Photonic micro-ring resonators can be used as sensor which finds promising bio-analytical applications. The light propagating through the micro-ring and the resultant evanescent field beyond the structure are sensitive to the refractive index of the environment. This sensing capability can be used for detection of biological targets. In this paper, a vertically coupled micro-ring resonator for biosensing is studied. Silicon-on-insulator (SOI) based resonator is preferred for its high index contrast, low cost, and compatibility. Analytical derivations and computations are done to study the effects of structural parameters of the resonator to increase the efficiency of the sensor. © 2019, Springer Nature Singapore Pte Ltd.