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O10.08 - MoS2 Transistors Operating at Gigahertz Frequencies 
April 10, 2015   11:45am - 12:00pm

The presence of a direct band gap and in an ultrathin form factor has caused a considerable interest in two-dimensional semiconductors from the transition metal dichalcogenides (TMD) family with molybdenum disulphide (MoS2) being the most studied representative of this family of materials. While diverse electronic elements1, integrated circuits2 and optoelectronic devices3 have been demonstrated using ultrathin MoS2 and related materials, very little is known about their performance at high frequencies where commercial devices are expected to function. We fabricated top-gated MoS2 transistors operating in the gigahertz range of frequencies. The presence of a band gap also gives rise to current saturation,4 allowing voltage gain higher than 1. The MoS2 FETs are fabricated from exfoliated MoS25. We have fabricated RF transistors based on MoS2 layers with different thickness. Electrical contacts were patterned using electron-beam lithography and by depositing Au electrodes. Atomic layer deposition (ALD) was used to deposit HfO2 as a gate dielectric. All our devices presented transconductance typical of n-type materials with on-state current reaching ~300 �A/�m for Vds = 2 V and gate voltage Vtg = 10 V in the case of monolayer MoS2. The current gain of the MoS2 FETs decreases with increasing frequency and shows the typical 1/f dependence for different thicknesses of 2D MoS2 crystals. We realized MoS2 FETs showing current saturation. We found the behavior of the cut-off frequency as a function of the number of layers of MoS2 FETs. The cut-off frequency rises with increasing number of layers in the ambient atmosphere. In conclusion, we studied top-gated MoS2 transistors with a 240 nm gate length. Our MoS2 RF-FETs show an intrinsic transconductance higher than 50 �S/�m and a drain-source current saturation with a voltage gain higher than 1. All these features allow the operation of MoS2 transistors in the GHz range of frequencies. Our devices show cut-off frequencies in the GHz range and are able not only to amplify current in this frequency range but also power and voltage, with the maximum operating frequency fmax = 8.2 GHz. 1 Bertolazzi, S., Krasnozhon, D. & Kis, A. ACS Nano 7, 3246-3252, (2013). 2 Radisavljevic, B., Whitwick, M. B. & Kis, A. ACS Nano 5, 9934-9938, (2011). 3 Lopez-Sanchez, O., Lembke, D., Kayci, M., Radenovic, A. & Kis, A. Nat Nano 8, 497-501, (2013). 4 Lembke, D. & Kis, A. ACS Nano 6, 10070-10075, (2012). 5 Benameur, M. M. et al. Nanotechnology 22, 125706, (2011).

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