“The steepness of a transistor’s turn on is characterised by the subthreshold swing, which cannot be lowered below a certain level in MOSFETs,” said Professor Kaustav Banerjee from UC Santa Barbara. This means a minimum gate voltage change of 60mV at room temperature is required to change the current by a factor of ten in MOSFETs.
Looking to address the problem, Prof Banerjee’s research group used the quantum mechanical phenomenon of band to band tunneling to design a tunnel field effect transistor (TFET) with a subthreshold swing of less than 60mV per decade.
“We restructured the transistor’s source to channel junction to filter out high energy electrons that can diffuse over the source/channel barrier, even in the off state, thereby making the off state current negligibly small,” Prof Banerjee noted.
The TFET designed by the UCSB team uses molybdenum disulphide (MoS2) as the current carrying channel, placed over a highly doped germanium (Ge) source electrode. Using the 2D properties of MoS2 is said to offer an ideal surface and a thickness of only 1.3nm. The resulting vertical heterostructure, says the team, provides a source-channel junction that is strain-free, has a low barrier for current-carrying electrons to tunnel through from Ge to MoS2 through an ultra-thin (~0.34nm) van der Waals gap, and a large tunnelling area.
“We have engineered what is, at present, the thinnest-channel subthermionic transistor ever made,” said Prof Banerjee. Their atomically-thin and layered semiconducting channel tunnel FET (or ATLAS-TFET) is the only planar architecture TFET to achieve subthermionic subthreshold swing (30mV/decade at room temperature) over four decades of drain current and the only one in any architecture to do so at a drain-source voltage of 0.1V.
