We have built a THz time-domain spectrometer driven by a sub-15fs pulse duration mode-locked Ti:Al2O3 laser. Using THz time-domain spectroscopy with photoconductive antenna for THz generation and electro-optic sampling for detection as well as photoexcited THz spectroscopy, we measured the carrier concentrations and mobilities of epitaxially grown undoped GaAs samples to be used in photoconductive antenna production. The samples were grown at 600 degrees C to 1 mu m effective layer thickness on top of a 650 mu m SI-GaAs wafer. The resistivities, mobilities and the carrier concentrations were measured and calculated by the van der Pauw method under the magnetic field. These Hall effect measurements and the THz probe studies were compared with each other. The measurements and calculations obtained electronically are compared optically using the Drude Model for the conductivity and mobility.