The waveguiding properties of high-resistivity float zone silicon slab waveguides are characterized over the spectral range from 0.5 to 7.5 THz. Waveguide modes and dispersion are observed for lengths of 1.2 cm and silicon thicknesses from 40 to 300 mu m. The influence of core thickness and cladding glass attenuation is characterized, and modeled transmitted pulse shapes compare well to the measured signals. Fused silica cladding allows propagation in the 40 mu m thick wafer, demonstrating the feasibility of developing flexible semiconductor core fibers for THz transmission.