Patent Number: 
Section: claims

1. A gas sensor comprising:a semiconductor substrate, which has an opening; anda sensing membrane, which is located at an end of the opening to make up a recess in combination with the opening and includes:a gas sensitive film;a heater made of polycrystalline silicon;a pair of gas-sensitive-film extension electrodes made of polycrystalline silicon, wherein 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; anda pair of heater extension electrodes made of polycrystalline silicon, wherein a first end of each heater extension electrode is in electric contact with the heater and a second end of each heater extension electrode extends outward from the sensing membrane,wherein the gas-sensitive-film extension electrodes and the heater are provided on a same layer of the semiconductor substrate. 2. The gas sensor in claim 1 including a plurality of circuit electrodes, which are made of a metal and located outside the sensing membrane, wherein the gas-sensitive-film extension electrodes and the heater extension electrodes are in electric contact with the circuit electrodes outside the sensing membrane. 3. The gas sensor in claim 2, wherein the circuit electrodes are made of aluminum or aluminum alloy. 4. The gas sensor in claim 2 further comprising circuit components, which are located outside the sensing membrane and are in electric contact with the circuit electrodes, wherein the circuit components electrify the heater through the circuit electrodes and are configured to facilitate processing an electric signal that is generated at the gas sensitive film to provide an output signal. 5. The gas sensor in claim 4, wherein the circuit components include a C-MOSFET. 6. The gas sensor in claim 5, wherein gate electrodes of the C-MOSFET are made of polycrystalline silicon. 7. A gas sensor comprising:a semiconductor substrate, which has an opening;a sensing membrane, which is located at an end of the opening to make up a recess in combination with the opening and includes:a gas sensitive film, which extends to an outside of the opening;a heater made of polycrystalline silicon; anda pair of heater extension electrodes made of polycrystalline silicon, wherein a first end of each heater extension electrode is in electric contact with the heater and a second end of each heater extension electrode extends outward from the sensing membrane; anda plurality of circuit electrodes, which are made of a metal and located outside the sensing membrane,wherein the heater extension electrodes are in electric contact with the circuit electrodes outside the sensing membrane, andwherein the gas sensitive film is provided with a pair of film electrodes positioned entirely outside the sensing membrane, which are directly connected to the circuit electrodes. 8. The gas sensor in claim 7, wherein the circuit electrodes are made of aluminum or aluminum alloy. 9. The gas sensor in claim 7 further comprising outside electric circuit components, which are located outside the sensing membrane and are in electric contact with the circuit electrodes, wherein the outside components electrify the heater through the circuit electrodes and are configured to facilitate processing an electric signal that is generated at the gas sensitive film and to produce an output signal. 10. The gas sensor in claim 9, wherein the outside electric components include a C-MOSFET. 11. The gas sensor in claim 10, wherein gate electrodes of the C-MOSFET are made of polycrystalline silicon. 12. The gas sensor in claim 7, wherein the gas sensitive film has a dimension in an extending direction, which is larger than a dimension of the sensing membrane in the extending direction. 13. The gas sensor in claim 1, wherein the sensing membrane includes a plurality of layers. 14. A gas sensor comprising:a semiconductor substrate, which has an opening portion;a gas sensitive film provided to cover at least a part of the opening portion;a heater made of polycrystalline silicon;a pair of film electrodes made of polycrystalline silicon, wherein each film electrode has a first end electrically connected to the gas sensitive film and a second end extending outward from the opening; anda pair of heater electrodes made of polycrystalline silicon, wherein each heater electrode has a first end electrically connected with the heater and a second end extending outward from the opening,wherein the film electrodes and the heater are provided on a single layer on the semiconductor substrate.