In this paper, the sensitivity (S) and figure of merit (FOM) of disk resonator based photonic biosensors as a function of the gap distance, are studied. These biosensors include a whispering gallery mode (WGM) silicon disk resonator (7μm external radius), coupled to a waveguide (500 nm wide). Gap distance is changed in the range of 150-350 nm. The target is to find the optimized parameters of biosensor device for obtaining high S and FOM. Calculations have been done based on the analytical conformal transformation method (CTM), and coupled mode theory (CMT). Our fully analytical methods have an effective role in fast design of WGM resonator based biosensors. Based on our calculations, the S and FOM parameters of optimized biosensor can be increased up to 134 nm/RIU and 2200 /RIU, at resonance wavelength of 1550 nm. The high FOM and the small size radius of this disk resonator biosensor, allow a high performance device with application in sensing of low amounts of analyte.
