[en] Exploring new 2D monolayer materials with intrinsic novel properties is of great significance from both fundamental and practical points of view. Using first-principles calculations based on density functional theory, we propose three stable hexagonal SiAs monolayers with the excellent thermal and dynamic stabilities, whose crystal structures are similar to the monolayer GaSe, named $\mathrm{\alpha <!--\; ??\; -->}$-, $\mathrm{\beta <!--\; ??\; -->}$- and $\mathrm{\gamma <!--\; ??\; -->}$-SiAs, respectively. Our results show that $\mathrm{\alpha <!--\; ??\; -->}$- and $\mathrm{\beta <!--\; ??\; -->}$-SiAs monolayers are wide-band-gap semiconductors with indirect band gaps, which have great potential applications in photoelectronic devices, especially used for catalytic splitting water. Further, the effective masses of electron transport in monolayers $\mathrm{\alpha <!--\; ??\; -->}$- and $\mathrm{\beta <!--\; ??\; -->}$-SiAs exhibit anisotropic characteristics. Intriguingly, $\mathrm{\gamma <!--\; ??\; -->}$-SiAs is a direct band gap semiconductor with a band gap of 1.13eV as bulk silicon, which results in a significant absorption in the visible-light region. More importantly, the effective masses in the $\mathrm{\gamma <!--\; ??\; -->}$-SiAs monolayer are only a little larger than 0.20 m₀, indicating that it may have high carrier mobility. Our findings provide three promising candidates for low-dimensional nanodevices with excellent performances in the future.