The ultrafast process in semiconductor was studied by using femtosecond pulse laser. The momentum relaxation of carriers excited by laser was detected by using ultrafast photo-voltage spectra. The time of momentum relaxation of carriers in semiconductor Si is about 70 femtosecond, which is related to the probability of carrier-carrier scattering. In SiGe quantum dot, it is about 130 femtosecond due to the decrease of carrier-carrier scattering. The energy relaxation and diffusion processes can be measured by ultrafast reflection spectra. The time of energy relaxation of carriers excited by high energy laser is about several picoseconds, which is related to the probability of carrier-phonon scattering, and the diffusion time of carriers excited by low energy laser is about several hundreds picoseconds.