Temperature sensing with optical devices is a very promising research field because of many attractive features common to all-optical sensing schemes. Many recent works have presented optical sensors made with fiber optic Bragg gratings where the temperature information is encoded into a wavelength shift of the transmitted or reflected Bragg spectrum. This calls for specialized readout schemes and it cannot be integrated with other electronic circuits needed for calibration and or compensation. On the other hand, all-silicon integrated sensors have many interesting features from their inherent low processing cost to integrability with signal-processing electronics. In this paper we present a novel approach to temperature sensing with optoelectronic devices which relies on the usage of bare silicon as the transducing material. The device is composed by a single mode input waveguide, a MMI region where the higher order modes are allowed to propagate and two output waveguides. The refractive index variation in the MMI section due to temperature shifts induces different phase velocities of the propagating modes. The position of the input and output waveguides together with the length and width of the MMI section are chosen in order to maximize the sensitivity of the device as it will be shown in the full paper. Analytical calculations are presented together with BPM simulations aimed to the maximization of the sensitivity of the sensor. Analytical calculations are presented together with BPM simulations aimed to the maximization of the sensitivity of the sensor.