A homojunction-structured oxide phototransistor based on a mechano-chemically treated indium–gallium–zinc oxide (IGZO) absorption layer is reported. Through this novel and facile mechano-chemical treatment, mechanical removal of the cellophane adhesive tape induces reactive radicals and organic compounds on the sputtered IGZO film surface. Surface modification, following the mechano-chemical treatment, caused porous sites in the solution-processed IGZO film, which can give rise to a homojunction-porous IGZO (HPI) layer and generate sub-gap states from oxygen-related defects. These intentionally generated sub-gap states played a key role in photoelectron generation under illumination with relatively long-wavelength visible light despite the wide band gap of IGZO (>3.0 eV). Compared with conventional IGZO phototransistors, our HPI phototransistor displayed outstanding optoelectronic characteristics and sensitivity; we measured a threshold voltage (V_th) shift from 3.64 to −6.27 V and an on/off current ratio shift from 4.21 × 10¹⁰ to 4.92 × 10² under illumination with a 532 nm green light of 10 mW/mm² intensity and calculated a photosensitivity of 1.16 × 10⁸. The remarkable optoelectronic characteristics and high optical transparency suggest optical sensor applications.