Black silicon (b-Si) has improved the performance of solar cells and photodetectors due to the excellent optics and surface passivation achieved with atomic-layer-deposited (ALD) dielectric films. One major reason for the success is the strong field-effect caused by the high density of fixed charges present in the dielectric. Depending on the device, the field-effect can be utilized also in a more active role than for mere surface passivation, including formation of floating and/or induced junctions in silicon devices. However, in order to utilize the field-effect efficiently, deeper understanding on thin film charge-induced electric field and its effects on charge carriers in b-Si is required. Here we investigate the field-effect in b-Si using Silvaco Atlas semiconductor device simulator. By studying the electric field and charge carrier profiles, we develop a model where the electrical properties of b-Si can be approximated with a planar surface, which significantly simplifies device level simulations. We validate the model by simulating the spectral response of a b-Si induced junction photodiode achieving less than 1% difference compared to experimental device performance in a wide range of wavelengths. Finally, we apply the model to study how variation in surface recombination velocity affects the short wavelength sensitivity and dynamic range in a b-Si photodiode.