Engineering researchers at the University of California Santa Barbara and Rice University performed the work and a paper written by the team has been published recently in Nature.
The steepness of a transistor’s turn-on is characterized by a parameter known as the subthreshold swing, which cannot be lowered below a certain level in MOSFETs," said Kaustav Banerjee, Professor of Electrical and Computer Engineering at UC Santa Barbara. For MOSFETs that steepness is 60mV/decade and is an effective limit on the energy efficiency of digital circuits in general.
To get round these problems in silicon the team built a transistor with germanium as the source and MoS2 as the channel that is capable of band-to-band tunneling. This tunnel field effect transistor (TFET) has sub-threshold swing of less than 60mV/decade.
The source to channel is designed to filter out high-energy electrons that could otherwise make it passed the source/channel energy barrier even in the off-state. This keeps the off-state current negligible the research team said in a statement.
At UCSB, Banerjee’s Nanoelectronics Research Lab includes Deblina Sarkar, Xuejun Xie, Wei Liu, Wei Cao, Jiahao Kang, and Stephan Kraemer, as well as Yongji Gong and Pulickel Ajayan of Rice University.
The approach is superior to TFETs built with silicon or III-V compound semiconductors as the channel materials because these materials have a high density of surface states that increase leakage current. The paper reports a subthermionic sub-threshold swing of approximately 30 millivolts/decade at room temperature over four decades of drain current and at operating at 0.1V.
"The use of 2D materials in tunneling transistors started only recently, and this paper gives the whole field yet another strong boost in improving the characteristics of such devices even further," commented Konstantin Novoselov, a professor of physics at the University of Manchester who was a co-recipient of the 2010 Nobel Prize in Physics, awarded for the discovery of graphene.
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