  The High-Granularity Timing Detector is a detector proposed for the ATLAS
Phase II upgrade. The detector, based on the Low-Gain Avalanche Detector (LGAD)
technology will cover the pseudo-rapidity region of $2.4<|\eta|<4.0$ with two
end caps on each side and a total area of 6.4 $m^2$. The timing performance can
be improved by implanting an internal gain layer that can produce signal with a
fast rising edge, which improve significantly the signal-to-noise ratio. The
required average timing resolution per track for a minimum-ionising particle is
30 ps at the start and 50 ps at the end of the HL-LHC operation. This is
achieved with several layers of LGAD. The innermost region of the detector
would accumulate a 1 MeV-neutron equivalent fluence up to $2.5 \times 10^{15}
cm^{-2}$ before being replaced during the scheduled shutdowns. The addition of
this new detector is expected to play an important role in the mitigation of
high pile-up at the HL-LHC. The layout and performance of the various versions
of LGAD prototypes produced by Hamamatsu (HPK) have been studied by the ATLAS
Collaboration. The breakdown voltages, depletion voltages, inter-pad gaps,
collected charge as well as the time resolution have been measured and the
production yield of large size sensors has been evaluated.
