  We present first-principles calculations of electronic structures of a class
of two-dimensional (2D) honeycomb structures of group-V binary compounds. Our
results show these new 2D materials are stable semiconductors with direct or
indirect band gaps. The band gap can be tuned by applying lattice strain.
During their stretchable regime, they all exhibit metal-indirect gap
semiconductor-direct gap semiconductor-topological insulator (TI) transitions
with increasing strain from negative (compressive) to positive (tensile)
values. The topological phase transition results from the band inversion at
$\Gamma$ point due to lattice strain and is irrelevant to spin-orbit coupling
(SOC).
