In this article, we report strong boron segregation to polycrystalline Si1-x-yGexCy from polysilicon during thermal anneals in the temperature range of 800–900 °C. This effect is larger than previous reports of segregation to single-crystal Si1-xGex and increases with carbon concentration. Segregation also occurs in polycrystalline Si1-yCy, revealing that carbon by itself can drive the segregation (without germanium present). This segregation is used to model the enhanced threshold voltage stability of p-channel metal oxide semiconductor field effect transistors with boron-doped polycrystalline Si1-x-yGexCy gates. We also study the electrical properties of polycrystalline Si1-x-yGexCy. For low carbon concentrations (0.4%), polycrystalline Si1-x-yGexCy has a similar level of dopant activation and mobility as polycrystalline Si1-xGex; increasing the concentration to 1.6% results in significant losses in both. Annealing the films for time scales similar to those needed for segregation causes no degradation of the electrical properties, indicating that electrically inactive defects are not driving the segregation. © 2004 American Institute of Physics.