  The replacement of the existing endcap calorimeter in the Compact Muon
Solenoid (CMS) detector for the high-luminosity LHC (HL-LHC), scheduled for
2027, will be a high granularity calorimeter. It will provide detailed
position, energy, and timing information on electromagnetic and hadronic
showers in the immense pileup of the HL-LHC. The High Granularity Calorimeter
(HGCAL) will use 120-, 200-, and 300-$\mu\textrm{m}$ thick silicon (Si) pad
sensors as the main active material and will sustain 1-MeV neutron equivalent
fluences up to about $10^{16}~\textrm{n}_\textrm{eq}\textrm{cm}^{-2}$. In order
to address the performance degradation of the Si detectors caused by the
intense radiation environment, irradiation campaigns of test diode samples from
8-inch and 6-inch wafers were performed in two reactors. Characterization of
the electrical and charge collection properties after irradiation involved both
bulk polarities for the three sensor thicknesses. Since the Si sensors will be
operated at -30 $^\circ$C to reduce increasing bulk leakage current with
fluence, the charge collection investigation of 30 irradiated samples was
carried out with the infrared-TCT setup at -30 $^\circ$C. TCAD simulation
results at the lower fluences are in close agreement with the experimental
results and provide predictions of sensor performance for the lower fluence
regions not covered by the experimental study. All investigated sensors display
60$\%$ or higher charge collection efficiency at their respective highest
lifetime fluences when operated at 800 V, and display above 90$\%$ at the
lowest fluence, at 600 V. The collected charge close to the fluence of
$10^{16}~\textrm{n}_\textrm{eq}\textrm{cm}^{-2}$ exceeds 1 fC at voltages
beyond 800 V.
