  The Auger Muons and Infill for the Ground Array (AMIGA) aims to both extend
the detection range of the Pierre Auger Observatory down to energies $\sim
10^{16.5}~\mathrm{eV}$ and to measure the muon content of extensive air
showers. To accomplish these goals, its detection system is composed of an
array of coupled water-Cherenkov and scintillation detectors deployed in a
graded triangular grid of 433 and 750\,m spacings. At each position, the
scintillation detector is buried $2.3~\mathrm{m}$ deep so as to shield it from
the air shower electromagnetic component and thus only measure the muon
component. These muon detectors have $30~\mathrm{m^2}$ area split into modules,
each of them highly segmented in 64 plastic-scintillator strips with an
embedded wavelength-shifter optical fiber to transport light to an optical
sensor located at the center of the module. During the engineering array phase
(finished in November 2017) two module areas ($5~\mathrm{m^2}$ and
$10~\mathrm{m^2}$) and two optical sensors (photo-multiplier tubes and
silicon-photomultipliers) were tested. In this work, we present the final
performance of the muon detectors equipped with silicon-photomultipliers which
were thereafter selected as the baseline design for the AMIGA production phase.
Analyses and results are based both on laboratory and field measurements.
