In this paper we present a detailed investigation on the influence of carbon co-implantation in the source/drain extension on the leakage current and defect density in PFET transistors. Carbon is used to reduce the transient enhanced boron diffusion, to decrease short channel effects and to control the overlap length of the transistor. The leakage currents are measured and separated in order to analyze the influence of the carbon on the different MOSFET regions. The carbon implantation dose is varied between 3.5 × 10¹⁴ atm/cm² and 4 × 10¹⁴ atm/cm ². This small increase in implantation dose leads to an enhanced source/drain extension leakage which is caused by carbon induced defects. No effect of the carbon implantation on the source/drain junction leakage was found as the co-implant is located above the source/drain depletion region. In addition an increase of the gate induced drain leakage with carbon dose was observed.