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Low-power gas sensing with organic transistors
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   Low-power gas sensing with organic transistors


Solution processed organic field effect transistors (OFET) exhibit attractive features such as mechanical flexibility, low temperature processing, printability and compatibility with arbitrary substrates. OFET are often considered a promising platform technology for developing ubiquitous sensor applications in digital health, environment monitoring and Internet of Things (IoT). However, achieving low voltage operation for low power and easy electrical integration has been challenging because it was often at the expense of stable device performance. Decreasing the operating voltage is seen as a requirement for the technology to become commercially viable.

 Recent progress by researchers from Shanghai Jiao Tong University in China shows low power OFET can exhibit excellent stability. First, they fabricated OFET using solution processing, which makes it compatible with printing techniques. Then they measured the electrical properties of the transistors after either being placed for weeks or continuously prolonged bias stressing for hours in ambient air.

The researchers observed that electrical properties remain stable even when the bottom gate transistor was not encapsulated and the channel was being used as a gas sensor. The results have been published in the journal Scientific Reports (Nature Publishing Group).



In the study, low voltage operation was achieved by reducing the sub-gap density of states (DOS) at the channel and using a proper low-k non-polar polymer dielectric layer. The fabricated OFET exhibits a steep subthreshold swing less than 100 mV/decade, enabling an ON/OFF ratio of 10^6 with a voltage swing of 3 V.
The combined low voltage and stable operation of the OFET enabled integration into a 2.4 V battery powered sensor tag which continuously sensed ammonia (NH3) vapour in ambient air for 12 hours. Power consumption of the sensor tag was only 50 nW.
"Printing organic transistors on plastic or paper will provide a low cost solution for a wide range of emerging large area and flexible sensing applications, and enable more friendly interfaces with the physical world for people," said Dr Xiaojun Guo, co-author of the paper. "However, the constraints in both materials and processes cause printed transistors to have large operation voltage and poor power efficiency, which becomes a significant performance hurdle for the technology to be commercially viable. The developed device technology based on available materials is able to address the material and process issues for making fully printed low voltage and power efficient organic transistors towards a commercially competitive technology platform for the envisioned ubiquitous sensing applications."
According to Dr Guo, this is the first demonstration of an OFET being operated in a battery powered low voltage electronic system for long term and reliable vapour sensing. The design of the device addresses the power and stability issues associated with printable OFET.
Link to paper (open access): www.nature.com/articles/srep20671






Copyright: 2013 Department of Electronic Engineering Shanghai Jiao Tong University       Technical support: Shanghai EChao