In this work, we present a numerical analysis comparing the field-effect passivation provided by fixed dielectric charges on planar and textured undiffused silicon surfaces. We focus particularly on the ratio of effective surface recombination velocities (S-eff) of planar and textured surfaces (sigma=S-eff,S-textured/S-eff,S-planar) utilizing the same fixed dielectric charge density (Q(f)) under the same injection conditions. To ensure a difference between the space-charge region profiles of textured and planar surfaces, we specifically analyze structures with large aspect ratios and submicron feature sizes. A wide range of Q(f) (1 x 10(7) to 1 x 10(13)cm(-2)) having both negative and positive polarities is covered in the analysis. We show that sigma approximately follows the surface area enhancement for very small and large Q(f), which induce weak band-bending and strong accumulation or inversion conditions, respectively. For moderately large Q(f), on the other hand, sigma shows a stronger deviation from the surface area enhancement. To elucidate the underlying physical mechanisms leading to the nonmonotonous relation between sigma and Q(f), we analyze the electrostatics of opposing charged surfaces. We show that an enhanced field-effect passivation is not realized along a large portion of a textured surface having a large Q(f) due to the significant carrier localization near the surface.