Two-dimensional ballistic transistors for advanced-node integrated circuits

As the feature size of metal-oxide-semiconductor-field-effect-transistors (MOSFETs) in integrated


Jia Li and Xidong Duan *
As the feature size of metal-oxidesemiconductor-field-effect-transistors (MOSFETs) in integrated circuits (ICs) is reduced to sub-10-nm, the traditional channel materials are plagued by enhanced scattering and short-channel effects, leading to the deterioration of switching characteristics and the increase of power consumption.Twodimensional atomic crystals (2DACs) with atom-scale thickness and dangling bond-free surfaces have shown great potential as promising channel materials for extreme device scaling.According to the 2021 International Roadmap for Devices and Systems (IRDS), 2DACs could potentially be a strong competitor for silicon FETs at the sub-1 .0-nmnode [1 ].
To explore the ultimate potential of 2DAC FETs, there are three prerequisites: (i) producing high-quality 2D semiconductors with a small effective mass and a small scale length; (ii) con-structing ultra-thin high-k dielectrics and high-quality gate-stack interfaces to improve the electrostatic gate control; (iii) constructing high-quality semiconductor-metal (MS) ohmic contact to decrease contact resistance ( R C ) [2 ].Many studies have shown that the actual device performance is mainly limited by the Fermi pinning effect (FPE) in non-ideal MS interfaces.Various strategies, including van der Waals integration of 2DACs and metals [3 ,4 ], semi-metal Bi or Sb contacts [5 ,6 ], low energy deposition of 3D metals [7 ], and phase transition [8 ] have been developed to alleviate FPE.Although R C obtained some significant improvement, the overall performance of 2D FETs is sti l l far from the theoretical prediction, lagging far behind that of state-of-the-art silicon MOSFETs.
Recently, writing in Nature , J. Jiang and colleagues have reported the first realization of 2D ballistic transistors with n-type indium selenide (InSe) as channel material (Fig. 1 a) [9 ].To obtain ohmic contact in InSe FETs, the key is introducing a semi-metal yttrium-doping InSe as the buffer layer between the MS interface by a phase-transition process (Fig. 1 b).Notably, the InSe FET has a constant on-state current (1 mA•μm −1 ) over a wide temperature range (300-100 K), providing strong evidence for the achievement of ballistic transistors with ohmic contacts (Fig. 1 c).As-fabricated InSe FETs with a 10-nm channel length and double-gate configuration (2.6-nmthick HfO 2 dielectrics) can effectively suppress short-channel effects and show nearly perfect switching behav iour w ith a low supply voltage of 0.5 V, a record high transconductance 6 mS•μm −1 and room-temperature ballistic ratio in the saturation region of 83% (Fig. 1 c and d).Furthermore, a low R C of 62 •μm was  (a, c and d) Reprinted with permission from [9 ].(b) Reprinted with permission from [12 ].
Natl Sci Rev, 2024, Vol. 11, nwad315 reliably extracted in 10-nm ballistic InSe FETs, leading to a faster speed and much lower energy consumption than the predicted silicon limit.
This work [9 ] for the first time fabricates 2D ballistic transistors with whole performance surpassing those of state-ofthe-art silicon FETs.However, this work relies on the mechanically exfoliated InSe with small sizes, large-scale integration of such devices for ICs needs the production of wafer-scale high-quality InSe single crystals, which require more effort.The stability of semi-metal Y-doping contacts for heat/wet resistance also needs further exploration.Encouragingly, this work, together with several representative works published recently, such as 2D FETs with ultra-high on-state current density (1.72 mA/ μm for bilayer WSe 2 ) [10 ], 2D ICs operating at gigahertz fre-quencies [11 ], strongly demonstrate the superiorit y, feasibilit y and enormous potential of 2DACs in the post-Moore electronics.

Figure 1 .
Figure 1.Two-dimensional ballistic InSe transistors.(a) Transmission electron microscopy image showing a cross-section of an InSe FET with a doublegate structure.(b) Schematic of a semi-metal Y-doping contacted InSe FET.(c) Transfer characteristics comparison of five typical ballistic 2D InSe FETs (coloured dots), 10-nm-node silicon FinFET (Intel, solid black line), and 20-nm gate InGaAs FinFET normalized by state-of-the-art Fin Pitch = 34 nm (IBM, dashed black line).(d) Scaling trends of V DD of the ballistic 2D InSe FETs compared with those of silicon FETs (the silicon data are from IRDS 2022 and Intel).(a, c and d) Reprinted with permission from [9 ].(b) Reprinted with permission from [12 ].