The Tauc plot method is widely used to determine the bandgap of
semiconductors via UV-visible optical spectroscopy due to its simplicity and
perceived accuracy. However, the actual Tauc plot often exhibits significant
baseline absorption below the expected bandgap, leading to discrepancies in the
calculated bandgap depending on whether the linear fit is extrapolated to zero
or non-zero baseline. In this study, we show that both extrapolation methods
can produce significant errors by simulating Tauc plots with varying levels of
baseline absorption. To address this issue, we propose a new method that
involves idealizing the absorption spectrum by removing its baseline before
constructing the Tauc plot. Experimental verification of this method using a
gallium phosphide (GaP) wafer with intentionally introduced baseline
absorptions shows promising results. Furthermore, we apply this new method to
cubic boron arsenide (c-BAs) and resolve discrepancies in c-BAs bandgap values
reported by different groups, obtaining a converging bandgap of 1.835 eV based
on both previous and new transmission spectra. The method is applicable to both
indirect and direct bandgap semiconductors, regardless of whether the
absorption spectrum is measured via transmission or diffuse reflectance, will
become essential to obtain accurate values of their bandgaps