US team finds MoS2 promising for extreme temperature electronics
A research team from the University of California, Riverside and Rensselaer Polytechnic Institute have discovered that the semiconductor MoS2 may be a promising candidate to make thin-film transistors for extreme temperature applications.
MoS2 has attracted a lot of interest for device applications, but this is the first effort to investigate the material's potential for high-temperature electronics.
In a paper published this week in the Journal of Applied Physics, the researchers report the fabrication of MoS2 thin-film transistors and their functional performance at high temperatures, demonstrating the material's potential for extreme-temperature electronics.
"Our study shows that MoS2 thin-film transistors remain functional to high temperatures of at least 500K", said Alexander Balandin, the team leader and a professor at the Department of Electrical and Computer Engineering at the UC-Riverside. "The transistors also demonstrate stable operation after two months of aging, which suggests new applications for MoS2 thin-film transistors in extreme-temperature electronics and sensors."
Many industries are calling for electronics that can operate reliably in a harsh environment, including extreme temperatures above 200degC. Examples of the high temperature applications include turbine engine control in aerospace and electronics or sensors used for drilling operation in oil and gas industry.
"Although devices made of conventional large-band-gap-semiconductors, such as SiC or GaN, hold promise for extended high-temperature operation, they are still not cost-effective for high volume applications," Balandin said. "A single-layer MoS2 shows a band gap of 1.9 eV, which is larger than that of silicon and GaAs. This is beneficial for the proposed application." The presence of a larger band gap means that a device can be easily switched on and off, a crucial property for transistor's operation.
Using standard lithography techniques in a clean room environment, Balandin's team built MoS2 transistors on silicon substrates for high-temperature experiments. Some had just a few layers (1-3) and others had more, multiple-layers (15-18). The relatively thick films were more thermally stable and demonstrated a higher mobility at elevated temperatures, according to Balandin.
By conducting direct current measurement, a technique applying constant voltage or current through the device for a relatively long time, researchers studied the current-voltage characteristics or functional performance of the fabricated transistor at temperatures from 300 to 500K. They found that the device performed differently but remained functional as the temperature increased.
"Both mobility and threshold voltage decrease with temperature," Balandin said. "Decreasing mobility results in current decrease through the device channel, while decreasing threshold voltage leads to current increase. Therefore, the exact behaviour of current with increasing temperature would depend on the interplay of decreasing mobility and threshold voltage."
Another intriguing feature researchers observed is a characteristic "kink" on the current-voltage graph at the zero voltage for temperatures higher than 450 Kelvin. This 'memory effect' is similar to one observed in graphene transistors and electron glasses and suggests the material's potential for use in high-temperature sensors.
According to Balandin, practical application of MoS2 transistors in control circuits or sensors at high temperatures requires operation longer than one month. As the team studied after two months, the aged devices demonstrated a stable operation, and were characterized by a higher threshold voltage, lower mobility and weaker temperature dependence of the mobility.
The researchers' next step is to study the high-temperature function of MoS2 transistors and circuits, fabricated by industrial methods such as chemical vapour deposition.
'High-Temperature Performance of MoS2 Thin-Film Transistors: Direct Current and Pulse Current-Voltage Characteristics' by Chenglong Jiang et al appears in Journal of Applied Physics, 117, 064301 (2015).