Abstract: The innermost layers of the CMS tracker are built of pixel detectors arranged in multiple barrel layers and forward disks in each endcap. The original CMS detector was designed for the nominal instantaneous Large Hadron Collider (LHC) luminosity of 1 x 1034 cm-2 s-1. Under the conditions achieved by the LHC with a factor 2 increase in the instantaneous luminosity, the CMS pixel detector would have seen a dynamic inefficiency caused by data losses due to buffer overflows. For this reason, the CMS Collaboration installed a replacement pixel detector in 2017.
The Phase I upgrade of the CMS pixel detector operates at full efficiency with increased detector acceptance and additional redundancy for the tracking while at the same time reducing the material budget. These goals are achieved by using a new readout chip and modified powering and readout schemes, one additional tracking layer both in the barrel and in the disks, and new detector supports including a CO2-based evaporative cooling system, which contribute to the reduction of the material in the tracking volume. This contribution will review the design and technological choices of the Phase I detector with a focus on the challenges and difficulties encountered, as well as the lessons learned for future upgrades. Also the experiences of the first two years of operation are highlighted.