Avalanche gas ionization detector

Abstract

A gas detector is presented. The gas detector is comprised of: an ionization chamber, an ionization source, a pair of collection electrodes and a p-n junction. The ionization source is configured to ionize molecules of the gas sample residing in the ionization chamber. The pair of collection electrodes are arranged proximate to the ionization chamber and configured to apply a collection voltage across the pair of collection electrodes. The p-n junction is disposed adjacent to a first electrode of the pair of collection electrodes and electrically coupled to the first electrode. During operation, electrons (or negative ions) produced by ionization are collected by the first electrode electrically coupled to the p-n junction and then accelerated in the depletion region by the electric field of the reversely biased p-n junction to form avalanche electrons. Consequently, the signal can be amplified, which in turn increases the sensitivity of the gas detector.

Description

AVALANCHE GAS IONIZATION DETECTORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 541 ,025, filed on September 28, 2023. The entire disclosure of the above application is incorporated herein by reference.GOVERNEMNT CLAUSE

[0002] This invention was made with government support under R01 OH011082 awarded by the Centers for Disease Control and Prevention, and TR004066 awarded by the National Institutes of Health. The government has certain rights in the invention.FIELD

[0003] The present disclosure relates to ion-based gas sensors.BACKGROUND

[0004] In general, there are three categories of gas sensing methods. The first one is based on single gas sensors, such as electrochemical sensors, metal oxide sensors, and nanomaterial (like gold nanoparticles and carbon nanotubes) based sensors, to detect individual gas species, such as CO2, H2S, NO2, and alcohol (ethanol), etc. An array of the above gas sensors can also be constructed to form an eNose. These individual sensors and eNose are simple and of low cost but lack sufficient analytical capability to deal with complex matrices or identify unknown compounds. Furthermore, they are easily affected by ambient factors such as humidity, temperature, and the presence of other vapor molecules (interferents). The detection limit is usually on the order of sub-ppm to ppm.

[0005] Spectroscopic techniques have also been explored for gas sensing, such as deep UV absorption spectroscopy, infrared laser spectroscopy, photo-acoustic spectroscopy, Raman spectroscopy, cavity ring-down spectroscopy, Fourier transform infrared spectroscopy, and frequency comb. However, due to the possible spectral overlap among the gas analytes themselves and with the interferents, the gas identification and quantification ability drops significantly when a sample contains a large number of gas species (>20). Additionally, the related instruments are bulky andexpensive. The detection limit for most of the instruments is in the range of ppm to subppm. Usually, optical detectors are used in those instruments.

[0006] The third method uses analytical instruments, such as gas chromatography (GC), GC x GC (also known as 2D GC), mass spectrometry (MS), GC- MS, GC-IMS (gas chromatography in tandem with ion mobility spectrometry). Gas chromatography relies on the separation of different gas molecules ( / .e., their retention times) for gas analysis, whereas mass spectrometry and ion mobility spectrometry use mass / charge and size / charge spectra for gas analysis. Generally, these instruments have much higher analytical capabilities than those in the first two categories and can analyze complex samples, and therefore, are widely used in various gas analysis applications. Particularly, GC-MS is often regarded as the gold standard in gas analysis.

[0007] While non-ion-based gas sensors, such as chemiresistive I chemicapacitive sensors, optical interferometric sensors, thermal conductivity detectors (TCDs), and metal oxides are explored in the above analytical instruments, ion-based gas sensors are the most commonly used. In ion-based gas sensors, gas molecules are first ionized, and the resulting ions (positive / negative ions and electrons) are detected for analyte quantification. Ionization sources include flame (flame ionization), vacuum UV (VUV) light (~70 nm - 200 nm in wavelength, photoionization), radioactive materials (e.g., Nickel-63 that emits beta radiation), high-voltage discharge (corona ionization), high-energy (e.g., 70 eV) electrons (electron impact ionization), and charged molecules (e.g., proton transfer ionization). The corresponding gas sensors, include flame ionization detector (FID), photoionization detector (PID), electron capture detector (ECD, which ionizes the make-up gas), Faraday cup based detector, and electron multiplier tube (EMT). The ionization source can be located at the same place as the ion detector, such as in FID and PID, or some distance away from the ion detector (in which case the ions travel to the detector), such as in Faraday cup based detector and EMT used in IMS and MS, respectively. The captured ions (or electrons) form an electrical current that is subsequently converted to voltage. Faraday cup based gas detectors do not have internal or external amplification, FID and PID have external amplification. They perform analog detection and are unable to measure individual ions due to the low signal-to-noise ratio. In contrast, EMT, which is similar to PMT (photomultiplier tube) used in photon detection, has a large internal gain due to a series of voltage-biased dynodes. Therefore, EMT can count individual ions. However, anEMT requires high voltage (>1000 V) and vacuum to operate, and it is also very bulky and expensive.

[0008] This section provides background information related to the present disclosure which is not necessarily prior art.SUMMARY

[0009] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0010] In one aspect, a gas detector is presented. The gas detector is comprised of: an ionization chamber, an ionization source, a pair of collection electrodes and a p-n junction. The ionization source is configured to ionize molecules of the gas sample residing in the ionization chamber. The pair of collection electrodes are arranged proximate to the ionization chamber and configured to apply a collection voltage across the pair of collection electrodes. The p-n junction is disposed adjacent to a first electrode of the pair of collection electrodes and electrically coupled to the first electrode. During operation, electrons (or negative ions) produced by ionization are collected by the first electrode electrically coupled to the p-n junction and then accelerated in the depletion region by the electric field of the reversely biased p-n junction to form avalanche electrons. Consequently, the signal can be amplified, which in turn increases the sensitivity of the gas detector.

[0011] More specifically, the first electrode is interfaced with p side of the p-n junction and the p-n junction is reverse biased, such that the collection voltage drives negative ions or electrons generated from the gas sample towards the first electrode and then accelerated through the p-n junction. Alternatively, the first electrode is interfaced with n side of the p-n junction and the p-n junction is reverse biased, such that the collection voltage drives positive ions generated from the gas sample towards the first electrode and the positive ions are accelerated through the p-n junction.

[0012] In an example embodiment, the ionization source is further defined as a vacuum UV light source. The vacuum UV light source may be disposed adjacent to the pair of collection electrodes and the pair of collection electrodes are configured to pass light from the vacuum UV light source into the ionization chamber. In other embodiments, the vacuum UV light source is arranged adjacent to the ionization chamber but distal from the pair of collection electrodes.

[0013] In yet other embodiments, the ionization source is further defined as radioactive material.

[0014] In the example embodiment, the pair of collection electrodes are arranged on opposing sides of the ionization chamber and configured to apply a collection voltage across the ionization chamber.

[0015] The gas detector may further include a pump fluidically coupled to the ionization chamber as well as a current sensor configured to measure the current through the p-n junction, where magnitude of the current correlates to quantity of the molecules in the gas sample.

[0016] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.DRAWINGS

[0017] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0018] Figure 1A is a diagram depicting an example embodiment of a gas detector configured to detect negatively charged ions or electrons.

[0019] Figure 1A is a diagram depicting an example embodiment of a gas detector configured to detect positively charged ions.

[0020] Figure 2 is a diagram depicting an example embodiment of a gas detector with different ionization source.

[0021] Figure 3 is a diagram depicting an example embodiment of a gas detector with the ionization source arranged distal from the pair of collection electrodes.

[0022] Figures 4A and 4B are diagrams depicting two gas detectors implemented to test feasibility.

[0023] Figure 5 illustrates an example fabrication method for the gas detector.

[0024] Figure 6 is a diagram showing an experimental setup for a gas detector.

[0025] Figures 7A-7C are graphs showing test results for the gas detector.

[0026] Figure 8 is a diagram depicting another example embodiment of the gas detector.

[0027] Figures 9A and 9B are graphs showing the collection voltage dependency of the gas detector when the ionization chamber is filled with acetone and helium, respectively.

[0028] Figures 9Cand 9D are graphs showing zoomed in sections of Figures 9A and 9B.

[0029] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.DETAILED DESCRIPTION

[0030] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0031] Figures 1 A and 1 B illustrate example embodiments of a gas detector 10 in accordance with this disclosure. The gas detector 10 is comprised of: an ionization chamber 12, an ionization source 14, a pair of collection electrodes 16 and a p-n junction 18. The ionization chamber 12 has an inlet to receive a gas sample therein, and the ionization source 14 is configured to ionize molecules of the gas sample residing in the ionization chamber 12. In one example, the ionization source 14 is further defined as a vacuum UV light source although other types of sources are contemplated as discussed below. It is to be understood that only the relevant components of the gas detector are discussed in relation to Figures 1 A and 1 B, but that other components may be needed to control and manage the overall operation of the system.

[0032] The pair of collection electrodes 16 are arranged in the ionization chamber 12 and configured to apply a collection voltage across the pair of collection electrodes 12. In these example embodiments, the pair of collection electrodes arranged on opposing sides of the ionization chamber 12. The vacuum UV light source 14 is disposed adjacent to one of the collection electrodes which has one or more holes that permit light to pass from the vacuum UV light source 14 into the ionization chamber 12. In other embodiments, the pair of collection electrodes 16 may be arranged adjacent to each other in a coplanar arrangement. The p-n junction 18 is disposed adjacent to one of the electrodes and is electrically coupled thereto.

[0033] During operation, the p-n junction 18 is reverse biased. In Figure 1A, the electrons (or negative ions) produced by ionization are collected by the electrode electrically coupled to the p-n junction 18 and then accelerated in the depletion regionby the electric field of the reversely biased P / N junction to form avalanche electrons. Consequently, the signal (i.e., current) can be amplified via the internal avalanche process, which in turn increases the sensitivity of the gas detector 10.

[0034] Similarly, the positive ions can be amplified through the same avalanche process in the reversely biased P / N junction, which is engineered in a way that holes (instead of electrons) are used for positive ion avalanche amplification as seen in Figure 1 B. That is, one of the collection electrodes is interfaced with n side of the p-n junction, such that the collection voltage drives positive ions generated from the gas sample towards the electrode and the positive ions are accelerated through the p-n junction. The gas detector can be operated with a bias voltage below the breakdown voltage, which produces an internal gain of 50-100 ( / .e., linear amplification mode). It can also be operated near the breakdown voltage to achieve single ion counting ( / .e., Geiger mode).

[0035] Additionally, the gas detector 10 can be regarded as the ion detector counterpart of an avalanche photodiode (APD) that absorbs incident photons in its conversion (or intrinsic) layer and produces photon-induced electron-hole pairs. However, there are quite a few differences. First, the gas detector 10 does not have a photon conversion layer, instead, it has an ionization chamber. Second, collection electrodes 16 ( / .e., ion capture electric field) need to be implemented to capture ions (or electrons). In contrast, an APD does not have ion capture electrodes. Third, some ionization sources can potentially contribute to background current, which should be taken into account seriously when designing and fabricating the gas detector 10. For example, the VUV light may produce photoelectrons in the metal electrodes and / or semiconductor materials, as the metal work function and the semiconductor bandgap are lower than the VUV photon energy. In contrast, no ionization source is needed for an APD.

[0036] Figure 2 depicts an alternative embodiment for the gas detector 10. In this embodiment, the ionization source 14 is a radioactive material, such as nickel 63, or a corona discharge introduced into the ionization chamber 12. As noted above, the ionization source 14 ionizes molecules of the gas sample residing in the ionization chamber 12. Except with respect to the differences discussed herein, the gas detector may be substantially the same as the gas detector described above in relation to Figures 1A and 1 B. Other types of ionization sources suitable for use in the gasdetector include but are not limited to electron impact ionization source and a flame ionization source.

[0037] Figure 3 illustrates yet another embodiment for the gas detector 10. In this embodiment, the ionization source 14 is arranged adjacent to the ionization chamber 12 but is distal (or spatially separated) from the pair of collection electrodes 16 and the p-n junction 18. Except with respect to the differences discussed herein, the gas detector may be substantially the same as the gas detector described above in relation to Figures 1 A and 1 B.

[0038] For proof of concept, two gas detectors Gen-1 and Gen-2 were fabricated and tested as seen in Figures 4A and 4B. The gas detectors Gen-1 and Gen-2 are comprised of a Kryton VUV lamp, a transmission window made from magnesium fluoride, ionization chamber, collection electrodes, and a P / N junction. The two gas detectors Gen-1 and Gen-2 have an ionization chamber volume of 0.22 mL and 0.79 mL, respectively.

[0039] To avoid the potential photoelectron generation from the ion capture electrodes (made of gold) and semiconductor and caused by the ultraviolet illumination, a small aperture is made to limit the spread of the ultraviolet light from the VUV lamp. The collection electrodes are also offset from the direct VUV illumination. Furthermore, a dielectric layer (e.g., SiO2) is coated on top of the semiconductor, leaving only a small window for the P+to be in contact with the collection electrode. Results show that this design can significantly reduce the background current caused by the ultraviolet illumination.

[0040] In this implementation, the P / N junction is formed by shallow boron diffusion with a high doping concentration (~1019cm-3) into a lightly phosphorus doped (phosphorus, ~1016cm-3) N type silicon. The junction has a diameter of 15 pm and a depth of ~250 nm. The N-type silicon has a thickness of 500 pm. N+is made of highly doped phosphorus (~1019cm-3) for ohmic contact with the metal electrode at the bottom. Fabrication of the P / N junction is further illustrated in Figure 5. The fabrication recipe is optimized to achieve an abrupt P / N junction. This is critical to avoiding a graded junction, which can significantly reduce the P / N junction breakdown voltage to <30 V.

[0041] Figure 5 illustrates an example fabrication process for the p / n junction. In Steps 1 -5, one side of the moderately doped N-type (phosphorous) silicon wafer (0.3- 0.5 Q*cm, i.e., ~1016 cm-3) was heavily doped (~1019 cm-3) with phosphorous to formthe ohmic contact with the metal layer at later stages. Thermal oxide was used as the diffusion mask in these steps to reduce interfacial defect states. Phosphorous diffusion was carried out at 950 oC for 2 hours. After diffusion, the mask oxide as well as the phosphorous silicate glass (PSG) were stripped by buffer hydrofluoric (BHF) wet etch. Then, the alignment marks on each die used for lithography exposure using a stepper were etched by reactive ion etch (RIE) on the silicon substrate for a depth of 100 nm.

[0042] In Step 6-9, the P+ / N junction was formed. Thermal oxide could no longer be used for masking due to the N+ layer formed on the backside, which would otherwise be consumed by oxidation. Instead, low pressure chemical vapor deposition (LPCVD) was used to grow 600 nm of high temperature oxide (HTO) at 900 oC for 1 hour as mask oxide on the wafer. Next, a 15 pm circular opening defining the P+ well was patterned with photoresist and the mask oxide was dry-etched by RIE for 500 nm before BHF wet-etch the final 100 nm of mask oxide to protect the Si surface from potential RIE damage. The boron diffusion was carried out at 1 ,000 oC for 2 hours to form a ~250 nm shallow junction with a diffusion profile shown in Figure S4. Note that it was experimentally determined that the diffusion oxide mask could not be used as the self-aligned dielectric layer for metallization, as the boron had also diffused into the oxide and a thin boron silica glass (BSG) layer was formed during diffusion, which would result in a high leakage current. After stripping mask oxide and boron silica glass (BSG), a 500 nm LPCVD HTO was freshly regrown, and wafers were immediately annealed in forming gas (5% H2 / 95% N2) at 450 oC for 4 hours to reduce interfacial trap states in dielectrics post-LPCVD. All processes up to this step had been CMOS- grade for contamination control.

[0043] In Steps 10-14, after the backside oxide was fully stripped in BHF and a 5 pm circular opening was patterned in the middle of the P+well (15 pm in diameter), the metallization was carried out by e-beam evaporation and lift-off processes. A tri-layer metal stack of 10 nm Titanium (Ti) / 30 nm Platinum (Pt) / 500 nm Gold (Au) was deposited on top of P+well. The Pt layer serves as the diffusion barrier of gold to prevent potential leakage currents over long term operation of the P+ / N junction. On the other side (N+), 10 nm Ti / 500 nm Au stack was deposited without a barrier layer. It was experimentally found that a metal contact area on a P+well larger than ~1 mm resulted in leaky current that quenched the avalanche effect. This was hypothetically attributed to the fact that LPCVD SiO2 had surface defect states and a larger contact could pick them up more easily than a smaller contact. Next, a window of AI2O3 was etched-stopon the underlying metal with a dimension smaller than the metal contact itself, and it was followed by a larger dimension metal contact lift-off process to define the ion collection electrode. This process is merely exemplary and other methods for fabricating the gas detector are contemplated by this disclosure.

[0044] Figure 6 depicts an experimental setup 60 for the gas detector. In addition to the gas detector 10 described above, the setup includes a pump 62 fluidically coupled to the ionization chamber of the gas detector and a current sensor 64. The pump 62 operates to move the gas sample in and out of the ionization chamber. The current sensor 64 is configured to measure the current through the p-n junction, where the magnitude of the current correlates to quantity of the molecules in the gas sample.

[0045] Figure 7A shows the l-V curve for the P / N junction in the Gen-2 gas detector described above, in which case the VUV illumination is turned off, Vi is set to be zero, and V2 is scanned. When V2 < 15V, the P / N junction shows a high ohmic resistance. The current is on the order of 10 pA. When V2 is increased to 15-26.8 V, avalanche processes take place, and the current becomes 100 pA. When V2 > 26.8 V (the breakdown voltage), an extremely high current (-5 mA) is observed. The l-V curve shows the characteristics commonly seen in an avalanche photodiode. However, compared to a typical P / N junction in an avalanche photodiode (like Hamamatsu APD) whose breakdown voltage is -150 V, the breakdown voltage in the gas detector is quite low due to the unique abrupt P / N junction.

[0046] Figure 7B shows the sensing signal ( / .e., current) from the gas detector when the ionization chamber is filled with helium (background, zero analyte) and analyte (acetone). This time, the Krypton lamp is turned on to produce 10.6 eV photons as the ionization source. Helium cannot be ionized by the 10.6 eV photons and therefore serves as the background, where acetone, whose ionization potential is 9.7 eV, can be ionized by the 10.6 eV photons. In addition, Vi=24 V is applied to the ion capture electrodes to collect the electrons. Taking a look at the black curve (helium) first. When V2 < 15 V, there is a background current of -1 nA, which is caused mainly by photoelectrons when the metal electrodes are not completely shielded from the VUV illumination. When V2 is increased to 15-26 V, those photoelectrons are amplified by the avalanche process and produce a current of -10 nA. It is envisioned that the gas detector can be further optimized to minimize the exposure of the electrodes to the VUVillumination. The background current is expected to be around 100 pA when V2=15 - 26 V.

[0047] The upper curve in Figure 7B shows the signal when the ionization chamber is filled with acetone. When V2 < 15 V, the acetone signal is buried within the background (black curve). When V2 is increased to 15-26 V, the current increases to ~200 nA, which is ~20X higher than the background current with helium, showing that the primary electrons generated by photoionization of acetone molecules are amplified by the avalanche process. This region is called the linear amplification region (or analog region). In analogy to the gain definition used in avalanche photodiodes, the gain of the gas detector, which is approximately 200 (200 nA vs. 1 nA), can be estimated by comparing the acetone signal when V2=25 V and V2=1 V, assuming that the signal at V2=1 V is 1 nA in the presence of acetone.

[0048] Figure 7C shows the zoom-in portion when the gas detector is operated in the Geiger mode when V2 is larger than 26.8 V. A ratio exceeding 104between acetone and helium ( / .e., zero acetone) is reached, even though there still exists a background current of ~10 nA due to VUV-induced photoelectrons. With further improvement, the background current is expected to be reduced to 100 pA, and therefore, a signal-to- background ratio as high as 106can be reached. Such a high ratio allows one to easily set a threshold to discriminate a real pulse generated by the analyte (after photoionization) and the noise from the background, thus enabling single electron counting.

[0049] Figure 8 illustrates yet another example embodiment of the gas detector 10. In this embodiment, a semitransparent mesh deflector electrode 16A was implemented, preferably as a honeycomb structure. The mesh design preserves the ideal parallel-plate electric field for ion / electron capture while allowing in-situ VUV photon ionization. This mesh design maximizes photoionization and capture efficiency with the same deflection voltage. However, it also introduces a significant VUV photocurrent background. Despite this background, net acetone current was detected. It is worth noting that the meshed design shows higher ionization and capture efficiencies with a stronger ion collection field within the ionization region ( / .e., direct overlap), with a ~3 nA PIN mode current sensed. Other arrangement for the deflector electrode are envisioned including an interdigitated figures arrangement.

[0050] Lastly, the tunability of the gas detector’s sensitivity by varying the ion collection bias voltage, i.e., Vi, was investigated as Vi determines the collectionefficiency of the primary electrons generated by ionization of gas analytes. Figure 9A shows the l-V curves of the gas detector with headspace acetone gas molecules filled in the microfluidic chamber when Vi was varied from 3 V to 96 V. There are no differences in signal strength in the PIN mode, indicating that the unamplified signals were all buried under the background current. In the analog mode, however, ion collection voltage dependent signals start to emerge, with a larger Vi producing a higher acetone current at the same reverse bias voltages (V2) of the P+ / N junction (see the inset of Figure 9A). More primary electrons are collected at the higher Vi, leading to an increase in avalanche electrons generated at the same V2. The data in Figure 9A presented in a linear scale show that at a low Vi (3 - 12 V), corresponding to a lower electron capture efficiency, the acetone current increases almost linearly with V2. At a higher Vi (24 - 96 V), corresponding to a higher electron capture efficiency, the trend evolves from super-linear to sub-linear before eventual breakdown, suggesting saturation of avalanche events. This can be attributed to the depletion of available silicon lattice sites for additional impact ionization events.

[0051] In contrast, a different phenomenon was observed in the Geiger mode. As depicted in Figure 9C, the breakdown voltage shifts to a lower V2 by as much as 300 mV when Vi is increased from 3 V to 96 V. Unlike the progressively higher avalanche current with increased Vi in the analog mode, the resulting avalanche currents at different Vi’s are similar (~10-4A) once it reached the Geiger mode, implying its binary ( / .e., digital) detection nature regardless of the captured electron quantities.

[0052] As a control experiment, when the gas detector was filled with helium, almost no ion collection voltage (V-i) dependence was observed in the analog mode (Figure 9B). In the Geiger mode (Figure 9D), the breakdown voltage (V2) is centered around 27.15 V and exhibits minimal dependence on Vi, being indistinguishable within measurement errors. This suggests that the larger capturing electric field inside the microfluidic ionization chamber does not capture any additional electrons from the chamber apart from the photoelectrons generated directly on the gold electrodes. These photoelectrons are generated in-situ on the physical surfaces of the gold electrodes without going through the capturing process by the ion collection electrodes ( / .e., no ion collection voltage dependence). From another perspective, this ion collection voltage independence also suggests that the quantity of photoelectrons generated by the dielectric stack ( / .e., Al2O3 / SiO2) due to the VUV illumination is insignificant inside the microfluidic chamber, which would otherwise producedistinguishable avalanche currents since they would be captured by the collection field to the collector for amplification, hence showing ion collection voltage Vi dependence. Lastly, comparison of Figures 9C and 9D shows that the Geiger window can be enlarged ( / .e., operation voltage range for counting mode) with increased Vi.

[0053] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0054] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0055] When an element or layer is referred to as being "on," “engaged to,” "connected to," or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," “directly engaged to,” "directly connected to," or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). Asused herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0056] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0057] Spatially relative terms, such as “inner,” “outer,” "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0058] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

CLAIMSWhat is claimed is:1 . A gas detector, comprising: an ionization chamber having an inlet to receive a gas sample therein; an ionization source configured to ionize molecules of the gas sample residing in the ionization chamber; a pair of collection electrodes arranged proximate to the ionization chamber and configured to apply a collection voltage across the pair of collection electrodes; and a p-n junction disposed adjacent to a first electrode of the pair of collection electrodes and electrically coupled to the first electrode.

2. The gas detector of claim 1 wherein the first electrode is interfaced with p side of the p-n junction and the p-n junction is reverse biased, such that the collection voltage drives negative ions or electrons generated from the gas sample towards the first electrode and then accelerated through the p-n junction.

3. The gas detector of claim 1 wherein the first electrode is interfaced with n side of the p-n junction and the p-n junction is reverse biased, such that the collection voltage drives positive ions generated from the gas sample towards the first electrode and the positive ions are accelerated through the p-n junction.

4. The gas detector of claim 1 wherein the ionization source is further defined as a vacuum UV light source.

5. The gas detector of claim 4 wherein the vacuum UV light source is disposed adjacent to the pair of collection electrodes and the pair of collection electrodes are configured to pass light from the vacuum UV light source into the ionization chamber.

6. The gas detector of claim 4 wherein the vacuum UV light source is arranged adjacent to the ionization chamber but distal from the pair of collection electrodes.

7. The gas detector of claim 1 wherein the ionization source is further defined as radioactive material.

8. The gas detector of claim 1 further comprises a pump fluidically coupled to the ionization chamber.

9. The gas detector of claim 1 further comprises a current sensor configured to measure the current through the p-n junction, where magnitude of the current correlates to quantity of the molecules in the gas sample.

10. A gas detector, comprising: an ionization chamber having an inlet to receive a gas sample therein; an ionization source configured to ionize molecules of the gas sample residing in the ionization chamber; a pair of collection electrodes arranged on opposing sides of the ionization chamber and configured to apply a collection voltage across the ionization chamber; and a p-n junction disposed adjacent to a first electrode of the pair of collection electrodes and electrically coupled to the first electrode.11 . The gas detector of claim 1 wherein the first electrode is interfaced with p side of the p-n junction and the p-n junction is reverse biased, such that the collection voltage drives negative ions or electrons generated from the gas sample towards the first electrode and then accelerated through the p-n junction.

12. The gas detector of claim 1 wherein the first electrode is interfaced with n side of the p-n junction and the p-n junction is reverse biased, such that the collection voltage drives positive ions generated from the gas sample towards the first electrode and the positive ions are accelerated through the p-n junction.

13. The gas detector of claim 1 wherein the ionization source is further defined as a vacuum UV light source.

14. The gas detector of claim 4 wherein the vacuum UV light source is disposed adjacent to a second electrode of the pair of collection electrodes and the second electrode having one or more holes that permit light to pass from the vacuum UV light source into the ionization chamber.

15. The gas detector of claim 1 further comprises a pump fluidically coupled to the ionization chamber.

16. The gas detector of claim 1 further comprises a current sensor configured to measure the current through the p-n junction, where magnitude of the current correlates to quantity of the molecules in the gas sample.

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