Sustained Sub-60 mV/decade switching via the negative capacitance effect in MoS₂ transistors

It has been shown that a ferroelectric material integrated into the gate stack of a transistor can create an effective negative capacitance (NC) that allows the device to overcome "Boltzmann tyranny". While this switching below the thermal limit has been observed with Si-based NC field-effect transistors (NC-FETs), the adaptation to 2D materials would enable a device that is scalable in operating voltage as well as size. In this work, we demonstrate sustained sub-60 mV/dec switching, with a minimum subthreshold swing (SS) of 6.07 mV/dec (average of 8.03 mV/dec over 4 orders of magnitude in drain current), by incorporating hafnium zirconium oxide (HfZrO₂ or HZO) ferroelectric into the gate stack of a MoS₂ 2D-FET. By first fabricating and characterizing metal-ferroelectric-metal capacitors, the MoS₂ is able to be transferred directly on top and characterized with both a standard and a negative capacitance gate stack. The 2D NC-FET exhibited marked enhancement in low-voltage switching behavior compared to the 2D-FET on the same MoS₂ channel, reducing the SS by 2 orders of magnitude. A maximum internal voltage gain of 28× was realized with 12 nm thick HZO. Several unique dependencies were observed, including threshold voltage (V_th) shifts in the 2D NC-FET (compared to the 2D-FET) that correlate with source/drain overlap capacitance and changes in HZO (ferroelectric) and HfO₂ (dielectric) thicknesses. Remarkable sub-60 mV/dec switching was obtained from 2D NC-FETs of various sizes and gate stack thicknesses, demonstrating great potential for enabling size- and voltage-scalable transistors.