This invention relates to sensors which measure a characteristic of an environment in which the sensor is placed, particularly, but not exclusively, the temperature of the environment or species present in the environment.

Sensing of characteristics of an environment has many important applications. For example, the applications of temperature sensors include the computer and automotive industries for e.g. activating cooling fans or power shut down when necessary. Indeed in a laptop computer alone, several sensors are employed to monitor the temperatures of the central processing unit (CPU), battery, AC adaptor and other components. The global market for temperature sensors is large, and improvements in temperature sensing could therefore be of value. For example, an improvement may constitute greater sensitivity over a given temperature range, greater stability or repeatability in measurement, or an extension of the temperature range that can be measured using a conventional sensor. In addition to temperature monitoring, applications may extend to sensing one or more changes in an environment such as gas sensing (e.g. the detection of hydrogen or carbon monoxide constitute critical health and safety issues) or stress measurement.

According to a first aspect of the invention there is provided a sensor which measures a characteristic of an environment in which the sensor is placed, the sensor comprising a semiconductor substrate, a metallic structure attached to and in electrical contact with the semiconductor substrate, the metallic structure having a higher resistance to electrical current flow than the semiconductor substrate. The term "attached" includes directly attached or indirectly attached. For example, the metallic structure may be attached to the semiconductor substrate such that it is in direct contract with the semiconductor substrate, or, an additional layer(s) which is sufficiently thin that current may still pass between the metallic structure and the semiconductor substrate, may be interposed between the metallic structure and the semiconductor substrate.

The metallic structure may have any suitable thickness and geometry. Preferably, the metallic structure has a thickness of between 2 and 20 nm depending on the choice of metal.

The metallic structure may be configured to have a higher resistance to electrical current flow than the semiconductor substrate. Preferably, the semiconductor is sufficiently doped so that the metallic structure offers a higher resistance to electrical current flow than the semiconductor substrate.

At least two contact electrodes may be attached to, and in electrical contact with the metallic structure wherein the sensor obtains a

measurement of the characteristic by application of a known voltage between the contact electrodes, measurement of electrical current flow between the contact electrodes along a current path through the sensor and, using a pre-determined relationship between the current flow through the sensor and the characteristic, to obtain a measurement of the characteristic of the sensor's environment.

Alternatively, four contact electrodes may be attached to and be in electrical contact with the metallic structure. The sensor may obtain a measurement of the characteristic by driving a known current between two of the contact electrodes, measurement of voltage developed between the other two contact electrodes attached to the metallic structure and, using a pre-determined relationship between the resistance of the sensor and the characteristic to obtain a measurement of the characteristic of the sensor's environment.

The sensor may measure a characteristic of the environment comprising the temperature of the environment. There are three possible components of resistance to the flow of electrical current through the sensor. The first component is the resistance of the metallic structure and the second component is the resistance of the semiconductor substrate, where the resistance of the metallic structure is greater than that of the semiconductor substrate. The third resistance component in the sensor is a continuously-distributed resistance between the metallic structure and semiconductor substrate associated with a Schottky barrier that is formed between the metallic structure and semiconductor substrate. Preferably, this third resistance component is the most strongly temperature dependent component of resistance and may predominantly determine the sensor operation. As the temperature of the environment in which the sensor is placed changes, the resistance of the individual components of resistance change, which in turn changes the combined resistance of the components of resistance; the path taken by the current will be distributed between the semiconductor substrate and the metallic structure accordingly. Thus, measurements of the flow of electrical current between the contact electrodes, under a given, fixed voltage, may be used as a measure of the temperature of the environment (a two-probe arrangement). Alternatively, if a constant current source is used the potential difference developed along the length of the metallic structure may be used as a measure of the temperature of the environment (a four-probe arrangement). In either case, known voltage and current measurements may be used to determine the resistance of the sensor which is related to the temperature of the environment. The changes are explained in detail below, where, for simplicity, the description discloses a two-probe arrangement.

At low temperatures of the sensor's environment, the charge carriers in the metallic structure have insufficient thermal energy to overcome the Schottky barrier. In effect the distributed resistance of the barrier is infinite and the current flow is constrained to a path in the metallic structure only. Since the metallic structure has a high resistance, relative to that of the semiconductor substrate, the measured current flow is low. As the temperature of the environment increases the charge carriers gain sufficient thermal energy to cross the Schottky barrier and flow in the semiconductor substrate. In effect the distributed resistance of the Schottky barrier becomes progressively lower with increasing temperature. The current path through the sensor will now comprise a first current path portion through the metallic structure and a second, parallel, current path portion through the semiconductor substrate. The value of the continuously distributed resistance of the Schottky barrier determines the relative current flow between the first current path portion and the second current path portion. The overall resistance to the flow of electrical current is determined by the resistance of the metallic structure in parallel with the resistance of the semiconductor substrate, connected by the continuously distributed Schottky barrier resistance. As the resistance of the semiconductor substrate is less than that of the metallic structure, the overall resistance will be lower, and the measured current flow will be higher. As the temperature of the environment increases further, a majority of charge carriers gain sufficient thermal energy to cross the Schottky barrier and flow in the semiconductor substrate. The effective distributed resistance of the Schottky barrier becomes even lower, and the current path through the sensor will now predominantly comprise a current path portion through the semiconductor substrate. The overall resistance to the flow of electrical current is determined predominantly by the low resistance of the Schottky barrier and the relatively low resistance of the semiconductor substrate, and the measured current flow is then high. Between the regimes of current flow via the metallic structure only and predominantly via the semiconductor substrate, occurs the regime of primary interest where the sensor resistance falls rapidly with increasing temperature of the sensor's environment.

The sensor may operate over an operative temperature region where the current flow is along a current path through the sensor comprising a first current path portion through the metallic structure and a second current path portion through the semiconductor substrate. The sensor may provide in the operative temperature region, a relationship between the current flow through the sensor and the temperature of the environment in which the sensor is placed where a small change in the temperature will produce a relatively large change in the current flow.

A measurement of the temperature of the sensor's environment in the operative temperature region may be obtained using the relationship between the current flow through the sensor and the temperature of the environment in which the sensor is placed. The relationship may be obtained over the operative temperature region by placing the sensor in an environment, applying a plurality of known temperatures in the region to the environment, measuring the electrical current flow through the sensor at each known temperature, and using the measured current flows and known temperatures to obtain the relationship. The sensor may comprise a Schottky barrier which provides a desired operative temperature region. The operative temperature region may be determined by the height of the Schottky barrier. The desired operative temperature region may be selected by the choice of materials for the semiconductor substrate and the metallic structure.

The semiconductor substrate may comprise any semiconducting material, for example silicon or a lll-V semiconducting material such as gallium arsenide or indium phosphide. The semiconducting material may be doped n-type or p-type, to provide a Schottky barrier of a desired height.

The metallic structure may comprise any metal or metal alloy material that forms a Schottky barrier with the semiconductor substrate, such as platinum (or platinum silicide) or palladium (or palladium silicide) on a silicon substrate.

The sensor may provide in the operative temperature region, a

relationship between the current flow through the sensor and the temperature of the environment in which the sensor is placed where a small change in the temperature will produce a relatively large change in the current flow by control of the geometrical properties of the metallic structure. Control of the geometrical properties of the metallic structure may thus control the resistance of the metallic structure. Thus the sensitivity of the sensor, i.e. the total change in resistance over the operative temperature region may be enhanced. The metallic structure may be thin; the thickness is preferably in the range 2-20 nm. Such a thickness will increase the resistance of the metallic structure. (The thickness of the metallic structure may depend on the choice of metal which the forms the metallic structure). The metallic structure may be elongate, i.e. it may have a predetermined length to width ratio which will also increase the resistance of the metallic structure. The predetermined length to width ratio may be any value from 1- 000. The metallic structure may have a reduced width to increase the length to width ratio. Preferably, the width of the metallic structure is 1 μΐη or less. Such a width will also ensure that the sensor can take advantage of the depletion region in the semiconductor substrate becoming of non-planar geometry. The, elongate, metallic structure may be substantially straight. The, elongate, metallic structure may be patterned to a non-straight shape. For example, the elongate metallic structure may be U-shaped zig-zag, or a Fermat spiral shape. A first contact electrode may be attached to a first end of the patterned elongate metallic structure and a second contact electrode may be attached to a second end of the patterned elongate metallic structure. The patterned elongate metallic structure may be shaped to place the contact electrodes in close proximity.

The sensor may provide in the operative temperature region, a

relationship between the current flow through the sensor and the

temperature of the environment in which the sensor is placed where a small change in the temperature will produce a relatively large change in the current flow by control of the morphology and physical structure of the metallic structure, e.g. the granularity or a nanoparticulate structure of the metallic structure.

The sensor may provide in the operative temperature region, a relationship between the current flow through the sensor and the temperature of the environment in which the sensor is placed, where a change in the temperature of the environment will produce a change in the current flow by control of the doping level of the semiconductor substrate. An increase in the doping level of the semiconductor will reduce the resistance minimum at an upper end of an operative temperature range of the sensor, thereby increasing the overall resistance change and thus increasing the sensor sensitivity over the operative temperature range.

The sensor may measure a characteristic of the environment comprising the presence of a species in the environment. The species may be an atomic or molecular species.

As stated above, there are three possible components of resistance to the flow of electrical current through the sensor i.e. resistance of the metallic structure, resistance of the semiconductor substrate, and the continuously- distributed resistance of the Schottky barrier between the metallic structure and semiconductor substrate. Each of these resistance components may be affected by the presence of a species in the environment in which the sensor is placed. Interaction of atomic or molecular species in the environment with the metallic structure can cause surface and/or 'bulk' modification of the metallic structure, which will influence the resistance of the metallic structure. The operative mechanism may be that of increased surface or bulk scattering of charge carriers in the metallic structure, leading to increased resistance. Ingress of a species in the environment, into the semiconductor substrate, can alter the semiconductor substrate's properties, and change the resistance of the semiconductor substrate. Ingress of a species in the environment, into the Schottky barrier region, can change properties of the Schottky barrier, and affect the Schottky barrier resistance at a fixed temperature. Preferably, it is the response of this third resistance component that will be most sensitive to species in the environment and will predominantly determine the overall sensor response.

The presence of a species in the environment in which the sensor is placed affects the resistance experienced by the current and therefore the measurement of the flow of current between the contact electrodes. Thus, measurements of the flow of electrical current between the contact electrodes may be used as a measure of the presence of a species in the environment. Alternatively, if a constant current source is used, the potential difference developed along the length of the metallic structure may be used as a measure of the presence of a species in the environment.

The metallic structure may comprise a material that responds to at least one species present in the environment. Preferably, the metallic structure will comprise a material that responds uniquely to one atomic or molecular species present in the environment or to more than one such species present in the environment. The metallic structure may comprise a material that adsorbs at least one species present in the environment. The metallic structure may comprise a material that undergoes one or more chemical reaction events with at least one species present in the environment.

The metallic structure may comprise a plurality of island-type metallic grains or nanoparticles, which may expand in volume, on exposure to a species present in the environment. Preferably, the plurality of island-type metallic grains or nanoparticles are sufficiently connected to be above an electrical percolation threshold. This will ensure that electric current is able to flow via the metallic structure.

There are three considerations following from the use of island-type metallic structures. First, the degree of electrical connectivity between the island-type metallic grains or nanoparticles may change in response to a species present in the environment - for example, expansion of the island- type metallic grains or nanoparticles will yield improved electrical connectivity between the island-type metallic grains or nanoparticles. This would lead to a decrease in the resistance of the metallic structure, and therefore an increase in the measured electrical current, thus offering a measurement of a species in the environment.

The metallic structure may comprise a plurality of island-type metallic Pd grains or Pd nanoparticles. A sensor comprising such grains or

nanoparticles can be used to sense hydrogen. The second consideration on the use of a granular or nanoparticulate metallic structure is to facilitate the transport of a species in the

environment to an interface between the metallic structure and the semiconductor substrate on a faster time scale than is possible with a fully continuous, or a less porous, metallic structure.

Thirdly, since the tendency of exposure of the Schottky barrier region to atomic or molecular species in the environment is to lower the barrier height and thus decrease the sensor resistance at a given temperature, the use of an island-type or nanoparticulate metallic structure for which exposure to an atomic or molecular species in the environment decreases its resistance means that the sense of the resistance change is the same due to both effects, i.e. one effect is not counteracting the other.

The sensor may measure a characteristic of the environment comprising the presence of hydrogen in the environment. The metallic structure may comprise a material that reacts with hydrogen. The metallic structure may comprise a metal material comprising, for example, Pd or Pt. The metallic structure may comprise a metal alloy material comprising, for example, Pd alloy or Pt alloy. The sensor may measure a characteristic of the environment comprising the presence of hydrogen in the environment by ingress of hydrogen into the Schottky barrier region of the sensor between the metallic structure and the semiconductor substrate, and measuring the electrical current flow between the contact electrodes. Preferably, such ingress will alter the properties of the Schottky barrier at a given temperature, thus changing the resistance of the Schottky barrier and the measured electrical current.

The metallic structure may be provided with at least one adlayer which responds to at least one species present in the environment. The metallic structure may be provided with at least one adlayer which adsorbs at least one species present in the environment. The metallic structure may be provided with at least one adlayer which undergoes one or more chemical reaction events with at least one species present in the environment. The adlayer may be provided on a surface of the metallic structure to come in contact with the environment.

The sensor may comprise a first element which measures a characteristic of the environment comprising the presence of a species in the environment, and a second element provided with a housing to protect it from species in the environment which does not measure a characteristic of the environment comprising the presence of a species in the environment, wherein the first element and the second element are connected together in a bridge circuit to annul the effect of temperature changes of the environment.

The sensor may comprise an output unit, for example to display the resistance of the sensor. The metallic structure may be deposited on the semiconductor substrate. Deposition may be by any available technology that leads successfully to Schottky barrier formation. The metallic structure may be deposited on the semiconductor substrate using, for example, any of thermal evaporation, electron-beam evaporation, sputtering techniques, electrochemical plating technique. The method of deposition used for the metallic structure will determine the morphology and micro-structure of the metallic structure and thus the resistance of the metallic structure. The method of deposition of the metallic structure on the semiconductor substrate will also, in general, influence the properties of the Schottky barrier itself. Post-deposition techniques may be used to control the physical properties and/or the chemical composition of the metallic structure. The post-deposition treatments may comprise, for example, annealing of Pt to form PtSi or annealing of Pd to form PdaSi. The silicides are of different chemical composition from the starting metallic materials, and tend to be rather more granular and electrically resistive.

According to a second aspect of the invention there is provided a method of measuring a characteristic of an environment, comprising placing a sensor according to the first aspect of the invention in the environment, measuring the electrical current flow through the sensor and using the measured current flow to determine the characteristic of the environment.

According to a further aspect of the invention there is provided a method of measuring a characteristic of an environment, comprising placing a sensor according to the first aspect of the invention in the environment, measuring the potential developed along a length of the sensor and using the measured potential to determine the characteristic of the environment. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross sectional view of a first embodiment of the sensor of the invention;

Figure 2 is a schematic plan view of the sensor of Figure 1 ;

Figure 3 is a schematic view of a second embodiment of the sensor of the invention, and,

Figures 4a-4c is a schematic cross-sectional view of three configurations of a comparator element of the hydrogen sensor of figure 3. Figure 1 shows a sensor which measures a characteristic of the

environment comprising the temperature of the environment. The temperature sensor 1 comprises a semiconductor substrate 3, a layer of insulator material 5, a metallic structure 7 comprising a resistive metal, contact electrodes in the form of contact electrodes 9 and an optional ohmic contact structure 11.

In this embodiment, the sensor 1 is fabricated from a wafer of semiconductor material, comprising p-type Si, which forms the semiconductor substrate 3. The structure 3 is oxidised to form the layer of insulator material 5, comprising S1O2, on a first surface of the structure 3. Using a photolithography (or electron-beam lithography) process, a window is opened in the layer of S1O2 insulator material 5, and the metallic structure 7, comprising Pt, is deposited in the window, such that a first surface of the Pt structure 7 is in contact with the first surface of the Si semiconductor substrate 3. Deposition is by any available technology that leads successfully to Schottky barrier formation. Optionally, at this stage the structure may undergo a conventional furnace annealing process or a rapid thermal annealing process to convert the Pt structure to platinum silicide, PtSi, if desired. A second lithographic process is then used to define the contact electrodes 9. The contact electrodes 9 may be formed by depositing Pt or by depositing a double-layer, comprising a thin (~10nm for example) underlying adiayer of titanium (Ti) and a thicker overlayer of Pt or other metal, such as aluminium or gold. The Ti adiayer may serve the function of enhancing the adhesion to the SiO ₂ layer 5. Four such contact electrodes 9 are formed (see Figure 2), each comprising a contact portion 13 which contacts the metallic structure 7, and extends to a larger pad 15 on the S1O2 insulator material 5, to facilitate contact to the external circuitry. The large contact pads 15 are exemplary only, for example in an encapsulated chip device, these would be replaced by micro-scale lead- offs to contact pins. The formation of the metallic structure 7 and the contact electrodes 9 is achieved by a 'lift-off process, in which the lithography step ideally uses a double layer of resist, such that a bottom layer of the resist (adjacent the S1O2) yields an undercut relative to an upper layer of the resist upon exposure to resist developer. In this embodiment of the temperature sensor 1 , the semiconductor substrate 3 is provided with an ohmic contact structure 11. This is formed on a second surface of the structure 3, opposite to that on which the insulator material 5 is formed. Formation of the ohmic contact structure may take place at any appropriate stage of the formation of the temperature sensor 1. The presence of the ohmic contact is not necessary for sensor operation. Here an ohmic contact is provided for additional, conventional measurements if such are considered desirable.

The temperature sensor 1 comprises a Schottky diode that is modified with respect to a conventional Schottky diode in a number of key respects. The contact electrodes 9 are in electrical contact with only the metallic structure 7, for delivery and measurement of-the flow of electric current through the sensor 1. The contact electrodes 9 can also measure the potential drop along the length of the metallic structure 7 of the sensor . In conventional Schottky diodes, contact electrodes are provided on a metallic structure and on a semiconductor substrate of the diode, i.e. via the ohmic contact, for measurement of electrical properties across the metal/semiconductor interface. The metallic structure 7 of the temperature sensor 1 of the invention, comprises a very thin layer of metal, Pt or PtSi, of a geometry which ensures a high length to width ratio (typically >10), such that the metallic structure has a resistance that is greater than that of the semiconductor substrate 3. In conventional Schottky diodes, the metallic structure would not be resistive as this would be undesirable for the operation of such diodes. The modifications provided in the diode of Figures 1 and 2, allow this diode to operate as a temperature sensor, as follows.

As per conventional Schottky diodes, the interface between the metallic structure 7 and the semiconductor substrate 3 of the temperature sensor 1 forms a barrier against the migration of charge carriers across the interface. Specifically, when the metallic structure 7 comprises Pt (or PtSi) and the semiconductor substrate 3 comprises p-type Si, the Schottky barrier is of the order of 380 meV (230 meV). As described above, there are three possible components of resistance to the flow of electrical current through the sensor 1 , i.e. the resistance of the metallic structure 7, the resistance of the semiconductor substrate 3, and distributed resistance associated with the Schottky barrier. As the temperature of the environment in which the sensor 1 is placed changes, the component resistances change and thus the overall sensor resistance changes. In the four contact electrode configuration of figures 1 and 2 a constant current is delivered to the sensor using the two outermost contact electrodes and the change in the overall sensor resistance is detected by measuring a change in the potential drop between the two innermost contact electrodes.

Using PtSi for the metallic structure 7 and p-type Si for the semiconductor substrate 3, the sensor 1 has an operative temperature region of approximately 100K to approximately 150K. In this region, a small temperature change results in a large change in electrical current flow between the contact electrodes 9. In the operative temperature region, charge carriers in the metallic structure 7 have sufficient energy to be able to scale the Schottky barrier, and pass across the interface between the metallic structure 7 and the semiconductor substrate 3. The current flow through the sensor 1 is along a first current path portion through the metallic structure 7 and a second current path portion through the semiconductor substrate 3.

To use the temperature sensor 1 , it is first of all calibrated using the four contact electrodes as described below. The sensor 1 is subjected to a range of temperatures, a constant, known, current is passed along the sensor 1 using the two outermost contact electrodes 9 while the potential drop between the two innermost contact electrodes 9 is measured. The resistance of the sensor 1 may thus be determined. Starting at low temperatures, the resistance of the sensor 1 is measured at a plurality of spaced temperatures. This allows identification of the temperature at which the transition occurs between current flow through the metallic structure 7 only and current flow through the metallic structure 7 and the semiconductor substrate 3. Thus the start of the operative temperature region of the sensor 1 is identified. Further measurements of sensor resistance are determined at increasing temperatures, and an approximate end of the operative temperature region identified. Once this has been done, measurements of sensor resistance are made at a plurality of closely-spaced temperatures within the operative temperature region. Thus a relationship between resistance and temperature in this region can be derived, and this can be used to determine a temperature to which the sensor 1 is exposed from a measurement of the resistance.

It will be appreciated that various modifications can be made to the embodiments of the temperature sensor described above. For example, the metallic structure can be made from any metallic material that forms a Schottky barrier with the underlying semiconductor substrate; specifically, metallic structure-semiconductor substrate combinations that yield an operative temperature range spanning typical room temperatures will be of particular use. As discussed, the geometry and physical structure of the metallic structure can be manipulated to control its resistance. The material of the semiconductor substrate 3 should have a resistance which is lower than that of the metallic structure, and can comprise not only p- type silicon but n-type silicon and other doped semiconductor materials. Temperature sensors according to this embodiment of the invention provide a means for obtaining very sensitive measurements, over a particular operative temperature region, and the sensitivity and the operative temperature region can be controlled by manipulation of the structure of the sensors. Such temperature sensors could offer a viable alternative in certain segments of the market to IC sensors currently offered on the market.

Referring now to Figure 3, this shows an implementation of a sensor according to the present invention, which measures a characteristic of the environment comprising the presence of a species. The sensor shown in figure 3 is a hydrogen sensor 21 which measures a characteristic of the environment comprising the presence of the species hydrogen. The hydrogen sensor 21 comprises a Schottky diode element 23 and a comparator element 25. A four-probe measurement technique is illustrated in figure 3 in which details of input/output resistors on

operational amplifier stages are omitted. Voltages developed across the Schottky diode element 23 and comparator element 25 are measured by operational amplifier stages 27 (in inverted configuration) and can be separately monitored, while the difference between the outputs of the operational amplifier stages 27 is measured (and amplified) by a differential amplifier stage 29.

The Schottky diode element 23 of the hydrogen sensor 21 comprises an elongated, resistive metallic structure electrically connected to a semiconducting substrate as described previously in relation to figures 1 and 2. The metallic structure-semiconductor combination (3 and 7 of figure 1) forms a Schottky barrier. The Schottky barrier has a height such that, at ambient temperature, the-resistance of the Schottky diode element 23 is near its maximum, and can drop to a lower resistance at higher temperatures. The Schottky barrier height is influenced (preferably lowered) by the diffusion of hydrogen into the Schottky barrier region; this can be due to a direct effect of hydrogen on the barrier interface region or by means of an interaction of the hydrogen with the metallic structure, changing its nature and thereby changing the height of the Schottky barrier. The metallic structure itself may thus also interact with hydrogen to change the metallic structure's electrical characteristics. The effect of exposure to hydrogen is reversible when hydrogen is removed from the environment. The semiconductor substrate (3 of figure 1) is made of n- type indium phosphide and the metallic structure (7 of figure 1 and figure 3) is made from palladium. In this embodiment the resistance of the Schottky diode element, as measured along the length of the metallic structure, may rise initially due to an increase in the resistance of the palladium itself, as it reacts with hydrogen to form palladium hydride. On a longer time scale the resistance of the Schottky diode element, as measured along the length of the palladium metallic structure will drop significantly (much more than the initial resistance increase) due to a decrease in the Schottky barrier height brought about by diffusion of hydrogen into the barrier region. Figures 4(a)-4(c) is a schematic cross-sectional view of three

configurations of the comparator element 25 of the hydrogen sensor of figure 3. The comparator element 25 comprises an identical metallic structure to that of the Schottky diode element 23. (The comparator element 25 is prepared under the same conditions as the Schottky diode element 23). The comparator element 25 may be (a) electrically isolated from the semiconductor substrate by an electrically insulating layer such as an oxide (e.g. silicon oxide) or a nitride (e.g. silicon nitride) material but exposed to the ambient environment (illustrated in Figure 4(a)), (b) in electrical contact with the semiconductor substrate (i.e. forming a second Schottky diode element) but isolated from the environment by the use of an appropriate coating material 31 (illustrated in Figure 4(b)) or (c) both electrically isolated from the semiconductor substrate and isolated from the environment (illustrated in Figure 4(c)). The purpose of the

comparator element 25 is to compensate for changes in the resistance of the Schottky diode element 23 that are not due to a lowering of the

Schottky barrier height upon exposure to hydrogen. In case (b), for example, subtraction of the output of the Schottky diode element 23 from that of the comparator element 25 (isolated from hydrogen exposure), will compensate for change in the resistance of the Schottky diode element 23 due to any change in temperature. In case (a), for example, subtraction of the output of the Schottky diode element 23 from that of the comparator metallic structure 25 (which is electrically isolated from the substrate), will yield a measure of the resistance drop due to hydrogen interaction with the Schottky barrier alone in Schottky diode element 23, while the resistance increase of the comparator device structure 25, due to hydrogen exposure, may be separately monitored. This implementation can offer the advantage of moderately sensitive detection of higher concentrations of hydrogen on a fast response time-scale via comparator element 25 combined with highly sensitive hydrogen detection at very low concentrations (10 ppm level or possibly less), though on a longer timescale, via the Schottky diode element 23. In case (c), for example, subtraction of the output of the Schottky diode element 23 from that of the comparator element 25, will compensate for any change in the resistance of the metallic structure only of the Schottky diode element 23 due to a change in the temperature of the environment; of the various components of the response of the Schottky diode element 23 that are not due to the effect of exposure of the Schottky barrier to hydrogen, this would be by far the least important. More than one of (a), (b) and (c) could be used in an extended comparator circuit.

To use the hydrogen sensor 21 , it is first of all calibrated. The hydrogen sensor 21 is subjected to a range of known hydrogen concentrations, and, at each concentration, a constant, known current is passed through the hydrogen sensor 21. The potential difference developed along the length of Schottky diode element 23 and comparator element 25 is measured and the resistance of each element and the sensor 21 as a whole determined. A relationship between resistance and hydrogen concentration is derived, and this can be used to determine a hydrogen concentration to which the sensor 21 is exposed from measurements of the sensor resistance (and the resistance of its Schottky diode element 23 and comparator element 25 individually).

It will be appreciated that various modifications can be made to the embodiments of the hydrogen sensor described above. For example, the metallic structure can be made from any resistive metallic material sensitive to hydrogen, such as Pt, and alloys of Pd and Pt. The metallic structure may also be composed of electrically connected island-type metallic grains or of metallic nanoparticles as described previously. With appropriate choice of the metallic structure material and/or semiconductor substrate material, sensors which measure the presence of other species in the environment may be fabricated.

Alternatively, the Schottky diode element 23 may comprise one arm in a Wheatstone bridge-type circuit, where the other arms of the same Wheatstone bridge-type circuit may comprise one (or two) of, a comparator element 25 (a) electrically isolated from the semiconductor substrate by an electrically insulating layer such as an oxide (e.g. silicon oxide) or a nitride (e.g. silicon nitride) material but exposed to the ambient environment, (b) in electrical contact with the semiconductor substrate (i.e. forming a second Schottky diode element) but isolated from the environment by the use of an appropriate coating material or (c) both electrically isolated from the semiconductor substrate and isolated from the environment , in combination with two (or one) calibrated resistor(s) and a variable (external) resistor to balance the circuit.

The detection and measurement of hydrogen in an environment has important applications, particularly in relation to hydrogen fuel-cell technology, which may well form a very substantive part of the future green energy landscape.