Thickness Considerations of Two-Dimensional Layered Semiconductors for Transistor

Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, Ion/Ioff . However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given Ion/Ioff as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS2 films. A difference in Ion/Ioff by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, Ion/Ioff approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of Ion/Ioff with t. This excellent agreement confirms that multilayer-MoS 2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff . Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors. The first successful demonstration of field-effect transistors (FETs) based on monolayer molybdenum disulfide (MoS2) with appealing performance 1,2 has stimulated intensive research on two-dimensional (2D) transition metal dichalcogenides (TMDs). The planar nature of these 2D semiconductor materials could potentially lead complementary metal-oxide-semiconductor (CMOS) technology to the ultimate size scaling envisioned by Moore’s law and beyond 3–5 . MoS2, a representative layered TMD, has a satisfactory bandgap in the range of 1.3 to 1.8 eV 6,7 , which is advantageous over the well-studied gapless graphene with respect to the standby leakage current of its FETs 8 . The bandgap of MoS 2 is thickness-dependent and it is 1.8 eV for monolayers. As a result, transistors of both single- and multilayer-MoS2 films have an exhibited high ratio of on-state to off-state current (Ion/Ioff > 10 6 ) with reasonable electron mobility 1,9–11 . All this makes the layered TMDs promising in fields of low-power switches/circuits 11,12 , nonvolatile memory devices 13,14 , ultrasensitive photodetectors 15,16 , etc.