Thickness Considerations of Two-Dimensional Layered Semiconductors for Transistor Applications

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, I(on)/I(off). However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given I(on)/I(off) 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-MoS(2) films. A difference in I(on)/I(off) by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in I(on)/I(off) 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, I(on)/I(off) approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of I(on)/I(off) 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 I(on)/I(off). Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.