  Arsenic doping in silicides has been much less studied compared with
phosphorus. In this study, superconductivity is successfully induced by As
doping in Mo$_5$Si$_3$. The superconducting transition temperature ($T_c$)
reaches 7.7 K, which is higher than those in previously known W$_5$Si$_3$-type
superconductors. Mo$_5$Si$_2$As is a type-II BCS superconductor with upper and
lower critical fields of 6.65 T and 22.4 mT, respectively. In addition, As
atoms are found to selectively take the 8$h$ sites in Mo$_5$Si$_2$As. The
emergence of superconductivity is possibly due to the shift of Fermi level as a
consequence of As doping, as revealed by the specific heat measurements and
first-principles calculations. Our work provides not only another example of As
doping, but also a practical strategy to achieve superconductivity in silicides
through Fermi level engineering.
