The electronic coupling between the photoactive proteins and semiconductors can be used for fabrication of a hybrid biosolid-state electrooptical devices. The robust cyanbacterial nanosized protein-chlorophyll complex photosystem I (PS I) can generate a photovoltage of 1 V with a quantum efficiency of ∼1 and can be used as a phototransistor gate. A functional dry-oriented junction was fabricated by covalently binding genetically engineered cysteine mutants of PS I to a chemisorbed small connecting molecules on the GaAs surface. Kelvin probe force microscopy measurements showed an induced photovoltage of 0.3 and -0.47 V in PS I-coated p- and n-type GaAs, respectively. The photovoltage resulted from an opposite direction of charge transfer between PS I and the semiconductors due to a difference of almost -0.8 eV in the Fermi level energy of the p- and n-GaAs, thus providing direct evidence of an electronically coupled junction useable as a photosensor.