Graphene, a one-atom-thick sheet of carbon atoms, is seen as a potential replacement for silicon in future transistors because it has an exceptional set of properties (high current density, mobility and saturation velocity). However, transistors made of graphene cannot be turned off because graphene has almost no band gap.

Researchers have begun to investigate a new 2D material—molybdenum sulfide (MoS)—which has similar characteristics but offers something graphene doesn’t: a wide energy bandgap, enabling transistors and circuits to be built from it directly. At the upcoming International Electron Devices Meeting (IEDM), an MIT-led team will describe the use of CVD processing to grow uniform, flexible, single-molecular layers of MoS, comprising a layer of Mo atoms sandwiched between two layers of S atoms. They exploited the material’s 1.8 eV bandgap to build MoS transistors and simple digital and analog circuits (a NAND logic gate and a 1-bit ADC converter). The transistors demonstrated record MoS mobility (>190 cm²/Vs), an ultra-high on/off current ratio of 10⁸,  record current density (~20 µA/µm) and saturation, and the first GHz RF performance from MoS. The results show MoS may be suitable for mixed-signal applications and for those which require high performance and mechanical flexibility.

The lattice structure of MoS.

A schematic of the CVD process for growing single-layer MoS.

An optical micrograph of single-layer MoS sheets grown by this process, showing great uniformity and coverage.