Patent Application
20070166832
A gas sensor may be used to detect the presence of one or more analytes in a gas. For example, a gas sensor might be used to detect the presence and/or concentration of nitrogen oxides (NOx), which are a group of highly reactive gases that contain varying amounts of nitrogen and oxygen. Such a sensor could be used, for example, to ensure that an industrial process or turbine engine complies with a governmental regulation (e.g., a regulation established by the US Environmental Protection Agency).
A gas sensor may need to selectively detect different species of an analyte. For example, a sensor might need to accurately distinguish between exposure to C2H2 and C2H4. Moreover, a sensor may need to operate in harsh environments, such as environments having relatively extreme vibration, temperature (e.g., 600° C.), chemical and/or pressure conditions. Also note that it may be impractical to use a sensor if it is too large, expensive, or unreliable.
According to some embodiments, an electronics based physical gas sensor includes a semiconductor layer, and at least one contact is electrically coupled to the semiconductor layer. A catalytic gate, having a property that changes when the gate is exposed to an analyte, and a variable bias from a voltage source are also provided.
A gas sensor may be used to determine if an “analyte” is present and/or to quantify an amount of the analyte. As used herein, the term “analyte” may refer to any substance to be detected and/or quantified, including a gas, a vapor, and/or a bioanalyte. For example,
A sensor may use a catalytic material to facilitate detection of an analyte. For example,
A dielectric layer 224 separates a catalytic gate 220 from the semiconductor layer 210. The dielectric layer 224 may comprise, for example, a layer of SiO2, SiN, HfO2 and/or a metal oxide or any combination thereof. The catalytic gate 220 may be, for example, a metallic contact. In this way, the gate 220 and dielectric layer 224 might form, for example, a metal/metal oxide stack on silicon nitride. A gate voltage source 222 may provide a fixed gate voltage VG to the catalytic gate 220 (e.g., on the side of the gate 220 opposite the dielectric layer 224 and the semiconductor layer 210).
A source contact 230 is electrically coupled to the semiconductor layer 210 and to ground. Similarly, a drain contact 240 is coupled to the semiconductor layer 210, remote from the source contact 230, as well as a drain voltage source 242. The drain voltage source 242 provides a fixed drain voltage VD to the drain contact 240. The source contact 230 and the drain contact 240 may comprise, for example, ohmic contacts made of nickel or aluminum.
The arrangement illustrated in
According to some embodiments, the sensor 200 is fabricated on a Wide Bandgap (WBG) semiconductor. For example, the catalytic gate 220, dielectric layer 224, and semiconductor layer 210 might be associated with a heterojunction wherein a surface of a heavily doped/high bandgap material interfaces with a surface of a lightly doped/low bandgap material. This heterojunction may be associated with a Schottky contact.
The gate 220 may be “catalytic” in that a property of the gate 220 changes when exposed to an analyte 250 (like hydrogen or NOx). For example, molecules of the analyte 250 may diffuse through the gate 220 and adsorb at the metal-dielectric interface. The adsorbed molecules may cause a change in the effective Schottky barrier height (and the direction and/or quantity of the change might depend on the amount and/or type of analyte that is adsorbed). This change in the Schottky barrier height may, for example, change the capacitance of the gate 220 and/or influence the region 212. The resulting change in current through the region 212 may then be correlated to the concentration of the analyte 250 in the sensor's environment.
Although the sensor 200 may be used to detect the presence of the analyte 250, it might be difficult to use the sensor 200 to selectively detect different types or species of analyte.
As used herein, a bias may be “variable” in that, for example, the bias changes over time - such as when an Alternating Current (AC) bias is provided to a sensor's gate contact. A bias might also be “variable” in that a first bias is applied to detect a first species of analyte while a second bias is later applied to the same sensor (e.g., to detect a second species of analyte). As another example, a first bias might be applied to a first sensor (or sub-sensor) while a second bias is applied to a second sensor at the same time (e.g., so that multiple species of analyte can be detected simultaneously). Several examples of variable biases are provided in connection with
At Step 304, an electrical characteristic associated with the sensor is measured to detect the analyte. The electrical characteristic might be associated with, for example, a source drain current level. As another example, the characteristic might be associated with a source drain current waveform. For example, a frequency and/or a time constant of a response signal waveform might be monitored to determine information associated with background concentrations of an analyte or other substance.
The sensor 500 includes a semiconductor layer 510, such as layer that includes silicon carbide, gallium nitride, and/or a WBG material. According to some embodiments, the layer 530 includes a metal, such as aluminum, gold, nickel, rhenium, tantalum, and or osmium. Moreover, according to some embodiments, the layer 520 is formed from a metal oxide such as gallium oxide, silver oxide, indium oxide, vanadium oxide, Mn2O3, CuO, Cr2O3, Co2O3, ZnO, Ge2O3, FeO2, and/or bismuth molybdate. According to other embodiments, the layer 520 is formed from a metal alloy, such as platinum/rhodium, palladium/iridium, platinum/titanium/gold, platinum/ruthenium, platinum/iridium, and/or platinum/gold.
A dielectric layer 524 separates a catalytic gate 520 from the semiconductor layer 510. The dielectric layer 524 might be, for example, a layer of SiO2, SiN, HfO2 and/or a metal oxide. The catalytic gate 520 may be, for example, a metallic contact such as one formed from a combination of oxides including platinum/tin oxide, platinum/indium oxide, zinc oxide/vanadium oxide, indium oxide/tin, or oxide/manganese oxide, Pt/Ga2O3, Pt/Ag/Ga2O3. According to some embodiments, the catalytic gate 520 comprises a material of the formula ABO3 where A is lanthanum and B is any transition metal or alkaline earth metal. In this way, the gate 520 and dielectric layer 524 might form, for example, a metal/metal oxide stack on silicon nitride.
Note that the catalytic gate 520 may be a multiple layer stack of catalytic material layers. Each layer might include, for example, a single catalytic material or a combination alloy of catalytic materials. According to some embodiments, each layer of material may have a thickness from about 50 Å to about 8000 Å.
A source contact 530 is electrically coupled to the semiconductor layer 510 and an electrical ground. Similarly, a drain contact 540 is coupled to the semiconductor layer 510, remote from the source contact 530, as well as a drain voltage source 542. The drain voltage source 542 provides a drain voltage VD to the drain contact 540. The source contact 530 and the drain contact 540 may be formed using, for example, nickel, titanium, aluminum, gold, chromium, and/or indium.
The arrangement illustrated in
According to some embodiments, the sensor 500 acts as a WBG based FET device. For example, the catalytic gate 520, dielectric layer 524, and semiconductor layer 510 might be associated with a heterojunction wherein a surface of a heavily doped/high bandgap material interfaces with a surface of a lightly doped/low bandgap material.
A property of the catalytic gate 520 may change when exposed to an analyte 550. For example, when the gate 520 is exposed to an analyte 550, molecules of the analyte may diffuse through the gate 520 and adsorb at the metal-semiconductor interface. The adsorption of the analyte by the catalytic gate 520 might, for example, change its capacitance and create a layer of ions between the catalytic gate 520 and a dielectric interface. This change may also change the capacitance of the gate 520 and/or influence a channel formed between the source contact 530 and the drain contact 540. The resulting change in current through the channel may then be correlated to the concentration of the analyte 550.
According to this embodiment, an AC voltage source 522 provides a bias that varies over time to the catalytic gate 520 (e.g., on the side of the gate 520 opposite the dielectric layer 524 and the semiconductor layer 510). Note that the AC bias may cause the adsorbed molecules in the gate 520 to move closer or further from the catalyst gate into the dielectric layer. Moreover, different types of molecules may move further up and down as compared to other molecules (e.g., based on the weight, mobility, and/or charge of each type of molecule) and the average displacement of a particular type of molecule might be based on the Root Mean Squared (RMS) value of the AC signal. In this way, applying an AC frequency to the sensor may improve the ability of the sensor 500 to detect a particular species of analyte (e.g., because other species may be moved further away from the junction).
According to some embodiments, the frequency associated with the AC voltage source 522 is varied to adjust the selectivity of the sensor 500 to different species of analyte. For example, a first AC frequency might be applied (and the source drain current monitored) to detect a first species of analyte while a second AC frequency might be used to detect a second species of analyte.
Although an AC bias is described with respect to
Moreover, although a variable bias is applied to the catalytic gate 520 in
As still another example, the FET device might be operated in a constant source drain current mode while the threshold value of the device is monitored (e.g., the level at which the device will turn “on” or “off”). A change in the threshold value may then be correlated to a concentration of analyte. Note that this mode of operation might be associated with constant power dissipation (and hence constant temperature operation).
According to some embodiments, an additional passivation layer is applied to a portion of the surface of the semiconductor layer 510. The passivation layer may comprise, for example, MgO, Sr2O3, ZrO2, Ln2O3, TiO2, AIN, and/or carbon and may act to improve the thermal stability and reproducibility of the sensor 500.
According to some embodiments, a heater may be provided proximate to the catalytic gate 520. The heater might comprise, for example, a wire of titanium and/or nickel and may be used to hold the device to a substantially constant temperature during operation. Such an approach might reduce any drift in operation of the sensor 500 due to changes in temperature. Another approach is to attach the die onto a ceramic board and deposit a metal line of Ti/Au on the backside to heat the device/
According to some embodiments, a “reset” signal may be applied to the sensor 500. Consider, for example, a catalytic gate 520 that has been exposed to (and therefore adsorbed) an analyte. In this case, a bias could be applied to the catalytic gate 520 in order to facilitate the expulsion of any adsorbed molecules (e.g., and reduce the device's “memory” that it was exposed to the analyte). Such a reset pulse might be applied, for example: periodically; after a threshold amount of an analyte has been detected; and/or when a different species of analyte is to be sensed by the sensor 500. Note that the polarity and magnitude of the reset signal may determine which types of analytes are expelled from the catalytic gate 520.
Note that a sensor might be creating using any type of FET device, including a Metal Oxide Semiconductor FET (MOSFET), a Heterostructure FET (HFET), and/or a Metal-Insulator Semiconductor Heterostructure FET (MISHFET).
Moreover, a sensor might be implemented using a device other than a transistor. For example,
According to another embodiment, a capacitor may be used to detect an analyte. For example,
According to some embodiments, a similar substrate and/or substrate bias may be combined with the approach described with respect to
Instead of (or in addition to) providing a bias the varies dynamically over time, according to some embodiments different biases may be simultaneously provided for different sensors or sub-sensors. For example,
A first gate voltage source provides a first gate voltage (VG1) to the first catalytic gate while a second gate voltage source provides a second t gate voltage (VG2) to the second catalytic gate (and VG1 does not equal VG2). By providing different biases to the gates 920, 922, the sensor may be used to detect multiple species of analytes. Although two FET devices are illustrated in
According to some embodiments, one or more devices in an array are prevented from adsorbing the analyte. For example,
In this case, a shielding layer 1070 is formed on the second catalytic gate 1922 to prevent it from being exposed to an analyte. The shielding layer 1070 might include, for example, silicon dioxide, silicon nitride and/or hafnium dioxide that will block molecules of analyte from being adsorbed by the second catalytic gate 1022. In this way, the source drain current associated with the first FET device might be monitored to detect a change in analyte concentration while the source drain current associated with the second FET device might be monitored to detect a change in, for example, a temperature.
According to some embodiments, a single voltage source may be used to provide variable biases for a sensor. For example,
Instead of (or in addition to) providing different biases to different catalytic gates, a sensor array could provide variable biases to source or drain contacts. For example,
Accordingly, embodiments described herein may provide sensors that are able to selectively detect different species of an analyte. Moreover, the sensors may appropriate for use in systems associated relatively harsh environments.
For example,
Note that wide bandgap material may be capable of withstanding the temperatures and corrosive conditions associated with harsh environments. For example, the materials may provide chemically stable, thermally stable, repeatable responses in wide temperature and pressure ranges. Moreover, such materials may be cost effective in that they might be manufactured into devices on a relatively large scale along the lines of well-established semiconductor devices. Note that computer programming or similar techniques may be used to adjust voltage levels and/or monitor characteristics for the sensor 1310 as appropriate.
According to some embodiments, the sensor 1310 is encapsulated. The encapsulation might, for example, protects the sensor 1310 from high temperatures and/or corrosive atmospheres. The encapsulant might, for example, cover the ohmic contact metals and peripheral areas of the sensor 1310 which do not benefit from exposure to the gases. This coverage may also be enhanced by forming a bond with the underlying layer which does not permit the flow of gases or other corrosive molecules which would be a detriment to the sensor 1310 over time. Examples of suitable materials for encapsulating include, but are not limited to, silicon carbide, ceramic-based epoxies such as those containing alumina, glass, quartz, silicon nitride, and/or silicon dioxide. The encapsulation layer might be deposited by any method, such as Plasma Enhanced Chemical Vapor Deposition (PECVD) or Low Pressure Chemical Vapor Deposition (LPCVD). Of course, at least a portion of one or more catalytic gate electrodes will remain exposed to ambient gases.
The system also includes a sensor dependent device 1320. The sensor dependent device 1320 might be associated with, for example, an air quality device, an oil quality device, an industrial process control device, an emissions management device, and/or a turbine sensor.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11137845 | May 2005 | US |
| Child | 11687872 | Mar 2007 | US |
