Hall-effect　in　semiconductors　has　wide　applications　for　magnetic　field　sensing.　Yet,　a　standard　Hall　sensor　retains　two　problems:　its　linearity　is　affected　by　the　non-uniformity　of　the　current　distribution;　the　sensitivity　is　bias-dependent,　with　linearity　decreasing　with　increasing　bias　current.　In　order　to　improve　the　performance,　we　here　propose　a　novel　structure　which　realizes　bias-free,　photo-induced　Hall　sensors.　The　system　consists　of　a　semi-transparent　metal　Pt　and　a　semiconductor　Si　or　GaAs　to　form　a　Schottky　contact.　We　systematically　compared　the　photo-induced　Schottky　behaviors　and　Hall　effects　without　net　current　flowing,　depending　on　various　magnetic　fields,　light　intensities　and　wavelengths　of　Pt/GaAs　and　Pt/Si　junctions.　The　electrical　characteristics　of　the　Schottky　photo-diodes　were　fitted　to　obtain　the　barrier　height　as　a　function　of　light　intensity.　We　show　that　the　open-circuit　Hall　voltage　of　Pt/GaAs　junction　is　orders　of　magnitude　lower　than　that　of　Pt/Si,　and　the　barrier　height　of　GaAs　is　smaller.　It　should　be　attributed　to　the　surface　states　in　GaAs　which　block　the　carrier　drifting.　This　work　not　only　realizes　the　physical　investigations　of　photo-induced　Hall　effects　in　Pt/GaAs　and　Pt/Si　Schottky　junctions,　but　also　opens　a　new　pathway　for　bias-free　magnetic　sensing　with　high　linearity　and　sensitivity　comparing　to　commercial　Hall-sensors.