(Invited) Progress in Flexible 2D Nanoelectronics

Over a decade, two-dimensional (2D) van der Waals crystal materials have been intensively studied in the electronic device community for their attractive physics and various electronic applications such as the Internet-of-Things (IoTs) and wearable electronics. Particularly, their robust mechanical properties with the intrinsic strain limit of ~20% and wide range of saturation velocity of ~108 cm/s facilitate the future flexible nanoelectronics. Graphene, the first pioneering 2D material, is believed to be promising for high frequency flexible applications due to its high mobility and flexibility. However, its semi-metallic band structure limits graphene in digital applications that require the evident current On/Off ratio. On the other hand, the Molybdenum disulfide (MoS2), which has sizable bandgap of 1.3 ~1.9 eV, has been demonstrated for high current On/Off ratio over 108, which indicates its promising digital/logic systems wth low power operation. However, its high frequency digital applications are limited by its low mobility and n-type unipolar transport. The black phosphorus has been intensively studied due to its sizable bandgap ranging from 0.3 to 2 eV and high carrier mobility reaching to 1000 cm2/Vs, which can facilitate the development of high frequency digital applications. In this work, we demonstrate the high frequency operation of flexible 2D materials-based transistors. Graphene transistors on flexible willow glass substrate shows outstanding 95 GHz intrinsic cut-off frequency, and its calculated saturation velocity is ~8.4 ×106 cm/s, which is the highest for any flexible transistor on any material system. RF measurement of MoS2 transistors with the 500 nm channel length on flexible polyimide substrate shows 5.6 GHz intrinsic cut-off frequency and 3.3 GHz power gain. Multilayered BP field-effect transistor with 500 nm channel length shows 17.5 GHz and 14.5 GHz for intrinsic cut-off frequency and power gain, respectively. All of these studies for 2D materials-based transistors on flexible substrates clearly represents the state-of-the-art progress of the GHz operation of flexible transistors as well as wide GHz range of frequency operations, which pave the path toward wireless communication flexible applications and IoTs. In addition, paper substrates have been investigated for future electronics due to its cost-effective, eco-friendly and flexible features. With polyimide coating layer on commercially available paper substrates, we could make wet-processable paper substrate with low surface roughness for short channel length fabrication. We represent CVD graphene and MoS2 transistors on paper substrate, which shows the first GHz FETs operation on paper substrates.