CMS outer tracker detector upgrade plans.

With the upgrade of the LHC to the high-luminosity LHC (HL-LHC), also the tracker system of the CMS experiment will have to be upgraded. It is planned to use information from the tracker in the first level trigger of CMS. For this purpose CMS plans to introduce $p_T$ modules which will provide trigger capability. The luminosity increase by a factor of five in the HL-LHC compared to the design luminosity will lead to higher occupancy and radiation levels in the sensors. In order to cope with these, the strip granularity will be significantly increased and the sensors of the upgraded tracker will have to be cooled to a temperature of -20$^{\circ}$C. Moreover, the sensor material and design have to be chosen carefully in order to withstand the anticipated radiation. This paper introduces the concept of the $p_T$ module and gives an overview of the ongoing R\&D activities concerning the silicon sensor material.