We design lattice matched InP/In0.53Ga0.47As mesa structured heterojunction p-n photodiodes with a novel passivation methodology based on a fully depleted thin p-InP layer. Mesa-structured detectors are targeted due to their competitive advantages for applications such as multicolor/hyperspectral imaging. Test detector pixels with different perimeter/area ratios are fabricated with and without etching thin InP passivation layer between pixels in order to comparatively examine passivating behavior. I-V characteristics of the test detectors are measured at room temperature. Based on the results from different sized pixel groups, bulk and surface dark current components are separated. Results show that thin InP layer decreases dark current by a factor of 3 while increasing photo current due to a higher carrier collection efficiency.