WO1999066319A1

WO1999066319A1 - Detector for one or several polluting gases and use thereof

Info

Publication number: WO1999066319A1
Application number: PCT/FR1999/001395
Authority: WO
Inventors: Gérard GUILLAUD; Jacques Simon
Original assignee: Grimm R & D, S.A.R.L.
Priority date: 1998-06-12
Filing date: 1999-06-11
Publication date: 1999-12-23
Also published as: CA2334849A1; AU4049599A; EP1086370A1; FR2779826A1; JP2002518673A; FR2779826B1

Abstract

The invention concerns a detector in the form of a field-effect transistor (13) arranged in a chamber (14). The transistor (13) electroattractive part is formed by a macromolecular material layer (4). The latter is exposed to a mixture containing at least one polluting gas, such as O3 and NO2, which causes it to be doped. A measuring device (10) comprises means for measuring the intensity of the current (IDS) which circulates between the transistor drain electrode (5) and source electrode (6) at least at one given time after the detector has been initialised, said at least polluting gas presence causing the layer (4) material to be doped depending on the concentration in polluting gas, the current (IDS)intensity and the intensity gradient being related to the concentration in polluting gas.

Description

DETECTOR FOR ONE OR MORE POLLUTANT GASES AND USE THEREOF

The present invention relates to a detector capable of detecting and measuring the quantity of at least one polluting gas, in particular the quantity of ozone or other polluting species, including N0 ₂ , present in a gas. The invention is intended both for industrial and environmental control uses as well as in the laboratory field. The object of the invention is to facilitate this detection and this measurement. The present invention relates more particularly to a detector of at least one polluting gas, comprising a field effect transistor comprising a source electrode and a drain electrode connected to each other by an electroactive part which consists of a layer of molecular material. , the source electrode and the drain electrode of this transistor being connected to a device for measuring the intensity of the current flowing between the drain electrode and the source electrode of the transistor when the layer of molecular material is subject to the influence of said at least one polluting gas.

Document DD 287 788 discloses a detector comprising a field effect transistor which uses a layer of metallophthalocyanine electrically connecting its drain and source electrodes. This document does not disclose a measurement device connected to the transistor and which is capable of carrying out measurements in two well-defined times, namely:

- a first stage, during which the transistor is exposed to a polluted atmosphere, - and a second stage of initialization of the detector, during which one or more of the polluting gases contained in this atmosphere is destroyed. The object of the detector according to the present invention is, among other things, to include such a measuring device.

To this end, the detector according to the invention comprises means for measuring the intensity of the current flowing between the drain electrode and the source electrode of the transistor at least at a given instant after initialization of the detector, the presence of said at least one polluting gas causing doping of the molecular material according to the concentration of polluting gas, the intensity and the slope of the growth of said current being linked to the concentration of polluting gas.

In preferred embodiments of the invention, it is optionally possible to have recourse in addition to one and / or the other of the following provisions: the electrical measurements are carried out by application of an electrical voltage between the electrode drain and the source electrode of the transistor and an electric voltage between the gate electrode and the source electrode of the transistor, these voltages being chosen from the group comprising direct voltages, voltage ramps, voltages impulse and alternating voltages;

- the transistor includes:

- a conductive substrate forming the gate of the transistor,

an insulating layer deposited on the substrate, and

- said layer of molecular material deposited on said insulating layer to ensure conduction between the source electrode and the drain electrode of the transistor; - the drain and source electrodes are deposited on the layer of molecular material by techniques chosen from the group comprising masking and screen printing and associated techniques;

- the drain and source electrodes are manufactured by microlithography; - The molecular units constituting the molecular material are substituted by atoms, paraffinic chains, polymer chains such as polyoxyethylene chains, anionic or cationic groups; the layer of molecular material is optionally deposited after dissolving in the solvent the molecular units constituting said layer, in particular by vacuum sublimation, by spinning method, by screen printing, by Langmuir Blodgett method, by photopolymerization after grafting or not the insulating layer, and associated techniques;

- the insulating layer consists of mineral insulators (Si0 ₂ , Si ₃ N _4, etc.) or else by thin layers of polymers, Langmuir Blodgett layers or else molecules having electrical insulating properties grafted onto the 'gate electrode of the transistor;

- The insulating layer is deposited by photopolymerization after grafting or not of said conductive substrate; - The molecular material is chosen in particular from the group comprising metallized or unmetallic phthalocyanine derivatives, metallized or unmetallized porphyrins, azaporphyrins, porphyrazines, rare earth bisphthalocyanines, their mixtures, and the superposition of several layers of them ; by "derivatives of metallized or non-metallized phthalocyanines" is meant the derivatives of formula PcM in which M represents in particular H ₂ , Cu, Ni, Zn, Fe and Pc represents the phthalocyanine. This type of phthalocyanine derivative is commercially available at low cost.

It has been discovered that the sensor properties of phthalocyanine derivatives depend on multiple parameters. Three of them are:

- the ability of molecular units to reduce or oxidize more or less reversibly,

- the propensity of the central metal to form adducts with various small molecules (0 ₂ , C0, EtOH, H ₂ 0 ...), - the morphology of the thin film used: polycrystallinity, grain size, type of defects, amorphous nature or almost amorphous.

The diffusion of gaseous species within the layer of molecular material is improved when it is of the amorphous or quasi-amorphous type instead of being polycrystalline. As a result, the response time of the detector is reduced very favorably.

Indeed, it turns out that polycrystalline materials lead to kinetics which are generally much slower for the adsorption and desorption of gases. As a result, the response times of the sensors are longer. It is possible to reduce this drawback by using mixtures of phthalonitrile and of phthalonitrile substituted by heteroatoms, for example fluorine, in order to promote the formation of thin amorphous (or almost amorphous) layers of molecular materials:

the detector comprises a light source for subjecting the transistor to a photonic flux during detection and measurement, said light source being intended to increase the sensitivity of the detector; - The detector includes a heat source to increase its temperature and promote the migration of the constituent species within the molecular material;

the transistor is arranged in an enclosure comprising controlled means for admitting at least one polluting gas and an internal coating at least part of which comprises a material capable of reacting with said at least one polluting gas to be detected, said controlled means being actuated, on the one hand, in intake to allow the introduction of said at least one polluting gas inside the enclosure and, on the other hand, in intake shutdown to prevent the introduction of said at least one polluting gas inside the enclosure and thus promoting the adsorption or chemical destruction of said at least one corresponding polluting gas, this last step constituting the initialization phase;

the internal coating material of the enclosure is chosen from the group comprising stainless steel, an alloy of aluminum, copper and magnesium among others, polymeric materials and comprises on at least part of its surface a compound capable of reacting with said at least one polluting gas to be detected, said compound being chosen as a function of said at least one gas to be detected from the group comprising in particular diphenylbenzidine, lead salts and indigo. Thus, indigo can be used, in the case where said at least one gas to be detected is ozone, diphenylbenzidine, in the case where said at least one gas to be detected is nitrogen dioxide, lead salts , in the case where said at least one gas to be detected is H ₂ S. It will of course be possible to use other compounds depending on the nature of the polluting gas or gases to be detected, such as, for example, Zr (OH) complexes ₄ / quinalizarin to detect HF, derivatives of Cr VI to detect aniline, NO or MeOH, desiccants such as molecular sieves, CaCl ₂ to detect H ₂ 0, solid acids such as R ₃ SiOH to detect amines, N ₂ H ₄ or ΦNHNH ₂ to detect C0 ₂ , 1 'o-toluidine to detect Cl ₂ , pyridine to detect C1CN, KMn0 ₄ to detect Me ₂ S, Cu II salts to detect RSH (R being alkyl), Ti III salts to detect 0 ₂ or ΦNH ₂ to detect C0C1 ₂ ;

- the controlled elements for admitting said at least one polluting gas are chosen from the group comprising a movable cover secured to the enclosure, and an electric pump connected to the enclosure;

- The enclosure further comprises elements generating convection current which are put into operation during the cut-off of the admission controlled elements of said at least one polluting gas to accelerate the flow of said at least one polluting gas towards the internal coating of said enclosure;

- The enclosure is connected to elements for the selective destruction of said at least one polluting gas; - Said at least one polluting gas is an element constituting a mixture of gases, this element being chosen from the group comprising in particular ozone, HF, CO, C0 ₂ , S0 ₂ , aniline, AsR ₃ R being a alkyl group, NO, N0 ₂ , Cl ₂ , C1CN, DMF, Me ₂ S, H ₂ S, RSH R being an alkyl group, H ₂ 0 ₂ , C0C1 ₂ , CS ₂ NO or their mixtures, toxic gases and dangerous gas.

The invention also relates to a method for measuring the concentration of at least one polluting gas constituting a mixture of gases captured by the detector according to the invention, method which is characterized in that it comprises the steps consisting at : a) admitting into the detector this mixture of gases between a first instant and a second instant following said first instant, and measuring the intensity of the current which circulates between the drain electrode and the source electrode of the transistor, between the first and second instants, b) cut off this admission between the second instant and a third instant following said second instant, and measure the intensity of the current flowing between the drain electrode and the source electrode of the transistor, in l time interval between the second and third instants, this interval constituting the initialization of the detector during which said at least one polluting gas contained in said gas mixture is selectively destroyed, c) admitting said mixture again into the detector of gas between the third instant and a fourth instant following said third instant, and measure the intensity of the current flowing between the drain electrode and the electrode source of the transistor, between the third and fourth instants, to deduce the concentration of said at least one polluting gas selectively destroyed in step b).

In an alternative embodiment of the method according to the invention, step b) is followed by a step consisting in comparing the value of the intensity of the current which flowed in the transistor between the first and second instants with a value of reference function of said at least one polluting gas, in order to determine the concentration of said at least one polluting gas.

In another variant embodiment of the method according to the invention,

- in step a) a value of the derivative with respect to time is calculated of the intensity of the current which has circulated between the drain electrode and the source electrode of the transistor between the first and second instants,

in step b) a value of the derivative with respect to time is calculated of the intensity of the current which has flowed between the drain electrode and the source electrode of the transistor between the second and third instants, and compares the derivatives calculated respectively in steps a) and b) with a reference value as a function of said at least one polluting gas in order to determine the concentration of said at least one polluting gas.

Preferably, the measurements carried out are repeated successively a plurality of times.

In preferred embodiments of the invention, it is possible optionally to have recourse to one and / or the other of the following arrangements:

- the voltage between the drain electrode and the source electrode of the transistor is applied, while the derivative with respect to time is determined of the intensity of the current flowing between the drain electrode and the electrode source of the transistor, said voltage being continuous and being equal to or different from the voltage between the gate electrode and the source electrode of the transistor;

- a voltage slot is applied during a period varying from zero to a few minutes between the drain electrode and the source electrode of the transistor, while the intensity of the current flowing between the drain electrode and l the source electrode of the transistor is measured according to a previously chosen sampling; a first voltage is applied between the first and second instants, a second voltage, different from the first voltage, is applied between the second and third instants, these steps being repeated periodically over time and the Fourier transform of the signal delivered by the detector being carried out;

- when the voltage between the drain electrode and the source electrode of the transistor is equal to the voltage between the gate electrode and the source electrode of the transistor, and these two voltages are applied for a short period of time, the current flowing between the drain electrode and the source electrode of the transistor appears after a time characteristic of the concentration of polluting gas;

- after application of a first voltage between the gate electrode and the source electrode of the transistor and measurement of the intensity of the current which circulates between the drain electrode and the source electrode of the transistor, a second voltage is applied between the gate electrode and the source electrode of the transistor so that said current intensity returns to zero, this second voltage having a sign opposite to said first voltage. Other characteristics and advantages of the present invention will appear during the following description of several of its embodiments, given by way of nonlimiting example, with reference to the accompanying drawings. In the drawings: - Figure 1 is a schematic view of a molecular transistor connected to a measuring device, this assembly being usable in a detector according to a first embodiment of the invention;

- Figure 2 is a view similar to Figure 1, for a second embodiment of the detector manufactured by microlithography; - Figures 3a and 3b are top views respectively representing two embodiments envisaged for the source and drain regions of a transistor usable in the detector of Figures 1 and 2; - Figure 4 is a partial perspective view of a detector according to a third embodiment of the invention;

- Figures 5 to 7 are schematic views similar to Figure 4, respectively for the fourth, fifth and sixth embodiments of the detector;

- Figure 8 is a partial perspective view of differential measurement means: a first detector according to the invention is in contact with an atmosphere of at least one polluting gas, while a second detector according to the invention is in contact with the same atmosphere in which one or more initial polluting gases have been removed;

FIG. 9 is a curve representing the evolution of the current flowing between the drain and the source of the transistor as a function of time, when the detector is subjected alternately to a confined atmosphere (static covering) and to ambient air (exposure static), and when the detector comprises, as a layer of molecular material, a quasi-amorphous layer of (F ₂ ) _x PcZn, for two consecutive experiments; - Figure 10 is a curve representative of the evolution of the current flowing between the drain and the source of the transistor as a function of time, when ozone is detected by the molecular transistor illustrated in Figure 2;

FIGS. 11 and 12 show comparatively the evolution of the current flowing between the drain and the source of the transistor as a function of time and the evolution of the derivative with respect to time of this current as a function of time, when the detector, comprising the transistor illustrated in FIG. 2, is exposed to air enriched with ozone;

- Figure 13 shows the detail of the response time of the derivative with respect to the time of the intensity of the current flowing between the drain and the source of the transistor, when the detector is exposed to a flow of natural air enriched in ozone;

- Figure 14 is a curve representative of the evolution over time of the current flowing between the drain and the source of the transistor as shown in Figure 1, when the detector is exposed to a flow of air enriched in ozone during 2 minutes, then subjected to a confined atmosphere for 8 minutes (static covering);

- Figures 15 and 16 show comparatively the evolution of the current flowing between the drain and the source of the transistor as a function of time, depending on whether the interior surface of the detector enclosure is coated or not with a material intended to neutralize the 'ozone during the initialization phase; - Figure 17 shows the evolution as a function of time of the derivative with respect to the time of the current flowing between the drain and source electrodes of the transistor illustrated in Figure 2 and used in the detector of Figure 4, the exposure of the polluted air detector and the static covering of the detector when the enclosure is closed, both being carried out without gas flow;

- Figure 18 shows the evolution over time of the current flowing between the drain and source electrodes of a transistor as shown in Figure 2, when a detector comprising this transistor is alternately exposed to 100 ppb of N0 ₂ in synthetic air, then in pure synthetic air; - Figure 19 shows the evolution as a function of time of the current flowing between the drain and source electrodes of the transistor, when the detector is exposed to increasing concentrations of N0 ₂ in synthetic air, then subjected to a flux synthetic air;

- Figure 20 shows the evolution as a function of time of the current flowing between the drain and source electrodes of the transistor, when the detector has been exposed to synthetic air containing N0 ₂ , while simultaneously the voltages V _GS = V _DS = -3V have been applied;

- Figure 21 shows the evolution as a function of time of the current flowing between the drain and source electrodes of the transistor, with a mode of application different from that used most commonly, the voltages V _GS = V _DS = -3V being applied in the form of one-second slots;

FIG. 22 shows the evolution as a function of time of the current flowing between the drain and source electrodes of the transistor, the measurement protocol involving a measurement of the response time of the detector; - Figure 23 shows the evolution as a function of time of the current flowing between the drain and source electrodes of the transistor, when the detector comprising the transistor of Figure 2 has a movable cover whose opening is mechanically controlled by a measurement "electric, the figure representing the way in which the properties of the detector are restored by the application of a voltage V ^ of opposite sign to the voltage V _G s applied for the measurement.

Referring to Figure 1, the transistor, designated by the reference numeral 13, is a field effect transistor. It includes: a metallic conductive substrate 1 forming the gate electrode G of the transistor and which is, for example, a highly doped silicon crystal;

one or more insulating layers 2, 3, for example of silicon oxide (Si0 ₂ ) and of silicon nitride (Si ₃ N ₄ ), deposited on the substrate 1;

a layer 4 of molecular material, for example metallophthalocyanine, deposited on layer 2, 3;

- And two drain 5 and source 6 electrodes deposited on layer 4, and constituting the rest of the transistor.

This transistor is connected to a device 7 for measuring the intensity of the current flowing between the drain electrode 5 and the source electrode 6, which will be designated subsequently by I _DS . In this example, this measuring device is a device which measures the intensity I _DS after application of a drain-source voltage V _DS and of a gate-source voltage V _GS , while a gas to be detected is in contact layer 4 of metallophthalocyanine. The voltages V _DS and V _GS generally applied are continuous, but it will be seen in the following description that voltages V _DS and V _G s of another type can be used.

Referring to Figure 2, the transistor 13 shown is obtained by microlithography, which is not the case of the transistor of Figure 1 which is produced by masking. In a similar way to the transistor of figure 1, the transistor represented in figure 2 comprises:

- A metallic conductive substrate 1 forming the gate electrode of the transistor and which is, for example, a highly doped silicon crystal. - an insulating layer 2, for example of silicon oxide (Si0 ₂ ), deposited on the layer 1; an insulating layer 3, for example of silicon nitride (Si ₃ N ₄ ), deposited on the layer 2;

a layer 4 of molecular material, for example metallophthalocyanine, deposited on layer 3; - And two drain 5 and source 6 electrodes placed below the layer 4, and constituting the rest of the transistor.

In this example, the drain 5 and source 6 electrodes are connected respectively by respective connections 8 ′ and 8 to the terminals of an electrical circuit constituted by an electrical supply 9 and a device 10 for measuring I _DS . The power supply 9 can deliver, for example, a DC, AC, pulse voltage, or even a voltage ramp. The gate electrode 1 is, for its part, connected, via a connection 11, to a supply 12 which makes it possible to vary the gate-source voltage V _GS . This gate electrode 1 can also be directly connected to earth. The transistor 13 thus formed is sensitive to the action of one or more polluting gases. In general, the polluting gas or gases admitted into the detector are elements constituting a mixture of gases, these elements being chosen from the group comprising in particular ozone, HF, CO, C0 ₂ , S0 ₂ , l aniline, AsR ₃ R being an alkyl group, NO, N0 ₂ , Cl ₂ , C1CN, DMF, Me ₂ S, H ₂ S, RSH R being an alkyl group, H ₂ 0 ₂ , C0C1 ₂ , CS ₂ , NO or their mixtures.

Layer 2 can be obtained by oxidation of the upper layer of substrate 1. It can also be obtained by deposition. The thickness of layer 2 is for example 500 Å. Layer 3 of silicon nitride can be obtained by an SiH ₄ / NH ₃ plasma or by chemical vapor deposition. Si ₃ N ₄ can also be deposited by spraying. The thickness of layer 3 is for example 600 Å. The layer 4 of metallophthalocyanine has a thickness equal to for example 500 Å.

The drain 5 and source 6 electrodes are respectively constituted, for example, by layers of gold each having a thickness of, for example, 500 Å, themselves deposited on layers of chromium each having a thickness of, by example, 100 Â, to improve contacts.

FIGS. 3a and 3b show preferred examples of embodiment of the drain and source electrodes 5 and 6.

In the example of FIG. 3a, the electrodes 5 and 6 are interdigitated. Each electrode comprises for example sixteen fingers whose length 1 is approximately 600 micrometers. Consequently, the width of the conduction channel, designated by, is of the order of 20,000 micrometers. The interelectrode distance L is, in this example, 10 micrometers. Under these conditions, the W / L ratio of the transistor formed is of the order of 2000.

FIG. 3b shows another embodiment in which the length 1 = W of the two elongate electrodes 5 and 6 is

3000 micrometers. The interelectrode distance L was obtained for values of 10, 20, 50 or 100 micrometers. The report

W / L is, in this case, 300 maximum.

The embodiments of FIGS. 3a and 3b relate to a fabrication of the microlithographic type transistor. The device shown in Figure 1 is more conventional and only requires the use of a cover

(wire, ribbon or mask). Referring to Figures 1 and 2, the measuring device according to the invention comprises a circuit for measuring the intensity of the current flowing between the drain 5 and the source 6 of the transistor when the layer 4 is subjected to the influence of one or more polluting gases. In practice, the circuit and the electrical means are produced by a microcontroller which will carry out an analog-digital conversion of the value of the measured current and which will include a clock, counters, as well as a prerecorded program for delivering a measurement signal at regular interval. or irregular.

If necessary, the microcontroller will be perfected and capable of processing the evolution of the measurements over time. These signal processing techniques are of known type.

Referring now to Figure 4, we will describe an advantageous embodiment of a detector of the invention and the measurement method associated with this detector.

In FIG. 4, the detector comprises a box 14 which contains a transistor 13 conforming to that of FIG. 1 or 2, an electronic circuit for processing information (not shown), the batteries (not shown) of the power supply electric and a liquid crystal screen (not shown). As an indication, the box 14 has for example overall dimensions equal to approximately 10 * 5 * 3 cm ³ .

The layer of molecular material 4 forms the upper face of the transistor 13, capable of being in direct contact with the atmosphere. The box 14 is equipped with a removable cover 15 which is, in this example, pivotable relative to the box 14. The box 14 has an inner surface 14a coated with a material capable of reacting with one or more of the polluting gases to be detected, such as for example Duralumin (known alloy of aluminum, copper and magnesium), stainless steel, polymeric materials. Said material can also comprise on a part of its surface a compound capable of reacting with one or more of the polluting gases to be detected, this compound being chosen as a function of the gas or gases to be detected.

One of the measurement principles is as follows (Figure 17). At a first instant tl, the opening of the box 14 is released to allow the admission of a gaseous mixture containing one or more polluting gases, such as for example ozone. The opening of the box 14 remains clear until a second instant t2, the difference t2-tl being equal to approximately 2 min in this example. The transistor 13 is subjected, during this time interval, to the influence of the polluting gas or gases, the exposure to this or these gases being for example static, that is to say without creation of flux thereof. During this time t2-tl, the measuring device 10 performs the measurement of Is and calculates dI _D s / dt. This constitutes the first phase of detection. At time t2, the cover 15 is folded so as to close the opening of the box 14 and the measurement of I _DS continues until a third time t3. In this time interval t3-t2, one or more of the polluting gases is destroyed or adsorbed by a constituent element of the chamber 14. For ozone, it may be duralumin or polymer products. This duration t3-t2 constitutes the initialization of the detector. This initialization takes approximately 8 minutes in this example. In a third phase starting at the third instant t3 with an opening of the cover 15 and ending at a fourth instant t4 with the closing of the cover 15, the transistor is again exposed to the gas mixture in its entirety. In the time interval t4-t3, we measure I _DS , dI _DS / dt or similar electrical parameters. The quantity of the polluting gas or gases is given either by I _DS or dI _D s / dt at a given instant, or by the maximum value of dI _DS / dt, or by a parameter logically arising from these measurements. A preliminary calibration makes it possible to give a quantitative value to the content of pollutant gas (s).

The steps described above can then be repeated for a plurality of cycles, better accuracy can be obtained by averaging for various measurements.

An example of use of the above detector consists in placing the box 14 in a glove box which is supplied with ambient air charged with ozone (about 40 ppb). Furthermore, a lamp 16 is disposed above the box 14 so as to illuminate the thin layer of metallophthalocyanine of the transistor 13. The use of such a lamp has the advantage of causing an increase in the sensitivity of the detector and also to broaden the range of products detected. As an indication, the lamp 16 provides approximately an illumination corresponding to 100 milliwatt per cm ² .

As can be seen in FIG. 4, a heating resistor 17 can also be used so as to bring the detector to a given temperature, for example of the order of 100 ° C., this in order to favor the migration of the constituent species of the molecular material of layer 4. Preferably, the heating resistor 17 consists of a resistor etched and electrically insulated on the rear face of the substrate 1 of the transistor 13. However, other geometries already known can be used for the heating resistor (radiator) . The use of a platinum resistance allows to obtain, in addition to the measurement of I _DS the temperature measurement of the detector. The results may be available on a conventional interface.

In an alternative embodiment shown diagrammatically in FIG. 5, an enclosure is used in the form of a tube with walls made, for example, of polymeric materials, the tube having, in this example , a capacity of 25 ml. A controlled element for the admission of one or more polluting gases, for example an electro-pump P, is connected downstream (in solid lines in FIG. 5) or upstream (in dotted lines in FIG. 5) of the tube 14 The transistor 13 is placed inside the tube 14.

During a period which is worth, in this example, approximately 2 min, a flow of polluted air, of which it is desired for example to titrate the ozone, is admitted into the tube 14 by actuation of the electro-pump P, and the device 10 performs the measurement of I _D s (and then, if necessary, the derivative dI _DS / dt). As an indication, this air flow is between 0.4 1 / min and 1 1 / min. Unlike the previous example shown in FIG. 4, the exposure of the detector to polluted air is dynamic here, taking into account the electro-pump P. Once the 2 min have elapsed, the pump P is stopped for approximately 8 min in this example, duration during which the ozone is destroyed on the walls of the tube 14. This step is comparable to the static covering of the box of FIG. 4. During this duration, the measuring device 10 performs the measurement of I _D s (and then the derivative dI _DS / dt). The process described above is then repeated for a plurality of cycles.

In the example shown in FIG. 6, the detector is identical to that represented in FIG. 5, with the difference that a means for generating convection current, such as an agitator or a turbine T, is placed in the tube. 14, so as to accelerate the flow of polluted air inside the tube, only during the moments when the pump P is stopped.

In the example shown in Figure 7, the detector is identical to that shown in Figure 5, with the difference that at least one circuit, consisting of a solenoid valve connected in series with a means of selective destruction of one or more several polluting gases, such as for example a filter, is connected upstream of the tube 14. This type of detector is particularly well suited in the case where it is exposed to a mixture of polluting gases. More particularly in this example, two circuits are provided, one of which is constituted by a solenoid valve VI and a filter IF, and the other is constituted by a solenoid valve V2 and a filter F2. These two circuits are connected upstream of the tube 14 so as to selectively admit into the tube 14 one or more polluting gases constituting the gas mixture to which the transistor 13 is exposed.

Another embodiment of the detector of the invention is now proposed with reference to FIG. 8. According to this figure, two detectors Dl and D2, in accordance with the invention and both identical, receive via a respective inlet orifice 01 , 02, a gas whose ozone content is to be measured. One of the detectors, for example D1, further comprises a filtering device 18. This filtering device 18 comprises for example a filter made of duralumin chips, causing a rapid decomposition of the ozone present in the injected polluting atmosphere. Unlike the detector D1, the detector D2, which is not connected to a filtering device, directly admits, through its orifice 02, the polluting atmosphere as it is injected. The electrical measurement of detector Dl and that of detector D2 are connected in common to a comparator 19 which is intended to normalize the results delivered by the two detectors Dl and D2.

The particularity of the detector of the invention, of which various embodiments have just been described above, resides in the fact that it has a reactivity to the presence of ozone or other polluting gases in the atmosphere. measured. The phenomenon which occurs is a phenomenon of partial oxidation of the layer of molecular material 4 by the polluting gas or gases, which has the effect of boosting the material of said layer 4 and thus constituting a more or less conductive layer, more or less doped depending on the concentration of ozone or other oxidizing species. N-type doping can be envisaged with reducing gases. The phenomenon evolves over time. The invention is characterized in that a thin layer of molecular material constituting the electroactive part of a field effect transistor is used to detect and titrate one or more polluting gases constituting a mixture of gases. The selectivity of detection is ensured by the comparison between a measurement carried out on the gas mixture in its entirety and another carried out on this same gas mixture to which one or more constituents have been removed by adsorption or chemical destruction.

EXAMPLES

EXAMPLE 1: Process for the preparation of phthalocyanine derivatives making it possible to obtain amorphous (or almost amorphous) layers of molecular material.

The procedure in an example was, to synthesize zinc phthalocyanine, to constitute a mixture of 0.96 g phthalonitrile (Mw: 128.13; 7.5 mmol) /1.23 g of 4,5-difluorophthalonitrile (Mw: 164.13; 7.5 mmol) /1.96 g Zn and 18 ml of chloronaphthalene. The theoretical proportions of each of the products can be calculated by assuming equal reactivity for the two types of phthalonitriles.

Mass spectrometry (chemical ionization; NH ₃ ) verified that the expected products were well synthesized. It has been verified that sublimation does not alter the relative proportions of the various constituents of (F ₂ ) _x PcZn. The ultraviolet, visible and infrared absorption spectra show significant differences between the two types of phthalocyanine (PcZn and partially substituted derivative).

PcZn: 344.5 nm; 604 nm; 668.5 nm (F ₂ ) _x PcZn: 339 nm; 599 nm; 662 nm

IR spectra were made on the two previous materials. In particular, characteristic lines of the CF link were noted between 1050 and 1400 cm ^-1 .

X-ray diffraction images were taken on thin films deposited under vacuum of PcZn and (F ₂ ) _x PcZn. The size of the crystallites could be deduced from the width at mid-height of the diffraction lines (taking into account the natural width of the lines). It has thus been found that PcZn leads to a crystallite size of approximately 100 nm, which is in agreement with electron microscopy measurements. In the case of (F ₂ ) _x PcZn, the fact of having several isomers and several products leads to an almost amorphous material. We could later see the importance of this observation on the reactivity of the transistor.

EXAMPLE 2 Evolution over time of the intensity I _DS of the current flowing between the drain electrodes 5 and of source 6 of a transistor such as that shown in FIG. 2 when a thin, almost amorphous layer (F ₂ ) _x PcZn is exposed to ambient air.

The thin layer of molecular material is annealed at 120 ° C for 1 hour and has a thickness whose value is for example equal to 500 Å. The electrical measurements are made in such a way that V _GS = 0, V _DS = -3V. With reference to FIG. 9, it can be seen that the kinetics of obtaining equilibrium is approximately 10 times faster than for a thin layer of unsubstituted polycrystalline PcNi.

EXAMPLE 3 Evolution over time of the intensity I _DS of the current flowing between the drain 5 and source 6 electrodes of a transistor such as that shown in FIG. 2, when ozone is detected.

FIG. 10 represents the evolution of the intensity of the current I _Ds as a function of time, the measurement process having been carried out over 10 cycles.

In solid lines, we can observe the response (source drain current I _D s _. Of the detector of the invention in accordance with the principle of FIG. 5, when it is subjected to the following processing:

- for 2 min, the detector is exposed to filtered air enriched with ozone (part A of the curve in solid line in FIG. 10). The ozone concentration is indicated by the dotted curve (about 275 ppb) which is designated by the reference numeral 20 in Figure 10.

- for 8 min (so-called static rest period), the previous air circulation is interrupted and the enclosure is only in contact with the outside environment through a small diameter outlet orifice. Ozone is destroyed gradually on the walls of the box 14 (part B of the curve in solid lines in FIG. 10).

The measurements are made in direct current with V _GS = V _DS = -3V. The source electrode is grounded.

EXAMPLE 4: Comparison of the current I _D s and of the derivative dlos / dt when the detector comprises the transistor illustrated in FIG. 2 and is based on the principle shown in FIG. 5, this detector being exposed to air enriched in ozone.

Figures 11 and 12 show the evolution over time of I _D s and dI _DS / dt for different concentrations of ozone in air previously filtered on activated carbon. The ozone concentrations are indicated by the dotted curves 21, the right scale indicating the exact ozone contents as measured by a metrological device (environment SA 0 ₃ 41M). During the so-called dynamic exposure of the detector, the air flow is 1.6 l / min. This period is followed by a static recovery phase (8 min) during which the pump is stopped and the flow interrupted. The measurement conditions are such that V _DS = V _GS = -3V and V _s = 0. In the present case, the value of (dlos / dt) ^ "is taken as an indication of the ozone content - ie the maximum of this derivative during the exposure phase. This principle of measurement overcomes some of the limitations linked to the direct measurement of I _D s such as the drift of I _D s as a function of time, or a certain lack of reproducibility.

EXAMPLE 5: Detail of the response time of the detector on the curve representing the evolution over time of dlos / dt, when the detector is exposed to one or more polluting gases. FIG. 13 shows the response of a detector comprising a transistor such as that illustrated in FIG. 2.

Natural air is enriched with ozone by the use of a mercury lamp. A pump mounted downstream of the enclosure (25 ml) containing the detector allows exposure under flow for 2 min. At the same time, an air sample is taken to titrate the ozone using a metrological device. The recovery is static (8 min) and is obtained by stopping the gas flow generating pump. The ON or OFF state of the pump is indicated on the curve by white circles (curve 22). The value of dlos / dt is indicated by black circles (curve 23).

It is easy to notice that the response time of the detector is much shorter than the total exposure time. In Figure 13, we note a response time of about 10s. However, this time could be further reduced if the various dead volumes (tube, enclosure) were also reduced. The measurements of I _D s are such that V _GS = V _D s = -3V. The concentration of ozone generated is of the order of

40 ppb.

EXAMPLE 6 Evolution over time of I _DS of a detector comprising a transistor such as that illustrated in FIG. 1, this detector being exposed to filtered air enriched with ozone.

FIG. 14 shows the variation of I _DS of a detector manufactured according to techniques which do not require microlithography. A thin layer of nickel phthalocyanine (PcNi) is deposited on the insulating layers 2, 3 (Si0 ₂ and

If ₃ N ₄ ). Gold electrodes are then deposited, the interelectrode distance (50 μm) being guaranteed thanks to a mask made up of a wire.

The exposure is made under flow (for 2 min) of filtered air enriched with ozone. The overlap is static. The measurement detector is shown diagrammatically in FIG. 5.

Electrical measurements are made under conditions different from those used most often.

In this example, V _G = V _s = 0, V _DS = + 20V. The voltage V _GS is applied for one second and I _DS is measured after 0.2 s.

It is remarkable to note that the sensitivity of the transistors manufactured according to the diagrams of FIGS. 1 and 2, although very different in their mode of manufacture, is approximately the same.

EXAMPLE 7 Evolution over time of I _DS with the transistor illustrated in FIG. 2 and used in the detector of FIG. 5, when this detector is exposed to a flow of filtered air enriched with ozone (30 ppb).

The pump is mounted upstream of the enclosure (25 ml) containing the detector. As in most previous examples, the exposure phase under flux lasts 2 min, while the static recovery lasts 8 min. Examination of FIGS. 15 and 16 shows the difference in response of the detector according to whether the enclosure is internally coated with indigo which is a reagent known to react selectively with ozone (FIG. 15), or according to whether the enclosure is left without any particular coating (figure 16). The comparison shows a very markedly improved response by the presence of the indigo layer. EXAMPLE 8 Evolution over time of the derivative with respect to time of the intensity I _D s when using the detector comprising the enclosure illustrated in FIG. 4 and the transistor illustrated in FIG. 2. FIG. 17 indicates the evolution over time of the derivative with respect to time of the intensity I _DS , when the detector is exposed to natural air enriched in ozone by a mercury lamp (approximately 40 ppb). The detector is placed in a box 14 having overall dimensions of approximately 10 * 5 * 3 cm ³ and containing all the components (batteries, display, information processing circuits, etc.) necessary for autonomous operation of the detector. . It is therefore a portable detector. The exposure is said to be static because no gas flow is necessary for detection, the only opening of the movable cover 15 secured to the box 14 allowing said static exposure.

The static covering phase is done by simply closing the cover 15, thus isolating the detector from the outside air and allowing the ozone, contained in the air inside the box 14, to adsorb or to decompose on the interior walls of the box 14.

In FIG. 17, only the derivative with respect to the time dI _D s / dt is represented during the exposure phase (2 min) followed by the static recovery phase (8 min).

The ozone concentration as measured by a commercial metrological device is indicated by the dotted curve 24.

EXAMPLE 9 Evolution over time of the intensity I _DS for a detector comprising a transistor as shown in FIG. 2, the detector being exposed to a flow of air Synthetic containing N0 _2, measuring the N0 ₂ concentration being carried out by comparison with a measurement carried out under a flow of pure synthetic air.

FIG. 18 shows the evolution over time of I _DS for a detector subjected successively to a flow of synthetic air containing 100 ppb of N0 ₂ , at the instant designated by t '1 in FIG. 18 up to a subsequent time indicated by t'2 (the interval t'2-t'l is approximately 80 min in this example). At instant t'2, the previous flow is replaced by a flow of pure synthetic air in order to allow the original properties of the detector to be recovered.

The layer of molecular material 4 used is a layer of nickel phthalocyanine with a thickness of approximately 300 Å. The electrical measurement conditions are as follows:

- continuous bias of the transistor at V _GS = V _DS = -5V,

- duration between each determination of I _Ds ⁼ 5s,

- no voltage is applied when the detector is subjected to the flow of pure synthetic air.

EXAMPLE 10 Evolution over time of the intensity I _D s for a detector comprising a transistor such as that shown in FIG. 2 and subjected to increasing concentrations of N0 ₂ in synthetic air under air flow, these periods being followed by a dynamic recovery phase during which the detector is subjected to a single flow of synthetic air.

Figure 19 shows 6 dynamic exposure / dynamic recovery measurement cycles. During the various exposure phases, the N0 ₂ concentration varies. In FIG. 19, the concentrations 50 have been reported successively, 100, 150, 200, 250 and 300 ppb of N0 ₂ . It is remarkable to note the proportionality observed between the maximum intensity of I _D s ^and the ozone content. It is also remarkable to note that the return to the original state of the detector (ie before its exposure to NO ₂ ) is excellent, although in this example requiring a fairly long time (2 hours).

The use of polycrystalline thin layers of PcNi (300 Å) was made in this example.

The use of amorphous or quasi-amorphous molecular materials would greatly improve the kinetics of recovery.

EXAMPLE 11 Evolution over time of the intensity I _os of the current flowing between the drain 5 and source 6 electrodes of a transistor such as that shown in FIG. 2, when the detector is exposed to a flux of synthetic air containing 100 ppb of N0 ₂ while simultaneously applying the voltages V _GS = V _DS = -3V. The dynamic exposure / dynamic recovery cycles are 2 min / 8 min. No bias of the transistor is applied during recovery under lux of pure synthetic air.

With reference to FIG. 20, it is possible to observe that I _D S first reaches a maximum at short times after the application of the voltage V _GS = V _D s, then that it decreases to increase again towards the end of the dynamic exposure phase. However, the shape of the curves varies and there is only a monotonous decrease after 4 to 5 cycles.

This measurement protocol is important because it makes it possible to distinguish between various polluting gases considering their response time which is itself different vis-à-vis that of the detector. Information processing by Fourier transform is then possible.

EXAMPLE 12 Evolution over time of the intensity I _DS of the current flowing between the drain 5 and source 6 electrodes of a transistor such as that shown in FIG. 2, for a variant of the previous measurement protocols.

FIG. 21 shows in full line the evolution of the intensity I _D s over time, when the exposure (2 min) of the detector is done under a flow of filtered air enriched with ozone, the covering being static and no air flow sweeping over the detector.

The ozone concentration is measured with a commercial metrological device and the ozone content is indicated by the right scale and the dotted curve 25.

This example illustrates the use of a measurement protocol according to which the voltages V _GS = V _DS = -3V are applied for ls only in the form of voltage slots, these voltage slots being spaced from each other by 10s.

I _DS is measured 0.2s after the application of said voltages.

EXAMPLE 13 Evolution over time of the intensity I _DS of the current flowing between the drain 5 and source 6 electrodes of a transistor such as that shown in FIG. 1, for another variant of the previous measurement protocols, in the case of an ozone determination.

The detector used comprises a thin layer 4 of nickel phthalocyanine (distance between electrodes = 50 μm) and is placed inside a duralumin enclosure which can be close to subject the transistor to a confined atmosphere, or open to expose the detector to ambient air.

The electrical measurement conditions are as follows: - a voltage pulse of 10V at V _DG ≈V _DS ≈-IOV and at

V _GS = 0 is applied for 200 ms, said pulses being separated from one another by about one minute.

- I _D s does not appear immediately after application of the voltage pulse, and a delay Δt necessary for the appearance of the intensity current I _DS is measured accordingly.

Depending on whether the detector is exposed to ambient air

(curve 26) or confined in the duralumin enclosure (curve

27), the observed delays Δtl and Δt2 are different, as are the intensities I _D s measured after these delays. This provides another means of discrimination against the various polluting gases without necessarily calling for the selective destruction of one of the constituents.

EXAMPLE 14: Evolution over time of the intensity I _D s of the current flowing between the drain 5 and source 6 electrodes of a transistor such as that shown in FIG. 1, the electrical measurement protocol being analogous to that of Example 13 (V _DG ≈V _DS ≈-IOV for 200 ms and measurement of I _DS every minute). The controlled opening of the enclosure cover containing the transistor made it possible to make differential measurements without manual intervention.

The measurement protocol is as follows:

- a 20V counter voltage is applied to neutralize the charge carriers at the interface, - We measure by the impulse technique described above the value of I _DS . The measurements are made in a closed duralumin enclosure containing the transistor,

- when I _DS > 10 ^"9 A and Δl _DS <5 10 ^{" 10} A (over 1 min), the enclosure cover is opened by mechanical means,

- a measurement is made with ambient air and applied immediately after a 20V back-voltage.

This way of doing things overcomes the problems associated with the drift of I _D s over time when the detector is exposed to one or more polluting gases.

Claims

1. Detector of at least one polluting gas, comprising a field effect transistor (13) comprising a drain electrode (5) and a source electrode (6) connected together by an electroactive part (4) which is constituted of a layer of molecular material, the source electrode and the drain electrode of this transistor being connected to a device for measuring the intensity (I _DS ) of the current flowing between the drain electrode and the source electrode of the transistor when the layer of molecular material is subjected to the influence of said at least one polluting gas, characterized in that the measuring device comprises means for measuring the intensity of the current (I _DS ) flowing between the drain electrode and the source electrode of the transistor at least at a given instant after initialization of the detector, the presence of said at least one polluting gas causing doping of this material according to the concentration of polluting gas, the intensity and the slope of the current growth (I _D S) being related to the concentration of polluting gas.

2. Detector according to claim 1, characterized in that the electrical measurements are carried out by application of an electrical voltage (V _DS ) between the drain electrode (5) and the source electrode (6) of the transistor (13 ) and an electrical voltage (V _GS ) between the gate electrode and the source electrode of the transistor, these voltages being chosen from the group comprising direct voltages, voltage ramps, pulse voltages and alternating voltages .

3. Detector according to claim 1 or claim 2, characterized in that the transistor (13) comprises: - a conductive substrate (1) forming the gate of the transistor,

- an insulating layer (2,3) deposited on the substrate (1), and - said layer of molecular material deposited on said insulating layer to ensure conduction between the drain electrode (5) and the source electrode (6 ) of the transistor.

4. Detector according to claim 3, characterized in that the drain (5) and source (6) electrodes are deposited on the layer of molecular material (4) by the techniques chosen from the group comprising masking and serigraphy.

5. Detector according to claim 3, characterized in that the drain (5) and source (6) electrodes are manufactured by microlithography.

6. Detector according to any one of claims 3 to 5, characterized in that the insulating layer (2,3) consists of insulators (Si0 ₂ , Si ₃ N ₄ ...) or else by thin layers of polymers, Langmuir Blodgett layers or molecules with electrical insulating properties grafted onto the gate electrode of the transistor.

7. Detector according to any one of claims 3 to 6, characterized in that the insulating layer (2,3) is deposited by photopolymerization after grafting or not of said conductive substrate (1).

8. Detector according to any one of claims 1 to 7, characterized in that the molecular material is chosen in particular from the group comprising phthalocyanine derivatives, metallized or not, porphyrins metallized or not, azaporphyrins, porphyrazines, rare earth bisphthalocyanines, their mixtures, and the layering of several layers of them.

9. Detector according to claim 8, characterized in that the molecular units constituting the molecular material are substituted by atoms, paraffinic chains, polymer chains such as polyoxyethylene chains, anionic or cationic groups.

10. Detector according to claim 8 or 9, characterized in that the layer of molecular material (4) is optionally deposited after dissolution in a solvent of the molecular units constituting said layer, by vacuum sublimation, by the spinning method, by screen printing, by the Langmuir Blodgett method, and by photopolymerization.

11. Detector according to any one of claims l to 10, characterized in that the layer of molecular material (4) is amorphous or almost amorphous, whereby the diffusion of gaseous species within the layer of molecular material is improved .

12. Detector according to any one of claims 1 to 11, characterized in that it comprises a light source (16) for subjecting the transistor to a photonic flux during detection and measurement, said light source being intended to increase the detector sensitivity.

13. Detector according to any one of claims 1 to 12, characterized in that it comprises a heat source (17) to increase its temperature and promote the migration of the constituent species within the molecular material.

14. Detector according to any one of claims 1 to 13, characterized in that the transistor (13) is arranged in an enclosure (14) comprising controlled means for admitting at least one polluting gas into a gas mixture and an internal coating (14a) at least part of which comprises a material capable of reacting with said at least one polluting gas to be detected, said controlled means being actuated, on the one hand, in intake to allow the introduction of said at least one polluting gas in a gas mixture inside the enclosure and, on the other hand, in intake shutdown to prevent the introduction of said at least one polluting gas in a gas mixture inside the enclosure and thus promote the adsorption or chemical destruction of said at least one corresponding polluting gas, this last step constituting the initialization phase.

15. Detector according to claim 14, characterized in that the internal coating material (14a) of the enclosure (14) is chosen from the group comprising stainless steel, an alloy of aluminum, copper and magnesium, polymeric materials and comprises on at least part of its surface a compound capable of reacting with said at least one polluting gas to be detected, said compound being chosen as a function of the gas to be detected from the group comprising in particular diphenylbenzidine, lead salts and indigo.

16. Detector according to claim 14 or claim 15, characterized in that said controlled means for admitting polluting gas are chosen from the group comprising a cover (15) movable and integral with the enclosure (14), and an electro -pump (P) connected to the enclosure.

17. Detector according to any one of claims 14 to 16, characterized in that the enclosure (14) further comprises means for generating convection current (T) which are put into operation when the intake of the controlled means for admitting said at least one polluting gas into a mixture of gases to accelerate the flow of said at least one polluting gas in a mixture of gases towards the internal coating (14a) of said enclosure.

18. Detector according to any one of claims 14 to 17, characterized in that the enclosure (14) is connected to means for selective destruction (FI, F2, 18) of said at least one polluting gas.

19. Detector according to any one of claims 1 to 18, characterized in that said at least one polluting gas is an element constituting a gas mixture, this element being chosen from the group comprising in particular ozone, HF, CO, C0 ₂ , S0 ₂ , aniline, AsR ₃ R being an alkyl group, NO, N0 ₂ , Cl ₂ , CICN, DMF, Me ₂ S, H ₂ S, RSH R being an alkyl group, H ₂ 0 ₂ , COCl ₂ , CS ₂ , NO or their mixtures, toxic gases and dangerous gases.

20. Method for measuring the concentration of at least one polluting gas constituting a mixture of gases captured by a detector according to one of claims 1 to 19, characterized in that it comprises the steps consisting in: a) admit into the detector this mixture of gases between a first instant (tl) and a second instant (t2) which follows said first instant, and measure the intensity of the current

(I _D s) which flows between the drain electrode (5) and the source electrode (6) of the transistor (13), between the first and second instants, b) cut off this admission between the second instant (t2) and a third instant (t3) which follows said second instant (t2), and measure the intensity of the current (I _DS ) flowing between the drain electrode (5) and the source electrode (6) of the transistor (13), in the time interval between the second and third instants, this interval constituting the initialization of the detector during which said at least one polluting gas contained in said gas mixture is selectively destroyed, c) admitting said gas mixture again into the detector between the third instant (t3) and a fourth instant (t4) which follows said third instant (t3), and measure the intensity of the current (I _D s) which flows between the drain electrode (5) and the source electrode (6 ) of the transistor (13), between the third and fourth instants, to deduce the concentration of said at least one polluting gas selectively destroyed in step b).

21. Method according to claim 20, characterized in that step b) is followed by a step consisting in comparing the value of the intensity back) of the current which circulated in the transistor (13) between the first (tl ) and second (t2) instants at a reference value as a function of said at least one polluting gas, in order to determine the concentration of said at least one polluting gas.

22. Method according to claim 20, characterized in that: - in step a) a value of the derivative with respect to time is calculated of the intensity of the current which has flowed between the drain electrode and the electrode source of the transistor (13) between the first and second instants,

- in step b) a value of the derivative with respect to time is calculated of the intensity of the current which has flowed between the drain electrode and the source electrode of the transistor (13) between the second and third instants , and we compares the derivatives calculated respectively in steps a) and b) with a reference value as a function of said at least one polluting gas in order to determine the concentration of said at least one polluting gas.

23. Method according to any one of claims

20 to 22, characterized in that steps a), b) and c) are repeated successively a plurality of times.

24. Method according to any one of claims 1 to 23, characterized in that the voltage (V _Ds ) between the drain electrode (5) and the source electrode (6) of the transistor is applied, while the derivative with respect to time (dI _D s / dt) of the intensity of the current flowing between the drain electrode (5) and the source electrode (6) of the transistor is determined, said voltage being continuous and being equal to or different from the voltage (V _GS ) between the gate electrode and the source electrode of the transistor (V _GS ).

25. Method according to any one of claims 1 to 23, characterized in that a voltage slot (V _GS ) is applied during a period varying from zero to a few minutes, while the intensity do _s ) the current flowing between the drain electrode (5) and the source electrode (6) of the transistor is measured according to a previously chosen sampling.

26. Method according to any one of claims 1 to 23, characterized in that a first voltage is applied between the first (tl) and second (t2) moments, a second voltage, different from the first voltage, is applied between the second (t2) and third (t3) instants, these steps being repeated periodically over time and the Fourier transform of the signal delivered by the detector being performed.

27. Method according to any one of claims 1 to 23, characterized in that when the voltage (V _DS ) between the drain electrode (5) and the source electrode (6) of the transistor is equal to the voltage (V _GS ) between the gate electrode and the source electrode (6) of the transistor, and that these two voltages are applied for a short period of time, the intensity current (I _D s) which flows between l the drain electrode and the source electrode of the transistor appear after a time (Δt) characteristic of the concentration of polluting gas.

28. Method according to any one of claims 1 to 23, characterized in that after application of a first voltage (V _GΞ ) between the gate electrode and the source electrode (6) of the transistor and measurement of the intensity of the current (I _D s) flowing between the drain electrode and the source electrode of the transistor, a second voltage (V _GS ) is applied between the gate electrode and the source electrode (6) of the transistor so that said current intensity returns to zero, this second voltage having a sign opposite to said first voltage.