Gas sensor, comprising: a substrate having a cavity with inner surfaces (120-A.. 120-D) for providing an optical interaction path for an interaction of a filtered infrared "IR" radiation having a center wavelength λ 0 with a target gas in the cavity, wherein the inner surfaces (120-A.. 120-D) of the cavity comprise a reflective coating or a metallization, and wherein the cavity is accessible for an environmental gas comprising the target gas component, wherein the substrate comprises a first partial substrate in form of a first wafer and a second partial substrate in form of a second wafer which are wafer bonded to each other, wherein at least one of the first and second partial substrates (110-1, 110-2) comprises a recess for providing the cavity between the first and second bonded partial substrates (110-1, 110-2), a thermal emitter arranged for emitting a broadband IR radiation (R 0), wherein the thermal emitter is formed on the second partial substrate and is optically coupled to the cavity through an optically transparent region or opening of the second partial substrate, a wavelength selective element arranged for filtering the broadband IR radiation (R 0) emitted by the thermal emitter and for providing the filtered IR radiation having the center wavelength λ 0 in the cavity, wherein the wavelength selective element is optically coupled between the thermal emitter and the cavity, or wherein the wavelength selective element is formed as a bound structure of the cavity, an IR detector formed on the second partial substrate and arranged to provide a detector output signal (S OUT) based on a signal strength of the filtered IR radiation having traversed the optical interaction path in the cavity and being received by the IR detector through an optically transparent region or opening of the second partial substrate, and a radiation directing element further to the reflective coating or metallization in the cavity for directing or coupling the filtered IR radiation into the optical interaction path and/or for focusing the filtered IR radiation to the IR detector, wherein the radiation directing element is formed on the first partial substrate. a substrate having a cavity with inner surfaces (120-A.. 120-D) for providing an optical interaction path for an interaction of a filtered infrared "IR" radiation having a center wavelength λ 0 with a target gas in the cavity, wherein the inner surfaces (120-A.. 120-D) of the cavity comprise a reflective coating or a metallization, and wherein the cavity is accessible for an environmental gas comprising the target gas component, wherein the substrate comprises a first partial substrate in form of a first wafer and a second partial substrate in form of a second wafer which are wafer bonded to each other, wherein at least one of the first and second partial substrates (110-1, 110-2) comprises a recess for providing the cavity between the first and second bonded partial substrates (110-1, 110-2), a thermal emitter arranged for emitting a broadband IR radiation (R 0), wherein the thermal emitter is formed on the second partial substrate and is optically coupled to the cavity through an optically transparent region or opening of the second partial substrate, a wavelength selective element arranged for filtering the broadband IR radiation (R 0) emitted by the thermal emitter and for providing the filtered IR radiation having the center wavelength λ 0 in the cavity, wherein the wavelength selective element is optically coupled between the thermal emitter and the cavity, or wherein the wavelength selective element is formed as a bound structure of the cavity, wherein the wavelength selective element is optically coupled between the thermal emitter and the cavity, or wherein the wavelength selective element is formed as a bound structure of the cavity, an IR detector formed on the second partial substrate and arranged to provide a detector output signal (S OUT) based on a signal strength of the filtered IR radiation having traversed the optical interaction path in the cavity and being received by the IR detector through an optically transparent region or opening of the second partial substrate, and a radiation directing element further to the reflective coating or metallization in the cavity for directing or coupling the filtered IR radiation into the optical interaction path and/or for focusing the filtered IR radiation to the IR detector, wherein the radiation directing element is formed on the first partial substrate.