This article explores terahertz time-domain spectroscopy (THz-TDS) for non-contact measurement of electrical properties of thin semiconductor layers in both the transmission and reflection geometries. 1 to 2 µm thick highly doped indium gallium arsenide layers on indium phosphide substrates with electrical sheet conductance between 10-7 and 10-1 S/sq were investigated. For the first time known to the authors, self-referenced and reference-based methods were applied and systematically compared to extract the electrical sheet conductance of the thin semiconductor layers. Self-referencing improved accuracy in the reflection geometry, and good agreement with contact-based four-point probe methods was validated.