We demonstrate an integrated evanescent-field multimode Mach–Zehnder interferometric chemical–biological sensor, fabricated on silicon, with sensitivity of parts per ${10}^9$ achieved by modal pattern tracking and analysis. This sensor is fully compatible with the fabrication constraints of the silicon–complementary-metal-oxide-semiconductor (Si-CMOS) process. Furthermore, using the separately measured ellipsometric response together with the mass uptake of agent by the polymer sensing layer, we validate sensor performance via simulation and measure an absolute index sensitivity of $2.5\times {10}^{-6}$ . We then extend this to a fully integrated chemical–biological sensor by considering the fundamental noise performance of CMOS detectors. We find that relatively short, $<5000\mu \mathrm{m}$ long, interferometric sensing elements, with modal pattern analysis, allow fully integrated optical sensors on Si-CMOS (assuming a $2.8\mu \mathrm{m}$ pixel pitch) with an index sensitivity of $\sim 9.2\times {10}^{-7}$ and a corresponding concentration sensitivity of $\sim 170$ parts per ${10}^9$ for methanol in ${\mathrm{N}}_2$ .