Abstract:
A semiconductor-based gas detector enhances the collection of gas molecules and also provides a self-contained means for removing collected gas molecules by utilizing one or more electric fields to transport the gas molecules to and away from a metallic material that has a high permeability to the gas molecules.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a gas detector and, more particularly, to a gas detector that utilizes an electric field to assist in the collection and removal of gas molecules. 
         [0003]    2. Description of the Related Art 
         [0004]    A semiconductor-based gas detector is a device that is sensitive to the presence of a gas species. When a gas detector is exposed to a gas species, the gas detector collects gas molecules and then measures the number of collected gas molecules to determine the concentration of the gas species. Carbon diode and other gas species can be detected by a gas detector. 
         [0005]      FIGS. 1A-1C  show views that illustrate a first example of a conventional gas detector  100 .  FIG. 1A  shows a plan view.  FIG. 1B  shows a cross-sectional view taken along line  1 B- 1 B of  FIG. 1A , while  FIG. 1C  shows a cross-sectional view taken along line  1 C- 1 C of  FIG. 1A . As shown in  FIGS. 1A-1C , gas detector  100  includes a p− substrate  110 , a shallow trench isolation region STI that is formed in substrate  110 , and an NMOS transistor  114  that is formed in and on substrate  110 . 
         [0006]    NMOS transistor  114 , in turn, includes spaced-apart source and drain regions  116  and  118  that are formed in substrate  110 , and a channel region  120  of substrate  110  that lies between the source and drain regions  116  and  118 . The source and drain regions  116  and  118 , in turn, each include an n+ region and an NLDD region. 
         [0007]    In addition, NMOS transistor  114  includes a gate dielectric layer  122  that touches the top surface of substrate  110  over channel region  120 , a gate  124  that touches the top surface of gate dielectric layer  122  over channel region  120 , and a side wall spacer  126  that touches the side wall of gate  124 . 
         [0008]    Gate  124  is implemented with a material that has a high permeability to the gas species to be detected. For example, lanthanum oxide, tin oxide, indium oxide, and zink oxide are materials which have a high permeability to carbon dioxide. Other materials are well known to have high permeabilities to other gas species. 
         [0009]    As further shown in  FIG. 1 , gas detector  100  also includes a first dielectric layer  130  that touches the top surface of substrate  110 , and a number of contacts  132  that extend through first dielectric layer  130 . The contacts  132  make individual electrical connections to source region  116 , drain region  118 , gate  124 , and a p+ region of substrate  110 . 
         [0010]    In addition, gas detector  100  includes a number of metal traces  134  that touch the top surface of first dielectric layer  130 , and a second dielectric layer  136  that touches the top surface of first dielectric layer  130  and the metal traces  134 . The metal traces  134  make electrical connections to the contacts  132 . Gas detector  100  further includes a window opening  140  that extends through the first and second dielectric layers  130  and  136  to expose the top surface of gate  124 . 
         [0011]    In operation, gas detector  100  begins with a calibration step, which is performed in an environment that is known to be free of, or have an insignificant concentration of, the to-be-measured gas species. The calibration step is utilized to determine a bias voltage for gate  124 , which is used in a subsequent measurement step. 
         [0012]    Gas detector  100  can be calibrated by first applying ground to substrate  110  and source region  116 , a VCC voltage to drain region  118 , and an initial calibration voltage to gate  124 . Once the voltages have been applied, the magnitude of the source current is measured. The initial calibration voltage is selected to ensure that a sub-threshold current flows out of source region  116 . 
         [0013]    Following this, the calibration voltage is incrementally increased, and the magnitude of the source current is re-measured. The process of incrementally increasing the calibration voltage and re-measuring the magnitude of the source current is repeated a number of times until the magnitude of the source current increases substantially, indicating the turn on of NMOS transistor  114 . 
         [0014]    After the source current has increased substantially, the process of incrementally increasing the calibration voltage and re-measuring the magnitude of the source current ends. Next, a calibration voltage is selected to be the bias voltage from the calibration voltages which were used to generate the source currents. For example, the calibration voltage selected to be the bias voltage can be the calibration voltage which lies just below the turn on voltage of NMOS transistor  114 . 
         [0015]    Once the bias voltage for gate  124  has been selected, the calibration step ends and a collection step begins. The collection step begins, for example, by grounding p− substrate  110 , source region  116 , and drain region  118 , and electrically floating the gate  124  for a predetermined period of time. 
         [0016]    When gas detector  100  is exposed to the gas species, random gas molecules of the gas species enter window  140  and hit the exposed top surface of gate  124 . When a gas molecule hits the exposed top surface of gate  124 , the gas molecule can bounce away from, or stick to, the exposed top surface of gate  124 . 
         [0017]    Due to the high permeability of the material used to form gate  124 , a number of gas molecules that stick to the exposed top surface of gate  124  are absorbed by gate  124 . The gas molecules that stick to gate  124  and are absorbed into gate  124  change the work function of the material used to form gate  124  which, in turn, has the effect of placing a positive charge on gate  124 . 
         [0018]    After the predetermined period of time, the collection step ends and a measurement step begins to determine the number of gas molecules which have been collected. The measurement step begins, for example, by applying the VCC voltage to drain region  118 , grounding substrate  110  and source region  116 , and applying the bias voltage to gate  124 . 
         [0019]    The total charge on gate  124  is the combination of the bias voltage and the effective charge placed on gate  124  by the gas molecules. As a result, when gas molecules have been collected, the total charge on gate  124  is greater than the bias voltage which, in turn, causes the source current to be larger than when no gas molecules have been collected. Thus, by evaluating the increase in source current when compared to the source current associated with the bias voltage, the effective charge placed on gate  124  by the gas species can be determined or accurately estimated. 
         [0020]    The concentration of the gas species that corresponds with the increase in source current or the effective charge placed on gate  124  can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of increased source currents and known gas concentrations. 
         [0021]      FIGS. 2A-2D  show views that illustrate a second example of a conventional gas detector  200 .  FIG. 2A  shows a plan view.  FIG. 2B  shows a cross-sectional view taken along line  2 B- 2 B of  FIG. 2A , while  FIGS. 2C and 2D  both show a cross-sectional view taken along line  2 C- 2 C of  FIG. 2A . Gas detector  200  is similar to gas detector  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. 
         [0022]    As shown in  FIGS. 2A-2D , gas detector  200  differs from gas detector  100  in that gas detector  200  eliminates window  140  and utilizes a floating gate structure  224  in place of gate  124 . Floating gate structure  224 , which is conductive and electrically isolated from all other conductive structures, includes a lower floating gate  230  that touches gate dielectric layer  122  and first dielectric layer  130 , an upper floating gate  232  that touches second dielectric layer  136 , and a vertical connection structure  234  that electrically connects upper floating gate  232  to lower floating gate  230 , and extends through the first and second dielectric layers  130  and  136 . 
         [0023]    Lower floating gate  230  can be implemented with polysilicon, and upper floating gate  232  can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected. The upper and lower portions of vertical conductive structure  234  can be implemented with a conventional via/contact material that has no or a very low permeability to the gas species to be detected, while the wider middle section of vertical conductive structure  234  can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected. 
         [0024]    Gas detector  200  also differs from gas detector  100  in that gas detector  200  has an inter-gate dielectric  240 , such as oxide-nitride-oxide (ONO), that touches the top surface of lower floating gate  230 , and a control gate  242  that touches inter-gate dielectric  240  and lies over a portion of the top surface of lower floating gate  230 . 
         [0025]    In addition, rather than a contact  132  making an electrical connection with gate  124 , the contact  132  instead makes an electrical connection with control gate  242 . (Rather than utilizing dielectric  240  and control gate  242  as shown in  FIG. 2C , a heavily doped region  244  that touches gate dielectric  122  and lies below a portion of lower floating gate  230  can alternately be formed as the control gate as illustrated in  FIG. 2D . Although not shown, a contact  132  makes an electrical connection to doped region  244 .) 
         [0026]    Gas detector  200  further differs from gas detector  100  in that gas detector  200  includes a third dielectric layer  250  that touches the top surface of second dielectric layer  136  and upper floating gate  232 . Gas detector  200  additionally differs from gas detector  100  in that gas detector  200  includes a detection structure  252  that touches the top surface of third dielectric layer  250 . 
         [0027]    Detection structure  252 , which is electrically isolated from all other conductive structures, is implemented with a material that has a high permeability to the gas species to be detected. For example, lanthanum oxide, tin oxide, indium oxide, and zink oxide are materials which have a high permeability to carbon dioxide. Other materials are well known to have high permeabilities to other gas species. 
         [0028]    The operation of gas detector  200  begins by calibrating gas detector  200  to determine a bias voltage for control gate  242  (or doped region  244 ). Gas detector  200  can be calibrated in the same manner as gas detector  100 , except that the source currents are measured in response to placing voltages on control gate  242  (or doped region  244 ). The voltages placed on control gate  242  (or doped region  244 ), in turn, are capacitively coupled to floating gate structure  224 . Due to the capacitive coupling, the bias voltage selected for control gate  242  (or doped region  244 ) is slightly larger than the bias voltage selected for gate  124 . 
         [0029]    Once the bias voltage for control gate  242  (or doped region  244 ) has been selected, the calibration step ends and a collection step begins. The collection step begins, for example, by grounding p− substrate  110 , source region  116 , drain region  118 , and control gate  224  (or doped region  244 ) for a predetermined period of time. 
         [0030]    When gas detector  200  is exposed to the gas species, random gas molecules of the gas species hit the exposed surface of detection structure  252 . When a gas molecule hits the exposed surface of detection structure  252 , the gas molecule can bounce away from, or stick to, the exposed surface of detection structure  252 . 
         [0031]    Due to the high permeability of the material used to form detection structure  252 , a number of gas molecules that stick to the exposed surface of detection structure  252  are absorbed by detection structure  252 . The gas molecules that stick to detection structure  252  and are absorbed into detection structure  252  change the work function of the material used to form detection structure  252  which, in turn, has the effect of placing a positive charge on detection structure  252 . 
         [0032]    After the predetermined period of time, the collection step ends and a measurement step begins to determine the number of gas molecules which have been collected. The measurement step begins, for example, by applying the VCC voltage to drain region  118 , grounding p− substrate  110  and source region  116 , and applying the bias voltage to control gate  224  (or doped region  244 ). 
         [0033]    The total potential on floating gate structure  224  is defined by the voltage on control gate  242  (or doped region  244 ) and the effective charge placed on detection structure  252  by the gas molecules, both of which are capacitively coupled to floating gate structure  224 . As a result, when gas molecules have been collected, the total potential on floating gate structure  224  is greater than the capacitively coupled potential of the bias voltage which, in turn, causes the source current to be larger than when no gas molecules have been collected. Thus, by evaluating the increase in source current when compared to the source current associated with the gate bias voltage, the effective charge placed on detection structure  252  by the gas species can be determined or accurately estimated. 
         [0034]    The concentration of the gas species that corresponds with the increase in source current or the effective charge placed on detection structure  252  can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of increased source currents and known gas concentrations. 
         [0035]    One of the advantages of gas detector  200  over gas detector  100  is that third dielectric layer  250  provides an environmental barrier to the components that lie below third dielectric layer  250 . By forming third dielectric layer  250  to be relatively thin, most of the effective charge placed on detection structure  252  can be capacitively coupled to floating gate structure  224 . 
         [0036]    In addition to using a transistor-based gas detector, resistor-based gas detectors can alternately be used. This is because in addition to effectively adding a positive charge to a material, gas molecules that stick to and are absorbed by a high permeability material also change the conductivity of the material. 
         [0037]      FIGS. 3A-3C  show views that illustrate a third example of a conventional gas detector  300 .  FIG. 3A  shows a plan view.  FIG. 3B  shows a cross-sectional view taken along line  3 B- 3 B of  FIG. 3A , while  FIG. 3C  shows a cross-sectional view taken along line  3 C- 3 C of  FIG. 3A . Gas detector  300  is similar to gas detector  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. 
         [0038]    As shown in  FIGS. 3A-3C , gas detector  300  differs from gas detector  100  in that gas detector  300  utilizes a resistive structure  310  in lieu of gate  124 , and a dielectric layer  312  in lieu of dielectric layer  122 . Gas detector  300  also differs from gas detector  100  in that gas detector  300  eliminates the source and drain regions  116  and  118 , and utilizes the contacts  132  to make connections to opposite sides of resistive structure  310 . Resistive structure  310  can be identical to gate  124 , while dielectric layer  312  can be thicker than gate dielectric layer  122 . 
         [0039]    Gas detector  300  can be calibrated by grounding substrate  110 , applying a set of voltages to the opposite sides of resistive structure  310 , and then measuring a baseline current through resistive structure  310 . Gas detector  300  can collect gas molecules by electrically floating resistive structure  310 , and grounding p− substrate  110 . During collection, gas molecules that stick to, and are absorbed by, resistive structure  310  change the conductivity of resistive structure  310 . 
         [0040]    Gas detector  300  can measure the number of collected gas molecules by grounding substrate  110 , applying the set of voltages to the opposite sides of resistive structure  310 , and then measuring a current through resistive structure  310 . Thus, by evaluating the change in current through resistive structure  310 , the number of gas molecules collected by resistive structure  310  can be determined or accurately estimated. The concentration of the gas species that corresponds with the change in current can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of currents and known gas concentrations. 
         [0041]    Although gas detectors  100 ,  200 , and  300  can be utilized to detect a number of gas species, there is a need for additional structures for detecting the presence of a gas species. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIGS. 1A-1C  are views illustrating a first example of a conventional gas detector  100 .  FIG. 1A  is a plan view.  FIG. 1B  is a cross-sectional view taken along line  1 B- 1 B of  FIG. 1A , while  FIG. 1C  is a cross-sectional view taken along line  1 C- 1 C of  FIG. 1A . 
           [0043]      FIGS. 2A-2D  are views illustrating a second example of a conventional gas detector  200 .  FIG. 2A  is a plan view.  FIG. 2B  is a cross-sectional view taken along line  2 B- 2 B of  FIG. 2A , while  FIGS. 2C and 2D  are cross-sectional view taken along line  2 C- 2 C of  FIG. 2A . 
           [0044]      FIGS. 3A-3C  are views illustrating a third example of a conventional gas detector  300 .  FIG. 3A  is a plan view.  FIG. 3B  is a cross-sectional view taken along line  3 B- 3 B of  FIG. 3A , while  FIG. 3C  is a cross-sectional view taken along line  3 C- 3 C of  FIG. 3A . 
           [0045]      FIGS. 4A-4E  are views illustrating an example of a gas detector  400  in accordance with the present invention.  FIG. 4A  is a plan view.  FIG. 4B  is a cross-sectional view taken along line  4 B- 4 B of  FIG. 4A , while  FIG. 4C  is a cross-sectional view taken along line  4 C- 4 C of  FIG. 4A ,  FIG. 4D  is a cross-sectional view taken along line  4 D- 4 D of  FIG. 4A , and  FIG. 4E  is a cross-sectional view taken along line  4 C- 4 C of  FIG. 4A . 
           [0046]      FIG. 5  is a flow chart illustrating an example of a method of operating gas detector  400  in accordance with the present invention. 
           [0047]      FIGS. 6A-6G  are views illustrating an example of a gas detector  600  in accordance with an alternate embodiment of the present invention.  FIG. 6A  is a plan view.  FIG. 6B  is a cross-sectional view taken along line  6 B- 6 B of  FIG. 6A , while  FIGS. 6C and 6D  are cross-sectional views taken along line  6 C- 6 C of  FIG. 6A ,  FIG. 6E  is a cross-sectional view taken along line  6 E- 6 E of  FIG. 6A ,  FIG. 6F  is a cross-sectional view taken along line  6 F- 6 F of  FIG. 6A , and  FIG. 6G  is a cross-sectional view taken along line  6 C- 6 C of  FIG. 6A . 
           [0048]      FIGS. 7A-7E  are views illustrating an example of a gas detector  700  in accordance with an alternate embodiment of the present invention.  FIG. 7A  is a plan view.  FIG. 7B  is a cross-sectional view taken along line  7 B- 7 B of  FIG. 7A , while  FIGS. 7C and 7D  are cross-sectional views taken along line  7 C- 7 C of  FIG. 7A , and  FIG. 7E  is a cross-sectional view taken along line  7 E- 7 E of  FIG. 7A . 
           [0049]      FIG. 8  is a flow chart illustrating an example of a method of operating gas detector  700  in accordance with the present invention. 
           [0050]      FIGS. 9A-9E  are views illustrating an example of a gas detector  900  in accordance with an alternate embodiment of the present invention.  FIG. 9A  is a plan view.  FIG. 9B  is a cross-sectional view taken along line  9 B- 9 B of  FIG. 9A , while  FIGS. 9C and 9D  are cross-sectional views taken along line  9 C- 9 C of  FIG. 9A , and  FIG. 9E  is a cross-sectional view taken along line  9 E- 9 E of  FIG. 9A . 
           [0051]      FIGS. 10A-10C  are views illustrating an example of a gas detector  1000  in accordance with the present invention.  FIG. 10A  is a plan view.  FIG. 10B  is a cross-sectional view taken along line  10 B- 10 B of  FIG. 10A , while  FIG. 10C  is a cross-sectional view taken along line  10 C- 10 C of  FIG. 10A . 
           [0052]      FIG. 11  is a flow chart illustrating an example of a method of operating gas detector  1000  in accordance with the present invention. 
           [0053]      FIGS. 12A-12F  are cross-sectional views illustrating a method of forming gas detector  400  in accordance with the present invention. 
           [0054]      FIGS. 13A-13K  are cross-sectional views illustrating a method of forming gas detector  700  in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0055]      FIGS. 4A-4E  show views that illustrate an example of a gas detector  400  in accordance with the present invention.  FIG. 4A  shows a plan view.  FIG. 4B  shows a cross-sectional view taken along line  4 B- 4 B of  FIG. 4A , while  FIG. 4C  shows a cross-sectional view taken along line  4 C- 4 C of  FIG. 4A ,  FIG. 4D  shows a cross-sectional view taken along line  4 D- 4 D of  FIG. 4A , and  FIG. 4E  shows a cross-sectional view taken along line  4 C- 4 C of  FIG. 4A . Gas detector  400  is similar to gas detector  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. 
         [0056]    As shown in  FIGS. 4A-4C , gas detector  400  differs from gas detector  100  in that gas detector  400  includes a metal grid  410  that touches the top surface of second dielectric layer  136  and lies over window opening  140 . Metal grid  410  can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected. 
         [0057]    Optionally, metal grid  410  can be implemented with a catalyzing metal, such as platinum or palladium. When metal grid  410  is implemented with a catalyzing metal, the catalyzing metal grid  410  can function as a reduction catalyst or an oxidization catalyst. For example, a catalyzing metal grid  410  can oxidize carbon monoxide to form carbon dioxide. 
         [0058]      FIG. 5  shows a flow chart that illustrates an example of a method of operating gas detector  400  in accordance with the present invention. As shown in  FIG. 5 , the method begins with a calibration step in  510  that determines a bias voltage for gate  124  of gas detector  400 . Gas detector  400  can be calibrated using the same bias voltages and method as gas detector  100 , except that metal grid  410  is grounded during the calibration step. 
         [0059]    Once the bias voltage for gate  124  has been determined, the method moves to a collection step in  512  to collect gas molecules for a predetermined period of time. Gas detector  400  collects gas molecules in the same manner and with the same bias voltages as gas detector  100 , except that a voltage is applied to metal grid  410  to set up an electric field from metal grid  410  to substrate  110 . 
         [0060]    For example, the electric field can be set up by applying a large positive voltage, such as 100V, to metal grid  410 . The magnitude of the voltage, and thereby the magnitude of the electric field, is dependent upon the maximum electric field that gate dielectric layer  122  can withstand without breaking down or leaking. 
         [0061]    In accordance with the present invention, the electric field significantly enhances the collection of gas molecules. The electric field that extends from metal grid  410  to substrate  110  transports polarized gas molecules and positively ionized gas molecules that enter window  140  down to gate  124 . (The electric field also vertically aligns the polarized gas molecules.) Thus, rather than relying on the random motion of the gas molecules, the electric field transports the gas molecules directly to gate  124 . 
         [0062]    As noted above, when a gas molecule hits the top surface of gate  124 , the gas molecule can bounce away from or stick to the top surface of gate  124 . The electric field, however, improves the sticking coefficient of the gas molecules that hit the top surface of gate  124 , thereby reducing the number of gas molecules that bounce off the top surface of gate  124  and escape from window  140 . 
         [0063]    This is because the energy required to bounce off the top surface of gate  124  and escape from window  140  must exceed the effect of the electric field. Thus, gas molecules which have sufficient energy to bounce off the top surface of gate  124 , but lack sufficient energy to escape from window  140 , will be again attracted to the top surface of gate  124  and eventually captured. 
         [0064]    In addition, the electric field captures and re-directs a number of gas molecules that would normally escape from window  140  as a result of random collisions with other gas molecules, and transports these gas molecules down to gate  124 . Further, when a gas molecule sticks to the top surface of gate  124 , the electric field limits the surface movement of the molecule. 
         [0065]    Due to the high permeability of the material used to form gate  124 , a number of gas molecules that stick to the exposed top surface of gate  124  are absorbed by gate  124 . The electric field also assists in the absorption of the gas molecules into gate  124 . The gas molecules that stick to, and are absorbed into, gate  124  change the work function of the material used to form gate  124  which, in turn, has the effect of placing a positive charge on gate  124 . Although to a lesser degree, the electric field is also expected to have the same effect on electrically neutral gas molecules. Experimental results have shown that water molecules, which are electrically neutral molecules, move under the influence of an electric field. 
         [0066]    Some of the gas molecules that are absorbed into gate  124  migrate through gate  124  into gate dielectric layer  122  under the influence of the electric field. The vertical alignment of the polarized gas molecules and the positively ionized gas molecules in gate dielectric layer  122  have the effect of placing a positive charge in gate dielectric layer  122 . 
         [0067]    Returning again to  FIG. 5 , after the predetermined period of time has ended, the method moves to a measurement step in  514  to determine the number of gas molecules collected by gas detector  400 . Gas detector  400  determines the number of collected gas molecules in the same manner and using the same bias voltages as gas detector  100 , except that metal grid  410  is grounded during the measurement step. 
         [0068]    Once the number of collected gas molecules has been determined, the method moves to an erase step in  516  to reset gas detector  400 . Gas detector  400  is erased by reversing the electric field for a predefined time. For example, the electric field can be reversed by electrically floating the gate  124 , grounding p− substrate  110 , source region  116 , and drain region  118 , and applying a large negative voltage, such as −100V, to metal grid  410 . 
         [0069]    These bias conditions reverse the direction of the electric field which, in turn, pulls the gas molecules out of gate dielectric layer  122  and gate  124 , and transports the gas molecules away from gate  124 . Thus, in addition to significantly enhancing the collection of gas molecules, the present invention also erases gas detector  400 . 
         [0070]    After the predefined time, the method moves to a check step in  518  to check the bias voltage. The bias voltage is checked by applying the VCC voltage to drain region  118 , grounding substrate  110 , source region  116 , and metal grid  410 , and applying the bias voltage to gate  124 . Following this, the source current is measured and compared to the source current associated with the original bias voltage. 
         [0071]    When the source current is equal to or within an error tolerance of the source current associated with the original bias voltage, the method returns to the collection step in  512  to perform another test. On the other hand, when the source current is greater than the error tolerance, the method returns to the calibration step in  510  to determine a new bias voltage for gate  124 . Thus, the check step in  518  allows the bias voltage to be adjusted to account for any gas molecules that were not removed from gate dielectric layer  122  and gate  124 , thereby ensuring that the original sensitivity of gas detector  400  is maintained. 
         [0072]    Optionally, as shown in  FIGS. 4D and 4E , gas detector  400  can also include a heating element  414  that generates heat. Heating element  414 , which is thermally coupled to gate  124 , is utilized to increase the temperature of gate  124  during the collection and erasure steps, thereby increasing the ability of gate  124  to absorb gas molecules during the collection step, and discharge gas molecules during the erase step. Heating element  414 , which lies over and is insulated from gate  124  by an isolation layer  416 , has a pair of opposing ends that touch a pair of contacts  132 . 
         [0073]    In operation, heating element  414  generates heat when a current is passed through heating element  414  in response to a set of voltages applied to the opposite ends of heating element  414 . Heating element  414 , which lies below the lowest metal trace, can be implemented as a doped strip of polysilicon, single-crystal silicon, or other conductive material which generates heat when a current is passed through heating element  414 . 
         [0074]      FIGS. 6A-6G  show views that illustrate an example of a gas detector  600  in accordance with an alternate embodiment of the present invention.  FIG. 6A  shows a plan view.  FIG. 6B  shows a cross-sectional view taken along line  6 B- 6 B of  FIG. 6A , while  FIGS. 6C and 6D  both show a cross-sectional view taken along line  6 C- 6 C of  FIG. 6A ,  FIG. 6E  shows a cross-sectional view taken along line  6 E- 6 E of  FIG. 6A ,  FIG. 6F  shows a cross-sectional view taken along line  6 F- 6 F of  FIG. 6A , and  FIG. 6G  shows a cross-sectional view taken along line  6 C- 6 C of  FIG. 6A . 
         [0075]    Gas detector  600  is similar to gas detector  400  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. As shown in  FIGS. 6A-6C , gas detector  600  differs from gas detector  400  in that gas detector  600  utilizes a floating gate  624  in place of gate  124 . Floating gate  624  is identical to gate  124  except that floating gate  624  is electrically isolated from all other conductive structures. 
         [0076]    Gas detector  600  also differs from gas detector  400  in that gas detector  600  includes an inter-gate dielectric  630 , such as oxide-nitride-oxide (ONO), that touches a portion of the top surface of floating gate  624 , and a control gate  632  that touches inter-gate dielectric  630  and lies over a portion of the top surface of floating gate  624 . 
         [0077]    In addition, rather than a contact  132  making an electrical connection with gate  124 , the contact  132  instead makes an electrical connection with control gate  632 . (Rather than utilizing dielectric  630  and control gate  632  as shown in  FIG. 6C , a heavily doped region  634  that touches gate dielectric  122  and lies below a portion of floating gate  624  can alternately be formed as the control gate as illustrated in  FIG. 6D . Although not shown, a contact  132  makes an electrical connection to doped region  634 .) 
         [0078]    Gas detector  600  operates the same as gas detector  400 , except for the following differences. In the calibration step in  510 , gas detector  600  can be calibrated in the same manner as gas detector  400 , except that the source currents are measured in response to placing voltages on control gate  632  (or doped region  634 ). The voltages placed on control gate  632  (or doped region  634 ), in turn, are capacitively coupled to floating gate  624 . Due to the capacitive coupling, the bias voltage selected for control gate  632  (or doped region  634 ) is slightly larger than the bias voltage selected for gate  124  of gas detector  400 . 
         [0079]    In the collection step in  512 , gas detector  600  collects gas molecules in the same manner and using the same bias voltages as gas detector  400 , except that control gate  632  (or doped region  634 ) is also grounded for the predetermined period of time. When gas detector  600  is exposed to the gas species, the gas molecules that enter window  140  are transported down to floating gate  624  by the electric field that extends from metal grid  410  to substrate  110 . When a gas molecule hits the exposed surface of floating gate  624 , the gas molecule can bounce away from, or stick to, the exposed surface of floating gate  624 . 
         [0080]    Due to the high permeability of the material used to form floating gate  624 , a number of gas molecules that stick to the exposed surface of floating gate  624  are absorbed by floating gate  624 . The electric field also assists in the absorption of the gas molecules into floating gate  624 . The gas molecules that stick to floating gate  624  and are absorbed into floating gate  624  change the work function of the material used to form floating gate  624  which, in turn, has the effect of placing a positive charge on floating gate  624 . 
         [0081]    Some of the gas molecules that are absorbed into floating gate  624  migrate through floating gate  624  into gate dielectric layer  122  under the influence of the electric field. The vertical alignment of the polarized gas molecules and the positively ionized gas molecules in gate dielectric layer  122  have the effect of placing a positive charge in gate dielectric layer  122 . 
         [0082]    In the measurement step in  514 , gas detector  600  determines the number of collected gas molecules in the same manner and using the same bias voltages as gas detector  400 , except that the bias voltage for control gate  632  (or doped region  634 ) is applied to control gate  632  (or doped region  634 ). The total potential on floating gate  624  is defined by the voltage on control gate  632  (or doped region  634 ), which is capacitively coupled to floating gate  624 , and the effective charge placed on floating gate  624  by the gas molecules. 
         [0083]    As a result, when gas molecules have been collected, the total potential on floating gate  624  is greater than the capacitively coupled potential of the bias voltage which, in turn, causes the source current to be larger than when no gas molecules have been collected. Thus, by evaluating the increase in the source current when compared to the source current associated with the bias voltage, the effective charge placed on floating gate  624  by the gas species can be determined or accurately estimated. 
         [0084]    The concentration of the gas species that corresponds with the increase in source current or the effective charge placed on floating gate  624  can then be determined by referencing a look-up table, where the entries in the look-up table are experimentally determined from a series of increased source currents and known gas concentrations. 
         [0085]    In the erase step in  516 , gas detector  600  is erased in the same manner and using the same bias voltages as gas detector  400 , except that control gate  632  (or doped region  634 ) is also grounded for the predefined period of time. These bias conditions reverse the direction of the electric field which, in turn, pulls the gas molecules out of gate dielectric layer  122  and floating gate  624 , and transports the gas molecules away from floating gate  624 . 
         [0086]    In the check step in  518 , gas detector  600  checks the bias voltage for control gate  632  (or doped region  634 ) in the same manner and using the same bias voltages as gas detector  400 , except that the bias voltage for control gate  632  (or doped region  634 ) is applied to control gate  632  (or doped region  634 ). 
         [0087]    As shown in  FIG. 6E , gas detector  600  can optionally include a notched region  640  in gate dielectric  122 , a heavily doped region  642  that lies below notched region  640 , and a programming gate  644  that touches inter-gate dielectric  630  and lies laterally adjacent to control gate  632  shown in  FIG. 6C . (Although not shown, a contact  132  makes an electrical connection to doped region  642 .) 
         [0088]    Programming gate  644  allows charge to be placed on or removed from floating gate  624  in a conventional manner by way of Fowler-Nordheim tunneling. Thus, when the method returns to the calibration step in  510  to again determine the bias voltage for control gate  632  (or doped region  634 ), the bias voltage for control gate  632  (or doped region  634 ) can remain unchanged if charge is injected onto floating gate  624  by way of programming to account for any gas molecules that were not removed during the erasure step in  516 . 
         [0089]    As shown in  FIGS. 6F and 6G , gas detector  600  can also optionally include a heating element  650  that generates heat. Heating element  650 , which is thermally coupled to floating gate  624 , is utilized to increase the temperature of floating gate  624  during the collection and erasure steps, thereby increasing the ability of floating gate  624  to absorb gas molecules during the collection step, and discharge gas molecules during the erase step. Heating element  650 , which lies over and is insulated from floating gate  624  by inter-gate dielectric  630 , has a pair of opposing ends that touch a pair of contacts  132 . 
         [0090]    In operation, heating element  650  generates heat when a current is passed through heating element  650  in response to a set of voltages applied to the opposite ends of heating element  650 . Heating element  650 , which lies below the lowest metal trace, can be implemented as a doped strip of polysilicon, single-crystal silicon, or other conductive material which generates heat when a current is passed through heating element  650 , but is preferably implemented with the same material as control gate  632 . 
         [0091]      FIGS. 7A-7E  show views that illustrate an example of a gas detector  700  in accordance with an alternate embodiment of the present invention.  FIG. 7A  shows a plan view.  FIG. 7B  shows a cross-sectional view taken along line  7 B- 7 B of  FIG. 7A , while  FIGS. 7C and 7D  show cross-sectional views taken along line  7 C- 7 C of  FIG. 7A , and  FIG. 7E  shows a cross-sectional view taken along line  7 E- 7 E of  FIG. 7A . 
         [0092]    Gas detector  700  is similar to gas detector  200  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. As shown in  FIGS. 7A-7D , gas detector  700  differs from gas detector  200  in that gas detector  700  utilizes a dielectric layer  708  in lieu of third dielectric layer  250 , and a detection structure  710  in lieu of detection structure  252 . Like detection structure  252 , detection structure  710  is electrically isolated from all other conductive structures. In addition, gas detector  700  also includes a metal structure  712  that touches the top surface of dielectric layer  708 . 
         [0093]    Detection structure  710  has a base  710 B and a number of fingers  710 F that extend away from base  710 B, while metal structure  712  also has a base  712 B and a number of fingers  712 F that extend away from base  712 B. In addition, the fingers  710 F of detection structure  710  and the fingers  712 F of metal structure  712  are interdigitated so that the fingers  710 F of detection structure  710  lie between the fingers  712 F of metal structure  712 . Further, the widths of the fingers  710 F are wider than the widths of the fingers  712 F. 
         [0094]    Detection structure  710  and metal structure  712  are implemented with a material that has a high permeability to the gas species to be detected. For example, lanthanum oxide, tin oxide, indium oxide, and zink oxide are materials that have a high permeability to carbon dioxide. Other materials are well known to have permeabilities that are selective to other gas species. Optionally, metal structure  712  can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected, or with a catalyzing metal, such as platinum or palladium. 
         [0095]      FIG. 8  shows a flow chart that illustrates an example of a method of operating gas detector  700  in accordance with the present invention. As shown in  FIG. 8 , the method begins with a calibration step in  810  that determines a bias voltage for control gate  242  (or doped region  244 ). Gas detector  700  is calibrated in the same manner and using the same bias voltages as gas detector  200 , except that metal structure  712  is grounded during the calibration step. 
         [0096]    Once the bias voltage for control gate  242  (or doped region  244 ) has been determined, the method moves to a collection step in  812  to collect gas molecules for a predetermined time. Gas detector  700  collects gas molecules in the same manner and with the same bias voltages as gas detector  200 , except that a voltage is applied to metal structure  712  during the collection step to set up a first electric field from metal structure  712  to detection structure  710 , and a second electric field from metal structure  712  and detection structure  710  to substrate  110 . Dielectric layer  708  is thicker than third dielectric layer  250  to accommodate the second electric field. 
         [0097]    For example, the first and second electric fields can be set up by applying a large positive voltage, such as 100V, to metal structure  712 . The large positive voltage placed on metal structure  712  is capacitively coupled to detection structure  710 . Thus, when a large positive voltage is applied to metal structure  712 , a smaller positive potential is present on detection structure  710  due to the capacitive coupling which, in turn, sets up the first electric field. In addition, the positive voltage on detection structure  710  and the positive potential on metal structure  712  set up the second electric field to extend from detection structure  710  and metal structure  712  to substrate  110 . 
         [0098]    In accordance with the present invention, the electric fields significantly enhance the collection of gas molecules. The first electric field transports polarized gas molecules and positively ionized gas molecules to the exposed surface of detection structure  710 . When a gas molecule hits the exposed surface of detection structure  710 , the gas molecule can bounce away from or stick to the exposed surface of detection structure  710 . The first electric field, however, improves the sticking coefficient of the gas molecules that hit the exposed surface of detection structure  710 , thereby reducing the number of gas molecules that bounce off the exposed surface of detection structure  710 . 
         [0099]    Due to the high permeability of the material used to form detection structure  710 , a number of gas molecules that stick to the exposed surface of detection structure  710  are absorbed by detection structure  710 . The first electric field also assists in the absorption of the gas molecules into detection structure  710 . The gas molecules that stick to detection structure  710  and are absorbed into detection structure  710  change the work function of the material used to form detection structure  710  which, in turn, has the effect of placing a positive charge on detection structure  710 . 
         [0100]    Some of the gas molecules that are absorbed into detection structure  710  migrate through detection structure  710  into dielectric layer  708  under the influence of the second electric field. The vertical alignment of the polarized gas molecules and the positively ionized gas molecules in dielectric layer  708  have the effect of placing a positive charge in dielectric layer  708 . 
         [0101]    Thus, the first electric field transports polarized gas molecules and positively ionized gas molecules to the exposed surface of detection structure  710 , improves the sticking coefficient of the gas molecules to detection structure  710 , and assists in the absorption of the gas molecules into detection structure  710 . In addition, the second electric field migrates the gas molecules through detection structure  710  and into dielectric layer  708 . 
         [0102]    Returning again to  FIG. 8 , after the predetermined period of time has ended, the method moves to a measurement step in  814  to determine the number of gas molecules collected by gas detector  700 . Gas detector  700  determines the number of collected gas molecules in the same manner and using the same bias voltages as gas detector  200 , except that metal structure  712  is grounded during the measurement step. 
         [0103]    Once the number of collected gas molecules has been determined, the method moves to an erase step in  816  to reset gas detector  700 . Gas detector  700  is erased by reversing the first and second electric fields for a predefined time. For example, the first and second electric fields can be reversed by grounding p− substrate  110 , source region  116 , drain region  118 , and control gate  242  (or doped region  244 ), and placing a large negative voltage, such as −100V, on metal structure  712 . 
         [0104]    These bias conditions reverse the directions of the first and second electric fields which, in turn, pull the gas molecules out of dielectric layer  708  and detection structure  710  and transport the gas molecules away from detection structure  710 . Thus, in addition to significantly enhancing the collection of gas molecules, the present invention also erases gas detector  700 . 
         [0105]    After the predefined time, the method moves to a check step in  818  to check the bias voltage for control gate  242  (or doped region  244 ). The bias voltage for control gate  242  (or doped region  244 ) is checked by applying the VCC voltage to drain region  118 , grounding to p− substrate  110 , source region  116 , and metal structure  712 , and applying the bias voltage to control gate  242  (or doped region  244 ). Following this, the source current is compared to the source current associated with the original bias voltage for control gate  242  (or doped region  244 ). 
         [0106]    When the source current is equal to or within an error tolerance of the source current associated with the original bias voltage for control gate  242  (or doped region  244 ), the method returns to the collection step in  812  to perform another test. On the other hand, when the source current is greater than the error tolerance, the method returns to the calibration step in  810  to determine a new bias voltage for control gate  242  (or doped region  244 ). Thus, the check step in  818  allows the bias voltage for control gate  242  (or doped region  244 ) to be adjusted to account for any gas molecules that were not removed from dielectric layer  708  and detection structure  710 , thereby ensuring that the original sensitivity of gas detector  700  is maintained. 
         [0107]    As shown in  FIG. 7E , gas detector  700  can optionally include a notched region  740  in gate dielectric layer  122 , a heavily doped region  742  that lies below notched region  740 , and a programming gate  744  that touches inter-gate dielectric  240  and lies laterally adjacent to control gate  242  in  FIG. 7C . (Although not shown, a contact  132  makes an electrical connection to doped region  742 .) 
         [0108]    Programming gate  744  allows charge to be placed on or removed from floating gate structure  224  in a conventional manner by way of Fowler-Nordheim tunneling. Thus, when the method returns to the calibration step in  810  to again determine the bias voltage for control gate  242  (or doped region  244 ), the bias voltage for control gate  242  (or doped region  244 ) can remain unchanged if charge is injected onto floating gate structure  224  by way of programming to account for any gas molecules that were not removed during the erasure step in  816 . 
         [0109]    As further shown in  FIG. 7B , gas detector  700  can optionally include an external heat source  750  that is thermally connected to detection structure  710 . In operation, heat source  750  increases the temperature of detection structure  710  during the collection and erasure steps, thereby increasing the permeability of detection structure  710 , and the ability of detection structure  710  to absorb gas molecules during the collection step, and discharge gas molecules during the erase step. 
         [0110]      FIGS. 9A-9E  show views that illustrate an example of a gas detector  900  in accordance with an alternate embodiment of the present invention.  FIG. 9A  shows a plan view.  FIG. 9B  shows a cross-sectional view taken along line  9 B- 9 B of  FIG. 9A , while  FIGS. 9C and 9D  show cross-sectional views taken along line  9 C- 9 C of  FIG. 9A , and  FIG. 9E  shows a cross-sectional view taken along line  9 E- 9 E of  FIG. 9A . 
         [0111]    Gas detector  900  is similar to gas detector  700  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. As shown in  FIGS. 9A-9E , gas detector  900  differs from gas detector  700  in that gas detector  900  utilizes a floating gate structure  924  in lieu of floating gate structure  224 . Floating gate structure  924 , in turn, differs from floating gate structure  224  in that a metal structure  930  is utilized in lieu of upper floating gate  232 . Gas detector  900  also differs from gas detector  700  in that gas detector  900  includes a metal structure  932 . 
         [0112]    Metal structure  930  has an identical shape as detection structure  710 , and lies directly below detection structure  710  between second dielectric layer  136  and dielectric layer  708 . Similarly, metal structure  932  has an identical shape as metal structure  712 , and lies directly below metal structure  712  between second dielectric layer  136  and dielectric layer  708 . In addition, the metal structures  930  and  932  can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected. 
         [0113]    Gas detector  900  operates the same as gas detector  700 , except for the following differences. During the calibration step in  810 , gas detector  900  is calibrated in the same manner and using the same bias voltages as gas detector  700 , except that both metal structures  712  and  932  are grounded during the calibration step. 
         [0114]    During the collection step in  812 , gas detector  900  collects gas molecules in the same manner and using the same bias voltages as gas detector  700 , except that voltages are applied to metal structures  712  and  932  during the collection step to set up a first electric field from metal structure  712  to detection structure  710 , a second electric field from metal structure  712  and detection structure  710  to floating gate structure  924  and metal structure  932 , and a third electric field from metal structure  932  to floating gate structure  924 . 
         [0115]    For example, the first, second, and third electric fields can be set up by applying a negative voltage, such as −100V, to control gate  242 , a positive voltage, such as 100V, to metal structure  932 , and a positive voltage, such as 300V, to metal structure  712 . (Other combinations of voltages on control gate  242 , metal structure  712 , and metal structure  932  can also be used to provide equivalent electric fields.) The positive voltage placed on metal structure  712  is capacitively coupled to detection structure  710 . Thus, when a positive voltage is applied to metal structure  712 , a smaller positive potential is present on detection structure  710  due to the capacitive coupling which, in turn, sets up the first electric field. 
         [0116]    The negative voltage placed on control gate  242  and the positive voltage placed on metal structure  932  are both capacitively coupled to floating gate structure  924 . The potential on floating gate structure  924  is the sum of these capacitively coupled values. Thus, for example, when −100V are applied to control gate  242  and 100V are applied to metal structure  932 , the potential on floating gate structure  924  is approximately 0V. The 100V on metal structure  932  and the 0V on floating gate structure  924  then set up the third electric field to extend from metal structure  932  to floating gate structure  924 . 
         [0117]    In addition, the positive potential on detection structure  710 , the positive voltage on metal structure  712 , the potential on floating gate structure  924 , and the positive voltage on metal structure  932  set up the second electric field to extend from detection structure  710  and metal structure  712  to floating gate structure  924  and metal structure  932 . Thus, the second electric field of gas detector  900  can be significantly larger than the second electric field of gas detector  700 . 
         [0118]    In accordance with the present invention, the electric fields significantly enhance the collection of gas molecules. As with gas detector  700 , the first electric field transports polarized gas molecules and positively ionized gas molecules to the exposed surface of detection structure  710 . When a gas molecule hits the exposed surface of detection structure  710 , the gas molecule can bounce away from or stick to the exposed surface of detection structure  710 . The first electric field, however, improves the sticking coefficient of the gas molecules that hit the exposed surface of detection structure  710 , thereby reducing the number of gas molecules that bounce off the exposed surface of detection structure  710 . 
         [0119]    Due to the high permeability of the material used to form detection structure  710 , a number of gas molecules that stick to the exposed surface of detection structure  710  are absorbed by detection structure  710 . The first electric field also assists in the absorption of the gas molecules into detection structure  710 . The gas molecules that stick to detection structure  710  and are absorbed into detection structure  710  change the work function of the material used to form detection structure  710  which, in turn, has the effect of placing a positive charge on detection structure  710 . 
         [0120]    Some of the gas molecules that are absorbed into detection structure  710  migrate through detection structure  710  into dielectric layer  708  under the influence of the second electric field. However, because the second electric field in gas detector  900  can be much larger than the second electric field in gas detector  700 , the number of gas molecules that migrate into dielectric layer  708  is greatly enhanced. The vertical alignment of the polarized gas molecules and the positively ionized gas molecules in dielectric layer  708  have the effect of placing a positive charge in dielectric layer  708 . After migrating into dielectric layer  708 , the third electric field migrates the gas molecules towards floating gate structure  924 . 
         [0121]    Thus, the first electric field transports polarized gas molecules and positively ionized gas molecules to the exposed surface of detection structure  710 , improves the sticking coefficient of the gas molecules to detection structure  710 , and assists in the absorption of the gas molecules into detection structure  710 . In addition, the second electric field migrates the gas molecules into dielectric layer  708 , while the third electric field migrates the gas molecules towards floating gate structure  924 . 
         [0122]    During the measurement step in  814 , gas detector  900  determines the number of collected gas molecules in the same manner and using the same bias voltages as gas detector  700 , except that metal structure  932  is also grounded. During the reset step in  816 , gas detector  900  is reset in the same manner and using the same bias voltages as gas detector  700 , except that the voltages placed on control gate  242 , metal structure  712 , and metal structure  932  are selected to reverse the first, second, and third electric fields, thereby pulling the gas molecules out of dielectric layer  708  and detection structure  710  and transporting the gas molecules away from detection structure  710 . During the check step in  818 , gas detector  900  determines if the control gate bias voltage needs to be reset in the same manner and using the same bias voltages as gas detector  700 , except that metal structure  932  is also grounded. 
         [0123]      FIGS. 10A-10C  show views that illustrate an example of a gas detector  1000  in accordance with the present invention.  FIG. 10A  shows a plan view.  FIG. 10B  shows a cross-sectional view taken along line  10 B- 10 B of  FIG. 10A , while  FIG. 10C  shows a cross-sectional view taken along line  10 C- 10 C of  FIG. 10A . Gas detector  1000  is similar to gas detector  300  and, as a result, utilizes the same reference numerals to designate the structures which are common to both detectors. 
         [0124]    As shown in  FIGS. 10A-10C , gas detector  1000  differs from gas detector  300  in that gas detector  1000  includes a metal grid  1010  that touches the top surface of second dielectric layer  136  and lies over window opening  140 . Metal grid  1010  can be implemented with a conventional metal trace material that has no or a very low permeability to the gas species to be detected. 
         [0125]    Optionally, metal grid  1010  can be implemented with a catalyzing metal, such as platinum or palladium. When metal grid  1010  is implemented with a catalyzing metal, the catalyzing metal grid  1010  can function as a reduction catalyst or an oxidization catalyst. For example, a catalyzing metal grid  1010  can oxidize carbon monoxide to form carbon dioxide. 
         [0126]      FIG. 11  shows a flow chart that illustrates an example of a method of operating gas detector  1000  in accordance with the present invention. As shown in  FIG. 11 , the method begins with a calibration step in  1110  that determines a baseline current for resistive structure  310 . Gas detector  1000  is calibrated in the same manner and using the same bias voltages as gas detector  300 , except that metal grid  1010  is grounded during the calibration step. 
         [0127]    Once the baseline current has been determined, the method moves to a collection step in  1112  to collect gas molecules for a predetermined time. Gas detector  1000  collects gas molecules in the same manner and with the same bias voltages as gas detector  300 , except that a large positive voltage, such as 100V, is applied to metal grid  1010  during the collection step to set up an electric field from metal grid  1010  to substrate  110 . 
         [0128]    In accordance with the present invention, the electric field significantly enhances the collection of gas molecules. The electric field transports polarized gas molecules and positively ionized gas molecules to the exposed surface of resistive structure  310 . When a gas molecule hits the exposed surface of resistive structure  310 , the gas molecule can bounce away from or stick to the exposed surface of resistive structure  310 . The electric field, however, improves the sticking coefficient of the gas molecules that hit the exposed surface of resistive structure  310 , thereby reducing the number of gas molecules that bounce off the exposed surface of resistive structure  310 . 
         [0129]    Due to the high permeability of the material used to form resistive structure  310 , a number of gas molecules that stick to the exposed surface of resistive structure  310  are absorbed by resistive structure  310 . The electric field also assists in the absorption of the gas molecules into resistive structure  310 . The gas molecules that stick to resistive structure  310  and are absorbed into resistive structure  310  change the conductivity of the material used to form resistive structure  310 . 
         [0130]    Some of the gas molecules that are absorbed into resistive structure  310  migrate through resistive structure  310  into dielectric layer  312  under the influence of the electric field. These gas molecules, however, have no effect on the conductivity of resistive structure  310  and, therefore, can be ignored. 
         [0131]    Returning again to  FIG. 11 , after the predetermined period of time has ended, the method moves to a measurement step in  1114  to determine the number of gas molecules collected by gas detector  1000 . Gas detector  1000  determines the number of collected gas molecules in the same manner and using the same bias voltages as gas detector  300 , except that metal grid  1010  is grounded during the measurement step. 
         [0132]    Once the number of collected gas molecules has been determined, the method moves to an erase step in  1116  to reset gas detector  1000 . Gas detector  1000  is erased by reversing the electric field for a predefined time. For example, the electric field can be reversed by electrically floating resistive structure  310 , grounding p− substrate  110 , and placing a large negative voltage, such as −100V, on metal grid  1010 . 
         [0133]    These bias conditions reverse the direction of the electric field which, in turn, pull the gas molecules out of resistive structure  310  and dielectric layer  312 , and transport the gas molecules away from resistive structure  310 . Thus, in addition to significantly enhancing the collection of gas molecules, the present invention also erases gas detector  1000 . 
         [0134]    After the predefined time, the method moves to a check step in  1118  to check the baseline current for resistive structure  310 . The baseline current for resistive structure is checked by grounding substrate  110 , applying the set of voltages to the opposite sides of resistive structure  310 , and then measuring the current through resistive structure  310 . 
         [0135]    When the current is equal to or within an error tolerance of the baseline current associated with the set of voltages, the method returns to the collection step in  1112  to perform another test. On the other hand, when the current is greater than the error tolerance, the method returns to the calibration step in  810  to determine a new baseline current for resistive structure  310 . Thus, the check step in  1118  allows the baseline current to be adjusted to account for any gas molecules that were not removed from resistive structure  310  and dielectric layer  312 , thereby ensuring that the original sensitivity of gas detector  1000  is maintained. 
         [0136]      FIGS. 12A-12F  show cross-sectional views that illustrate a method of forming gas detector  400  in accordance with the present invention. As shown in  FIG. 12A , the method utilizes a conventionally-formed structure  1210 . Structure  1210 , in turn, is similar to gas detector  100  and, as a result, utilizes the same reference numerals to designate the elements which are common to both structures. 
         [0137]    As further shown in  FIG. 12A , the method begins by forming a patterned photoresist layer  1240  on the top surface of second dielectric layer  136 . Patterned photoresist layer  1240  is formed in conventional manner, which includes depositing a layer of photoresist, projecting a light through a patterned black/clear glass plate known as a mask to form a patterned image on the layer of photoresist, which softens the photoresist regions exposed by the light, and removing the softened photoresist regions. 
         [0138]    As shown in  FIG. 12B , after patterned photoresist layer  1240  has been formed, the exposed regions of second dielectric layer  136  and the underlying regions of first dielectric layer  130  are etched to form a window  1242  that exposes the top surface of gate  124 . Following this, patterned photoresist layer  1240  is removed. 
         [0139]    As shown in  FIG. 12C , following the removal of the patterned photoresist layer  1240 , a layer of sacrificial material is deposited on second dielectric layer  136  and the exposed surface of gate  124  to fill up window  1242 . Once the sacrificial material has been deposited, the sacrificial material is planarized in a conventional manner to form a sacrificial region  1244 . 
         [0140]    As shown in  FIG. 12D , after sacrificial region  1244  has been formed, a metal layer  1246  is deposited on second dielectric layer  136  and sacrificial region  1244 . Next, a patterned photoresist layer  1250  is formed on the top surface of metal layer  1246  in a conventional manner. As shown in  FIG. 12E , after patterned photoresist layer  1250  has been formed, metal layer  1246  is etched to form a metal grid  1252 . Following this, patterned photoresist layer  1250  is removed. As shown in  FIG. 12F , following the removal of patterned photoresist layer  1250 , sacrificial region  1244  is removed in a conventional manner with a wet etchant that is highly selective to second dielectric layer  136 , first dielectric layer  130 , and gate  124  to form gas detector  400 . 
         [0141]    Heating element  414  shown in  FIGS. 4D and 4E  can be formed following the conventional formation of the source and drain regions  116  and  118  of transistor  114  (which may or may not include siliciding the top surface of the source and drain regions  116  and  118 ). The process begins by forming isolation layer  416  on shallow trench isolation region STI, the source and drain regions  116  and  118 , gate  124 , and side wall spacer  126  in a conventional manner. 
         [0142]    Next, a layer of conductive material is conventionally formed on isolation layer  416 . Following this, the layer of conductive material is masked, and then etched using conventional steps to form heating element  414 . After this, the process continues in a conventional manner with the formation of first dielectric layer  130 . 
         [0143]    Gas detector  600  is formed in the same manner as gas detector  400 , except that structure  1210  utilizes a conventionally-formed NMOS EEPROM transistor in lieu of NMOS transistor  114 . (Inter-gate dielectric  630 , which can be formed before or after side wall spacer  126  has been formed, is illustrated as being formed after side wall spacer  126  has been formed.) The floating gate of the NMOS EEPROM transistor is formed from a material that has a high permeability to the gas species to be detected. Further, heavily doped region  634  shown in  FIG. 6D  can be formed in a conventional manner at the same time that the source and drain regions  116  and  118  are formed. 
         [0144]    In addition, programming gate  644  shown in  FIG. 6E  can be formed at the same time that control gate  632  is formed. (Conventionally, a mask is formed and patterned on a second layer of polysilicon (poly2) to define control gate  632 . The mask can also be patterned to define programming gate  644  so that when the poly2 layer is etched to form control gate  632 , programming gate  644  is also formed at the same time.) 
         [0145]    Further, heavily doped region  642  shown in  FIG. 6E  can be formed at the same time that the source and drain regions  116  and  118  are formed. In addition, notched region  640  in gate dielectric  122  can be formed in a conventional manner, such as by masking and etching gate dielectric  122 . 
         [0146]    Heating element  650  shown in  FIGS. 6F and 6G  can also be formed at the same time that control gate  632  is formed. (The mask used to define control gate  632  can also be patterned to define heating element  650 . Thus, when the poly2 layer is etched to form control gate  632 , heating element  650  is also formed at the same time. Further, the mask can be patterned to define control gate  632 , programming gate  644 , and heating element  650  at the same time.) 
         [0147]    Gas detector  1000  is also formed in the same manner as gas detector  400 , except that structure  1210  utilizes a conventionally-formed resistive structure in lieu of NMOS transistor  114 . The resistive structure is formed from a material that has a high permeability to the gas species to be detected. 
         [0148]      FIGS. 13A-13K  show cross-sectional views that illustrate a method of forming gas detector  700  in accordance with the present invention. As shown in  FIG. 13A , the method utilizes a conventionally-formed structure  1310 . Structure  1310 , in turn, is similar to gas detector  200  and, as a result, utilizes the same reference numerals to designate the elements which are common to both structures. 
         [0149]    As further shown in  FIG. 13A , the method begins by forming a non-conductive layer  1312  in the same manner that third dielectric layer  250  is formed, except that non-conductive layer  1312  is formed to be thicker than third dielectric layer  250 . Next, as shown in  FIG. 13B , non-conductive layer  1312  is planarized in a conventional manner. 
         [0150]    Following this, a conductive layer  1314  is deposited on the top surface of non-conductive layer  1312 . Conductive layer  1314  is implemented with a material that has a high permeability to the gas species to be detected. For example, lanthanum oxide, tin oxide, indium oxide, and zink oxide are materials which have a high permeability to carbon dioxide. Other materials are well known to have high permeabilities to other gas species. 
         [0151]    Next, a patterned photoresist layer  1316  is formed on the top surface of conductive layer  1314  in a conventional manner. As shown in  FIG. 13C , after patterned photoresist layer  1316  has been formed, conductive layer  1314  is etched to form detection structure  710  and metal structure  712  shown in  FIGS. 7A-7D . Following this, as shown in  FIG. 13D , patterned photoresist layer  1316  is removed to form gas detector  700 . 
         [0152]    As shown in  FIG. 13E , if metal structure  712  is formed from a material that has no or a very low permeability to the gas species to be detected, a patterned photoresist layer  1320  is formed on the top surface of conductive layer  1314  in a conventional manner in lieu of patterned photoresist layer  1316 . As shown in  FIG. 13F , after patterned photoresist layer  1320  has been formed, conductive layer  1314  is etched to form an etched layer  1322  with an opening  1324 . Following this, as shown in  FIG. 13G , patterned photoresist layer  1320  is removed 
         [0153]    Next, as shown in  FIG. 13H , a conductive layer  1326  that has no or a very low permeability to the gas species to be detected is deposited on etched layer  1322  and non-conductive layer  1312  to fill up opening  1324 . As shown in  FIG. 131 , conductive layer  1326  is next planarized to remove conductive layer  1326  from the top surface of etched layer  1322  and form a conductive layer  1328  that includes a first region  1330  of high permeability material and a second region  1332  of a no or very low permeability material. 
         [0154]    Following this, a patterned photoresist layer  1334  is formed on the top surface of conductive layer  1328  in a conventional manner. As shown in  FIG. 13J , after patterned photoresist layer  1334  has been formed, conductive layer  1328  is etched to form detection structure  710  and metal structure  712  shown in  FIGS. 7A-7D . Following this, as shown in  FIG. 13K , patterned photoresist layer  1334  is removed to form gas detector  700 . 
         [0155]    In addition, programming gate  744  shown in  FIG. 7E  can be formed at the same time that control gate  242  is formed. (Conventionally, a mask is formed and patterned on a second layer of polysilicon (poly2) to define control gate  242 . The mask can also be patterned to define programming gate  744  so that when the poly2 layer is etched to form control gate  242 , programming gate  744  is also formed at the same time.) 
         [0156]    Further, heavily doped region  742  shown in  FIG. 7E  can be formed at the same time that the source and drain regions  116  and  118  are formed. In addition, notched region  740  in gate dielectric  122  can be formed in a conventional manner, such as by masking and etching gate dielectric  122 . Gas detector  900  is formed in the same manner as illustrated in  FIGS. 13A-13K , except that the steps illustrated in  FIGS. 13A-13K  are also utilized to form metal structures  930  and  932 . 
         [0157]    Thus, gas detectors in accordance with the present invention have been described which utilize metal structures to set up electric fields that substantially enhance the collection of gas molecules, and also provide a self contained means for removing collected gas molecules from the gas detector. 
         [0158]    In addition, methods of operating the gas detectors have been described. In steps  510 ,  810  and  1110 , the methods determine a magnitude of a current, such as a drain-to-source current and a resistor current, to define a first current magnitude. After the first current magnitude has been defined, in steps  512 ,  812 , and  1112 , the methods set up an electric field to transport gas molecules to a detection structure, such as gate  124  of gas detector  400 , floating gate  624  of gas detector  600 , detection structure  710  of gas detectors  700  and  900 , and resistive structure  310  of gas detector  1000 . 
         [0159]    Following this, in steps  514 ,  814 , and  1114 , the methods remove the electric field after a predetermined period of time, and determine the magnitude of the current to define a second current magnitude. The methods determine that gas molecules were transported to the detection structure during the predetermined period of time if a difference between the second current magnitude and the first current magnitude is greater than an error tolerance. As described above, the difference is used to then determine the concentration of the gas species. 
         [0160]    After this, in steps  516 ,  816 , and  1116 , the methods set up a reverse electric field to transport gas molecules away from the detection structure for a predefined time, and then remove the electric field after the predefined time. Once the reverse electric field has been removed, in steps  518 ,  818 , and  1118 , the methods determine the magnitude of the current to define a third current magnitude. 
         [0161]    When the difference between the third current magnitude and the first current magnitude is greater than an error tolerance, the methods return to step  510 ,  810  and  1110  to determine the magnitude of the current to redefine the first current magnitude. When the difference between the third current magnitude and the first current magnitude is less than the error tolerance, the methods return to step  512 ,  812  and  1112  to perform another test. 
         [0162]    It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.