(Invited) 2D Non-Volatile RF Switches
Authors: Kim Myungsoo , Akinwande Deji 
Published: February 2020

Radiofrequency (RF) switches are significant components in wireless telecommunication systems. Micro-electro-mechanical system (MEMS) based switches have been widely discussed to replace the conventional solid-state switches that consume static and dynamic energy from their operations. MEMS switches use a mechanical movement, stimulated by a voltage/current, to manipulate an ohmic or capacitive contact. Although MEMS switches have higher linearity and lower power consumption compared to solid-state counterparts, they have some issues such as contact interface degradation, complex fabrication and packaging processes, and large switching voltages (~10-100 V). Therefore, non-volatile switches are preferred because they need no DC voltage and consume zero-static power. Non-volatile resistance switching (NVRS) devices such as resistance-change or phase-change (PC) material based switches have been discussed for RF switch applications. PC materials show a low resistance at the crystalline state and a high resistance at the amorphous state. Compared to MEMS switches, PC switches do not have hermetic packaging requirements and show easier integration with CMOS technologies. However, the phase transition is achieved through Joule heating and thus, the switches need an integrated heater and have slow switching times due to heat transport. Lately, Pi et al. showed memristive RF switch using a pair of electrochemically asymmetric metal electrodes. While memristive switches have power efficiency advantages and smaller physical size than other counterparts, it is necessary to engineer the location and geometry of the metallic filaments for practical applications.

Lately, Non-volatile resistance switching (NVRS) phenomenon was first observed in a variety of monolayer semiconducting TMDs sandwiched between metal electrodes. NVRS represents the resistance modulation between a high-resistance state (HRS, ROFF
) and a low-resistance state (LRS, RON
) and then preserve the state without power consumption. Especially, these memristor effects in atomically thin nanomaterials or atomic sheets are labeled as atomristor. Although other solution-processed multilayer two-dimensional (2D) materials have shown NVRS effect, it was believed that monolayer materials can't have an NVRS effect due to its leakage current. However, Sangwan et al. discovered NVRS effects in monolayer MoS2 lateral device using the resistance modulation of its grain boundaries. Nevertheless, for the practical applications, vertical MIM structure is advantageous for smaller device dimension and denser integration. Toward this end, here, we report zero-static power non-volatile RF switches based on MoS2 atomristor. RF circuits for wide-band system require an RF switch having low ON-state resistance and low OFF-state capacitance to achieve a high figure of merits (FOM) cutoff frequency (fc
=1/2πRONCOFF
). Our MoS2 RF switch can be programmed with a voltage as low as 0.7 V and has an ON/OFF conductance ratio up to 104. Interestingly, we found that our devices show the one-dimensional area-invariant RON
(<10 Ω), and two-dimensional area-dependent COFF
, (~20 fF/μm2), yields a higher fc
by reducing device area, a defining advantage over PCM switches. We demonstrate the RF performance of the device up to 50 GHz and report 0.3 dB insertion loss, 20 dB isolation (both at 50 GHz), 10 THz cutoff frequency and power-handling capability. Although various RF switches based on MEMS and PC materials have been broadly reported, our non-volatile low-power RF switches based on MoS2 atomristor present a better solution in respect to the described RF performance, non-volatility, and area-dependency.