  A remarkable property of intrinsic graphene is that upon doping, electrons
and holes travel through the monolayer thick material with constant velocity
which does not depend on energy up to about $0.3$ eV (Dirac fermions), as
though the electrons and holes are massless particles and antiparticles which
move at the Fermi velocity $v_F$. Consequently, there is Klein tunneling at a
$p-n$ junction, in which there is no backscattering at normal incidence of
massless Dirac fermions. However, this process yielding perfect transmission at
normal incidence is expected to be affected when the group velocity of the
charge carriers is energy dependent and there is non-zero effective mass for
the target particle. We investigate how away from normal incidence the combined
effect of incident electron energy $\epsilon$ and band gap parameter $\Delta$
can determine whether a $p-n$ junction would allow focusing of an electron beam
by behaving like a Veselago lens with negative refractive index. We demonstrate
that there is a specific region in $\epsilon-\Delta$ space where the index of
refraction is negative, i.e., where monolayer graphene behaves as a
metamaterial. Outside this region, the refractive index may be positive or
there may be no refraction at all. We compute the ballistic conductance across
a $p-n$ junction as a function of $\Delta$ and $\epsilon$ and compare our
results with those for a single electrostatic potential barrier and multiple
barriers.
