NOVELTY - Gas sensor has a sensing membrane at an end of an opening in a semiconductor substrate to make up a recess in combination with the opening. The sensing membrane has a gas-sensitive film, a heater made of polycrystalline silicon, a pair of gas-sensitive-film extension electrodes made of polycrystalline silicon, and a pair of heater extension electrodes made of polycrystalline silicon.    USE - Gas sensor, e.g. for carbon monoxide, methane, nitric oxide or nitrogen dioxide.    ADVANTAGE - The sensing membrane of the gas sensor provides a better heat holding capability. The gas sensor can be manufactured by a process which is compatible with that of ordinary semiconductor microchips.    DETAILED DESCRIPTION - Gas sensor has a semiconductor substrate (10), and a sensing membrane (20) at an end of an opening in the substrate to make up a recess in combination with the opening. The sensing membrane includes a gas-sensitive film (26), a heater (22) made of polycrystalline silicon, a pair of gas-sensitive-film extension electrodes (23) made of polycrystalline silicon, and a pair of heater extension electrodes (24) made of polycrystalline silicon. A first end of each gas-sensitive-film extension electrode is in electric contact with the gas-sensitive film and a second end of each gas-sensitive-film extension electrode extends outward from the sensing membrane. A first end of each heater extension electrode is in electric contact with the heater and a second end extends outward from the sensing membrane.    An INDEPENDENT CLAIM is also included for a method for manufacturing a gas sensor, which comprises:    (i) forming simultaneously a heater and a pair of heater extension electrodes on a first surface of a substrate from a polycrystalline silicon film;    (ii) etching the substrate from a second surface to form an opening in the silicon substrate and a preliminary membrane at an end of the opening; and    (iii) forming a gas sensitive film on the preliminary membrane to form a sensing membrane and a gas detecting area on the same level as the first surface of the substrate.    DESCRIPTION OF DRAWING(S) - The figure is a schematic plan view of a gas sensor.    Substrate (10)    Sensing membrane (20)    Heater (22)    Gas-sensitive-film extension electrodes (23)    Heater extension electrodes (24)    Gas sensitive film (26)