Exploring the application of Janus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mrow> <mml:mi>Mo</mml:mi> <mml:mi>Si</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mrow> <m

The $M{A}_{2}{Z}_{4}$ family of materials exhibits excellent thermal and mechanical stability, unique electronic properties, and ultrahigh carrier mobility, rendering them highly promising for low-dimensional nanodevice applications. Using atomic substitution modeling, we computationally investigate the structural configurations of ${\ensuremath{\alpha}}_{1}$- and ${\ensuremath{\alpha}}_{2}$-phase in three monolayers: ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{2}{\mathrm{P}}_{2}$, $\mathrm{Mo}(\mathrm{Si}\mathrm{NP}{)}_{2}\text{\ensuremath{-}}\mathrm{A}$, and $\mathrm{Mo}(\mathrm{Si}\mathrm{NP}{)}_{2}$-$\mathrm{B}$. Our first-principles analysis reveals that these monolayers possess robust structural and mechanical stability alongside high carrier mobility, making them ideal candidates for next-generation nanoelectronics. Specifically, the simulations of pn-junction diodes based on Janus ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{2}{\mathrm{P}}_{2}$ monolayers demonstrate excellent rectification effects, while ${\ensuremath{\alpha}}_{1}$-phase ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{N}}_{2}{\mathrm{P}}_{2}$ pn-junction diodes exhibit negative differential resistance at finite bias. Furthermore, the pin-junction field-effect transistors show distinct behavior under positive and negative gate voltages, and the associated phototransistors display robust photovoltaic responses in the violet and ultraviolet regions, underscoring their significant optoelectronic potential. These results demonstrate the potential of Janus $M{A}_{2}{Z}_{4}$-based monolayers for future high-performance electronic and optoelectronic applications.