  Scalar perturbations during inflation can be substantially amplified by tiny
features in the inflaton potential. A bump-like feature behaves like a local
speed-breaker and lowers the speed of the scalar field, thereby locally
enhancing the scalar power spectrum. A bump-like feature emerges naturally if
the base inflaton potential $V_b(\phi)$ contains a local correction term such
as $V_b(\phi)\left[1+\varepsilon(\phi)\right]$ at $\phi=\phi_0$. The presence
of such a localised correction term at $\phi_0$ leads to a large peak in the
curvature power spectrum and to an enhanced probability of black hole
formation. Remarkably this does not significantly affect the scalar spectral
index $n_{_S}$ and tensor to scalar ratio $r$ on CMB scales. Consequently such
models can produce higher mass primordial black holes ($M_{\rm PBH}\geq 1
M_{\odot}$) in contrast to models with `near inflection-point potentials' in
which generating higher mass black holes severely affects $n_{_S}$ and $r$.
With a suitable choice of the base potential - such as the string theory based
(KKLT) inflation or the $\alpha$-attractor models - the amplification of
primordial scalar power spectrum can be as large as $10^7$ which leads to a
significant contribution of primordial black holes (PBHs) to the dark matter
density today, $f_{\rm PBH} = \Omega_{0,\rm PBH}/\Omega_{0,\rm DM} \sim O(1)$.
Interestingly, our results remain valid if the bump is replaced by a dip. In
this case the base inflaton potential $V_b(\phi)$ contains a negative local
correction term such as $V_b(\phi)\left[1-\varepsilon(\phi)\right]$ at
$\phi=\phi_0$ which leads to an enhanced probability of PBH formation. We
conclude that primordial black holes in the mass range $10^{-17} M_{\odot} \leq
M_{\rm PBH} \leq 100\, M_{\odot}$ can easily form in single field inflation in
the presence of small bump-like and dip-like features in the inflaton
potential.
