Monolayer Bi ₂ O ₂ Se is a promising post-silicon-era semiconductor candidate because of its simultaneous excellent device performance and high ambient stability.

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal–2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Stimulated by the recent experimental synthesis of a new layered carbon allotrope–graphdiyne film, we provide the first systematic ab initio investigation of the structural and electronic properties of bilayer and trilayer graphdiyne and explore the possibility of tuning the energy gap via a homogeneous perpendicular electric field. Our results show that the most stable bilayer and trilayer graphdiyne both have their hexagonal carbon rings stacked in a Bernal way (AB and ABA style configuration, respectively). Bilayer graphdiyne with the most and the second most stable stacking arrangements have direct bandgaps of 0.35 eV and 0.14 eV, respectively; trilayer graphdiyne with stable stacking styles have bandgaps of 0.18–0.33 eV. The bandgaps of the semiconducting bilayer and trilayer graphdiyne generally decrease with increasing external vertical electric field, irrespective of the stacking style. Therefore, the possibility of tuning the electronic structure and optical absorption of bilayer and trilayer graphdiyne with an external electric field is suggested.

Opening a tunable and sizable band gap in single-layer graphene (SLG) without degrading its structural integrity and carrier mobility is a significant challenge. Using density functional theory calculations, we show that the band gap of SLG can be opened to 0.16 eV (without an electric field) and 0.34 eV (with a strong electric field) when properly sandwiched between two hexagonal boron nitride single layers. The zero-field band gaps are increased by more than 50% when the many-body effects are included. The ab initio quantum transport simulation of a dual-gated field effect transistor (FET) made of such a sandwich structure reveals an electric-field-enhanced transport gap, and the on/off current ratio is increased by a factor of 8.0 compared with that of a pure SLG FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance SLG FETs.

By using first-principles calculations, we predict that a sizable band gap can be opened at the Dirac point of silicene without degrading silicene's electronic properties with n-type doping by Cu, Ag, and Au adsorption, p-type doping by Ir adsorption, and neutral doping by Pt adsorption. A silicene p–i–n tunneling field effect transistor (TFET) model is designed by the adsorption of different transition metal atoms on different regions of silicene. Quantum transport simulations demonstrate that silicene TFETs have an on–off ratio of 10³, a small sub-threshold swing of 77 mV dec⁻¹, and a large on-state current of over 1 mA μm⁻¹ under a supply voltage of about 1.7 V.