Abstract:
A gas sensor includes a light source for generating light at a wavelength &lt;550 nm, and a detector for determining a scattered radiation. The gas sensor further includes a field-effect transistor, a semiconductor diode or an ohmic resistance, which forms a unit together with the light source and/or the detector.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a gas sensor, and relates more particularly to a gas sensor used as a fire detector. 
   BACKGROUND INFORMATION 
   To be able to detect fires as early and reliably as possible, gas sensors are needed as fire detectors, which are able to detect fire gases in the smallest concentrations. A fire detector is described, e.g., in published German patent document DE 199 56 303, the measuring principle of which detector is based on a field-effect transistor (FET), whose gate electrode has at least one gas-sensitive coating such that the channel conductivity of the field-effect transistor changes as a function of the concentration of one or several fire gases to be detected. 
   The fire detector described in published German patent document DE 199 56 303 may also include, in addition to a measuring function on the basis of a FET, a scattered light detector. For this purpose, a light source is provided, which usually emits light at a wavelength in the infrared or near-infrared range into a measuring chamber, which is in contact with the surrounding atmosphere. The measuring chamber furthermore contains a detector unit, which allows for a determination of particles possibly contained in the gas phase. However, such a construction having a light source, a detector and a measuring function in the form of a FET, each as separate components, is expensive. 
   An object of the present invention is to provide a gas sensor which allows for a reliable and early detection of fires, but which nevertheless has a simple construction. 
   SUMMARY OF THE INVENTION 
   The gas sensor according to the present invention advantageously provides at least two of the components of the gas sensor in one unit. This allows for a fire to be detected in at least two mutually independent ways without resulting in an expensive construction of the fire detector. 
   It is advantageous if a scattered light detector or a light source of the gas sensor is provided on a common substrate having a field-effect transistor, a semiconductor diode or an ohmic resistance. In this manner, these components form a common and thus space-saving unit. 
   It is furthermore advantageous if at least one gate electrode of the field-effect transistor has a coating that is sensitive to fire gases. This coating may be manufactured on the basis of acidic or basic oxides such that it responds to the relevant basic or acidic fire gases, or on the basis of tin dioxide, resulting in a sensitivity to organic combustion products. The existence of a sensitive coating in contact with the gate electrode of the FET results in a sensitive detector for fire gases. 
   In an example embodiment of the present invention, the gas sensor has two light sources, which emit lights of different wavelengths, the wavelength of the light emitted by the first light source amounting to 1.1 to 3 times the wavelength of the light emitted by the second light source, for example. In this manner, it is possible to successfully detect in a very precise manner particles or aerosols having a diameter in the range of 10 to 500 nm, since based on the mutual proportion of the scattering intensities it is possible to distinguish fire-typical particles or aerosols from dust particles in the air. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic representation of a first exemplary embodiment of a sensitive element of a gas sensor. 
       FIG. 2  shows the schematic representation of a sensitive element of a gas sensor according to a second exemplary embodiment. 
       FIG. 3  shows the schematic representation of a sensitive element of a gas sensor according to a third exemplary embodiment. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a sensitive element  10  of a gas sensor, which sensitive element  10  includes a substrate  12 , which in turn is produced, for example, from silicon, silicon carbide or gallium nitride, which is optionally deposited on sapphire. A light source  14  and a field-effect transistor (FET)  16  are situated on substrate  12 . Additionally, a heating element  50  is optionally integrated into substrate  12 , which is used to heat substrate  12  temporarily or regularly to a temperature at which substances adsorbed on the surface of the FET  16  desorb and thus a regeneration of the sensitive element  10  can be accomplished. The heating element may be configured, for example, as a meander-shaped resistor track. As an alternative or in addition to the application of a FET, a semiconductor diode or an ohmic resistance may be provided on substrate  12 . 
   A light-emitting diode (LED), for example, is chosen as light source  14 , which emits light at a wavelength of ≦550 nm, e.g., in the range of 250 to 550 nm, or more particularly in the range of 350 to 550 nm. For this purpose, the LED may be manufactured on the basis of a doped semiconductor material having a band gap of 2 to approximately 5 eV. These are materials such as α- or β-gallium nitride, 6H- or β-silicon carbide, zinc oxide, α- or β-aluminum nitride, α- or β-indium nitride, zinc sulphide, zinc selenide, aluminum phosphide, gallium phosphide and aluminum arsenide, for example. 
   The use of an LED manufactured in this manner as light source  14  has the advantage that light of short wavelength is scattered markedly more strongly on particles in the gas phase than radiation in the NIR (near infrared) or IR (infrared) range. Another advantage of using an LED as light source  14  which provides light of short wavelength, is seen in the fact that a heating of substrate  12 , on which both light source  14  as well as FET  16  or a semiconductor diode or an ohmic resistance are situated, is only possible when an LED having a great band gap is used. Other LEDs, which emit radiation in the NIR or IR range, cannot be used on heated substrates due to their increased thermally dependent conductivity. Light source  14  is electrically connected by an electrical contact  20 . 
   FET  16  includes a source region  24 , which is provided with an electrical contact  26 , as well as a drain region  28 , which is connected with another electrical contact  30 , source region  24  and drain region  28  being mutually connected by a semiconductive, possibly doped channel region  32 . A gate electrode  34  is in contact with channel region  32 . 
   FET  16 , for example, has on its gate electrode  34  a gas-sensitive coating  18 , gate electrode  34  being provided with the gas-sensitive coating either partially or across its entire surface. Gas-sensitive coating  18  may be accomplished, for example, on the basis of basic oxides such as magnesium oxide, whose electrical charge distribution on the surface or in the interior changes in the presence of acidically reacting gas components such as nitrogen oxides, sulfur oxides, hydrogen halides or carbon dioxide in the surrounding atmosphere and thus results in a change of the electrical field emanating from the gate electrode. 
   Alternatively or additionally, gas-sensitive coating  18  may contain acidic oxides such as tantalum oxides or niobium oxides, which react particularly to the existence of basically reacting gaseous substances such as ammonia or volatile amines in the surrounding atmosphere by a change in their charge carrier distribution. Another possible option is to use semiconductors such as tin dioxide or titanium dioxide, or particles of a precious metal such as platinum, for example, or a precious metal alloy such as a platinum-gold alloy, for example, as the basis of sensitive coating  18 , the use of these substances resulting in a change of the electrical charge carrier distribution in sensitive coating  18  when organic gas components such as polycondensated aromatics, hydrocarbons and carbon monoxide are absorbed. For this purpose, precious metal or precious metal alloy particles may be used in the form of nanoparticles. 
   As an alternative or in addition to the arrangement of a FET  16 , a semiconductor diode or an ohmic resistance may also be provided on substrate  12 . In this case, at least a part of the large surface exposed to a surrounding atmosphere is provided with a sensitive coating  18 , sensitive coating  18  in this case being manufactured from comparable materials as were already described above as a coating for FET  16 . 
   The gas sensor furthermore includes a detector  40  for detecting the radiation emitted by light source  14 . This includes, for example, one of the doped semiconductor materials as a light-detecting material (as described above) as the base material of light source  14 . 
   Detector  40  is positioned in such a way that it detects only a scattered radiation of the light emitted by light source  14 ; that is, it is in particular situated outside of the radiation cone produced by light source  14 . For this purpose, detector  40  is constructed, for example, as a photodiode, a photo-sensitive resistor or as a phototransistor. Furthermore, the fire detector may contain a reflector or parabolic mirror, by which scattered light may be supplied to the detector in a bundled form. 
   When used as a fire detector, this construction of the gas sensor on the one hand allows for a fire to be detected by detecting scattered radiation, which is amplified particularly with the existence of soot or smoke particles or of aerosols. Moreover, a fire may be detected by determining an electrical current flowing through FET  16  or a semiconductor diode or a semiconductor resistor, the magnitude of which is dependent upon, among other things, the gases occurring in fires. This dual fire detection allows for a significantly greater system safety since both highly soot-emitting, slightly poisonous fires as well as slightly soot-emitting fires accompanied by poisonous gases are detected. Nevertheless, the total construction of the gas sensor is not expensive since at least two of components  14 ,  16 ,  40  form a unit on substrate  12  of sensitive element  10 . 
   Another exemplary embodiment of a sensitive elements  10  is shown in  FIG. 2 , identical reference numerals indicating identical components as in  FIG. 1 . 
   In this instance, instead of light source  14 , a detector  40  for the radiation emitted by light source  14  is situated on substrate  12  of sensitive element  10 . Detector  40  is connected by an electrical contact  42 . Light source  14  in this instance is positioned separately, but in such a way that detector  40  detects only the scattered portion of the light emitted by light source  14 . 
   Sensitive element  10  is configured in such a way that gate electrode  34  and the light-sensitive region of detector  40  are manufactured on the basis of the same semiconductor material since this substantially simplifies the manufacture of sensitive element  10 . For this purpose, gallium nitride, indium-gallium nitride, aluminum-gallium nitride or silicon carbide, for example, are used as the material of detector  40 . 
   Another exemplary embodiment of a sensitive elements  10  is shown in  FIG. 3 , identical reference numerals again indicating identical components as in  FIGS. 1 and 2 . 
   As can be gathered from  FIG. 3 , substrate  12  of sensitive element  10 , according to the third exemplary embodiment, has in addition to FET  16  both a light source  14  as well as a detector  40 . The special advantage of this setup lies in the fact that all components of the gas sensor are situated on a common substrate  12  of sensitive element  10  and that the gas sensor itself may be constructed in a cost-effective and space-saving manner. 
   As an alternative embodiment of the sensitive elements shown in  FIGS. 1 through 3 , it is possible to provide two light sources  14 , which generate radiations of different wavelengths. For this purpose, it is particularly advantageous if the additional light source generates a radiation having a wavelength that corresponds to 1.1 to 3 times, e.g., 1.1 to 1.5 times, the wavelength of the radiation generated by first light source  14 . 
   For this purpose, detector  40  detects two scattered light signals, having different light intensities, at the same time or at different times, depending on the control system of the light sources. Theoretically, the measured light intensity is a function of the wavelength of the scattered radiation according to the formula:
 
 I= 1/λ 4 .
 
   Consequently, intensity differences having a factor of 1.5 to 5 are to be expected between the two signals depending on the chosen wavelength of the two light sources. In this manner it is possible to detect the scattered portion of the detected light in a very precise manner, particularly the portion attributable to particles or aerosols having a particle size of 10-500 nm, as typically occur in fires. 
   In addition to a use as fire detector, the gas sensor according to the present invention is suitable for early detection of smoldering fires or cable fires, and as a sensor for determining gas components in exhaust gases of internal combustion engines, power plants, etc., or as air quality sensors. Furthermore, it is possible to detect harmful exhaust gas components in the exhaust gases of waste incinerators.