Three dimensional dopant distributions in polycrystalline Si gate of n -type (n-) and p -type (p-) metal-oxide-semiconductor field effect transistor (MOSFET) structure were investigated by laser-assisted three dimensional atom probe. The remarkable difference in dopant distribution between n -MOSFET and p -MOSFET was clearly observed. In n -MOSFET gate, As and P atoms were segregated at grain boundaries and the interface between gate and gate oxide. No diffusion of As and P atoms into the gate oxide was observed. On the other hand, in p -MOSFET, no segregations of B atoms at grain boundaries or the interface were observed, and diffusion of B atoms into the gate oxide was directly observed.