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-MoS2 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-MoS2 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.