  We report on a terahertz time-domain technique for measuring the momentum
relaxation time of charge carriers in ultrathin semiconductor layers. The phase
sensitive modulation technique directly provides the relaxation time.
Time-resolved THz experiments were performed on n-doped GaAs and show precise
agreement with data obtained by electrical characterization. The technique is
well suited for studying novel materials where parameters such as the charge
carriers' effective mass or the carrier density are not known a priori.
