High-performing heterojunction devices were achieved by bringing an aluminum-doped ZnO (AZO) into contact with a p-Si substrate. A thin transparent AZO film was directly coated on pillar-array patterned Si and spontaneously formed a rectifying junction without any intentional doping process. Si pillar-arrays were designed to have 5μm width with variation in periods (7μm and 10μm) used to modulate the surficial lengths. The light response is directly proportional to the surficial length enhancement. AZO/Si heterojunction devices showed strong dependence on the incident wavelengths. At a wavelength of 600nm, the highest response ratio of 70,900% was achieved. We found that the locational superposition of the space charge region and the photo-generated region is crucial for light-reactive responses. We suggest an efficient geometric design scheme for highly efficient light-absorbers.