  In this work, we investigate the physical and electronic properties of
$\beta_{12}$-borophene FET-based gas sensor using a theoretical quantum
capacitance model based on tight-binding approach. We study the impact of
adsorbed NH$_3$, NO, NO$_2$ and CO gas molecule on its density of states,
carrier concentration, quantum capacitance and I-V characteristics. We found a
remarkable variation in the energy band structure and the density of states
(DOS) of the $\beta_{12}$-borophene in the presence of the adsorbed gas
molecule. The appearance of non-identical Van-Hove singularities in the DOS in
the presence of adsorbed gas molecules strongly indicates the high sensitivity
of $\beta_{12}$-borophene. We found a significant increase in the carrier
concentration for NH$_3$ gas while it decreases for all other gases. Moreover,
a drastic change in quantum capacitance and current-voltage relation is also
observed in adsorbed gases. The different properties of the given gas molecules
are compared with the pristine borophene and found to exhibit distinct wrinkles
in each case, thereby indicating the strong selectivity of our proposed gas
sensor. Though $\beta_{12}$ - borophene is found to be highly sensitive for all
studied gases, the NO gas is found to be most sensitive compared to the others.
