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Nonlinear dynamics of free carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using the intense few-cycle source available at the Advanced Laser Light Source (ALLS). Techniques such as Z-scan and optical-pump/THz-probe are employed to explore nonlinear interactions in an n-doped InGaAs thin film and a photoexcited GaAs sample, respectively. The physical mechanism that gives rise to such interactions is found to be intervalley scattering. A simple Drude-based mathematical model that incorporates the intervalley scattering process is developed and agrees well with the THz response of free carriers in semiconductors.