A novel biochemical sensor based on a submicrometer size, high core–cladding index difference, silica core Si– $\mathrm{Si}{\mathrm{O}}_2$ waveguide with a Bragg grating written in its cladding region is proposed and analyzed. Waveguide parameters are optimized to obtain maximum sensitivity, and for lower refractive index samples, an optimum core width is found to exist for both the TE and the TM mode configurations. Owing to the high index contrast at the Si– $\mathrm{Si}{\mathrm{O}}_2$ interface, the structure is much more sensitive while operating in the TM mode configuration, showing extremely high sensitivity [200–740 nm refractive index units (RIU)] for the ambient refractive indices between 1.33 and 1.63, which is of the order of most surface plasmon polariton (SPP) based biosensors. Further, unlike SPP based sensors, the proposed structure is free from any metallic layer or bulky prism and hence easy to realize. Owing to its simple structure and small dimensions, the proposed device could be easily integrated with planar lightwave circuits and could be used for lab-on-a-chip applications.