  With exceptional electrical and mechanical properties and at the same time
air-stability, layered MoSi2N4 has recently draw great attention. However, band
structure engineering via strain and electric field, which is vital for
practical applications, has not yet been explored. In this work, we show that
the biaxial strain and external electric field are effective ways for the band
gap engineering of bilayer MoSi$_2$N$_4$ and WSi$_2$N$_4$. It is found that
strain can lead to indirect band gap to direct band gap transition. On the
other hand, electric field can result in semiconductor to metal transition. Our
study provides insights into the band structure engineering of bilayer
MoSi$_2$N$_4$ and WSi$_2$N$_4$ and would pave the way for its future
nanoelectronics and optoelectronics applications.
