The team further intends to study how molybdenum disulfide transistors and circuits that are produced using industrial methods function at high-temperature.
The paper titled "High-Temperature Performance of MoS2 Thin-Film Transistors: Direct Current and Pulse Current-Voltage Characteristics" has been published in the Journal of Applied Physics, from AIP Publishing.