A semiconductor device for measuring IR radiation is disclosed. It comprises a substrate and a cap enclosing a cavity, a sensor pixel in the cavity, comprising a first absorber for receiving said IR radiation, a first heater, first temperature measurement means for measuring a first temperature; a reference pixel in the same cavity, comprising a second absorber shielded from said IR radiation, a second heater, and second temperature measurement means for measuring a second temperature; a control circuit for applying a first/second power to the first/second heater such that the first temperature equals the second temperature; and an output circuit for generating an output signal indicative of the IR radiation based on a difference between the first and second power.