  We demonstrate saturable absorber behavior of n-type semiconductors GaAs, GaP
and Ge in THz frequency range at room temperature using nonlinear THz
spectroscopy. The saturation mechanism is based on a decrease in electron
conductivity of semiconductors at high electron momentum states, due to
conduction band nonparabolicity and scattering into satellite valleys in strong
THz fields. Saturable absorber parameters, such as linear and non-saturable
transmission, and saturation fluence, are extracted by fits to a classic
saturable absorber model. Further, we observe THz pulse shortening, and an
increase of the group refractive index of the samples at higher THz pulse peak
fields.
