200809189 九、發明說明： 【發明所屬之技術領域】 本發明係關於一種氫氣感測器，1 阻式氫氣感測器及其操作系統。’、寸；種半導體電 【先前技術】 用途氮要伽_職錢氣與贼㈣監控之 用述，近年末，虱乳感測器已應用於眾 用於化學工業、半導體製程、醫療處理、和wtiil方: 因此展，域 皆具;極電二t 低、财用期長、適合高氣體濃度檢測、可===成 ¥體電阻式虱風測态，其使用上包括下述缺點· _測器之電阻值變化係因其所感、氣體 還原性氣體)及該半導體種類之不同而有所不同的體=^^ 該感測器無法感測到較低的氫氣濃度（低於千,以下）； 該感測H於低溫環境下，其感魏力受麻織幾乎無感測 能力，且 該感測器需外加較大的偏壓才能產生明顯的電流電壓之特性 曲線變化； 此外，由於傳統型的半導體電阻式感測器屬於被動式元件， 其需要額外附加設備或轉換電路裝置才能進行分析數據或放大該 測量值，造成增加氫氣感測之成本價格且無法達到智慧化的要 求，因此改良氫氣感測器對於各領域之發展實屬重要。 5 200809189 【發明内容】 鑒於以上所知現有技術作法之缺點，本發明之主要目的係提 供一種有效探測低氫氣濃度、低溫感測能力佳、元件操作電壓小 故為三端主動式感測元件點的半導體電阻錢 及其操作系統。 器 ，達成上述目的，本發明提供—種半導體電阻式氫氣感測 包括： 一基板； 一緩衝層’其係成長於該基板之上； 一主動層’其係成長於該緩衝層之上； 一蕭特基接觸層，其係成長於該主動層之上； 一帽層，其係成長於該蕭特基接觸層之上； 成 且電㈣由—歐姆性金屬接觸層所構 層之上=層’錢成長於祕_和源㈣極間之蕭特基接觸 且其係成自1躲_金屬酬層所形成， 測器測供-種以半導體電阻式氫氣感 同去檢财卜蚊·偏壓不 端將問極接地，該感測器可做為兩 端二乏型之主動式操作方式，職制科加負的閘極 6 200809189 偏壓’其可做為三端空乏型之主動式氫氣感測元件。 經由實驗數據可知，該感測器對於低濃度含量的氫氣具有良 好的感測表現，且在低溫度範圍的感測能力極佳，只需外加較^ 的偏壓便可具有明顯的電流變化，因此本發明之感測器具備低功 率、省電的特性。同時，該感測器本身又是主動型電子元件，藉 由結合微機電製造之技術可製成微小化、積體化、可攜帶式及多 功能之智慧型感測器，以達成智慧化的要求。 【實施方式】 以下配合圖示對本發明做進一步的說明後當更能明瞭。 參照^第一圖，其係為本發明感測器之結構圖；其中該半導體 電阻式氫氣感測器100係於一半絕緣型砷化鎵（GaAs)基板比 上以有機化學氣相沈積法(111过&1 〇rganic v叩沉 t^ion ’ MOCVD)成長為··一緩制2〇，其係成長於該半絕 、、1砷化鎵（GaAs)基板1〇之上，該層係為口質良200809189 IX. Description of the Invention: [Technical Field] The present invention relates to a hydrogen sensor, a resistive hydrogen sensor and an operating system thereof. ', inch; a variety of semiconductor power [previous technology] use of nitrogen to gambling _ job money and thieves (four) monitoring of the use, at the end of recent years, 虱 milk sensor has been used in the chemical industry, semiconductor manufacturing, medical treatment, And wtiil side: Therefore, the exhibition, the domain has; the pole is low, the financial period is long, suitable for high gas concentration detection, can be === into the body resistance type hurricane measurement, its use includes the following shortcomings · The change in the resistance value of the detector varies depending on the sense, the gas-reducing gas, and the type of the semiconductor. ^^ The sensor cannot sense a lower hydrogen concentration (less than a thousand, The following); the sensing H in the low temperature environment, the sense of Wei force is almost non-sensing ability by the hemp, and the sensor needs to add a large bias voltage to produce a significant characteristic curve of current and voltage; Since the conventional semiconductor resistive sensor is a passive component, it requires additional equipment or conversion circuit devices to analyze the data or amplify the measured value, resulting in an increase in the cost price of hydrogen sensing and the inability to achieve intelligent requirements. change It is important for the development of hydrogen sensors in all areas of. 5 200809189 SUMMARY OF THE INVENTION In view of the above-mentioned shortcomings of the prior art, the main object of the present invention is to provide a three-terminal active sensing element that effectively detects low hydrogen concentration, low temperature sensing capability, and low component operating voltage. The semiconductor resistor money and its operating system. To achieve the above object, the present invention provides a semiconductor resistive hydrogen sensing comprising: a substrate; a buffer layer 'which grows on the substrate; an active layer' which grows above the buffer layer; a Schottky contact layer, the system is grown on the active layer; a cap layer is grown on the Schottky contact layer; and the (4) is formed by an ohmic metal contact layer. The layer 'money grows in the secret between the secret _ and the source (four), and the system is formed from the 1 hiding _ metal layer, the measuring device is measured and the semiconductor resistance hydrogen gas is used to check the mosquitoes. The bias voltage will be grounded to the pole. The sensor can be used as the active mode of operation at both ends. The gate of the system is negative. 6200809189 Bias' can be used as the active type of three-end depletion type. Hydrogen sensing element. According to experimental data, the sensor has good sensing performance for low-concentration hydrogen gas, and has excellent sensing capability in a low temperature range, and can have a significant current change only by applying a bias voltage. Therefore, the sensor of the present invention has low power and power saving characteristics. At the same time, the sensor itself is an active electronic component, which can be made into a smart, integrated, portable and multifunctional intelligent sensor by combining the technology of MEMS manufacturing. Claim. [Embodiment] The present invention will be further clarified by the following description in conjunction with the drawings. Referring to the first figure, which is a structural diagram of the sensor of the present invention; wherein the semiconductor resistive hydrogen sensor 100 is based on a semi-insulating gallium arsenide (GaAs) substrate ratio by organic chemical vapor deposition ( 111 over &1 〇rganic v叩 sinking t^ion 'MOCVD) has grown into a two-dimensional system, which is grown on the semi-absolute, 1 gallium arsenide (GaAs) substrate, which is above Good mouth quality
鎵（GaAs) _層2G ’其厚度為麵A 緩衝層2G之上；—蕭特基接觸層4G，其係成長於 ==基接觸層4G，越歧厚度分顺2xl()lw及細a ; ’其係成長於該半導輯特基接觸層4G之上，該声係 為-未摻雜型坤化鎵（GaAs)之帽層5G，其厚度為5qA。曰’、 型石申tiirrtt域利用澄式糊的方式韻刻至半絕緣 氓ί _觀件，接著傳統光罩 祕無極之圖職，且於轉雜碎化鎵 鋅合全之德雜真絲射、統蒸鍍一金-鍺-以金屬層，該層即做為汲極和源極電極6()，接 7 200809189 氛射，錢碌燥氧氣以 蕭特基接“屬 ,^ίί '- 通道層3i，其厚度為150A，且其包^^(=Ga_As) 係為-未摻雜之珅化銘鎵（Α1 ^ λ層，——層32 ’該層 50 A，且i5M^_^2f：76As)隔離層％ ’其厚度為 曰，載子棱供層33，該層係為一矽（Si) 雜之載子提供層33’其濃度為ww，且魏 二二主動層3G之結構亦可選擇性將此三層以不同方式堆属曰排 列形成，及其有(L+M+N)!種排列方式，曰吁τ Λ>1 χ '隹宜排 層數值係介於0至5之間，脚，若L=1 = ΜΝ— 層通道層、兩層隔離層、兩層載子提供層7因而、亥 主動層有(1+2+2㈣種排列方式，即其會有12〇種排列方式而 電4貴trs第=〜三c圖:、說明本系統操作製程之等效 v J、、及；朽D糸及亟电阻’Rs係源極電阻’ Rch係通道電阻， vDS係及-源極電壓，與vGS係閘_源極電壓。 电1 H，i=、f係為本系統操作方式之閘極偏壓接地之等效電路 圖，當该感測器之閘極偏壓接地時，其可視為__ 感測器外加正閉極偏壓時，其可視為一三端辦強型 —舳二乏型之主動式感測元件。當上述感 =性區時’該元件之通道電阻济可==二 依據外加的氫氣濃度不同及外加的閘極偏麗不同而產生不同的 8 200809189 電阻值變化。 第^四圖係為本發明感測器於感測到氩氣時之能帶圖，其中該 ，之橫軸虛線係代表費米能階；如圖所示，當氣體中有含有氫^ ^刀子存在時，由於閘極8〇係為一觸媒金屬—鈀金屬，其對於^氣 ，有良好的催化作用，因此氫氣分子被吸附於鈀金屬時會解離成 氫原子並經由擴散穿透把金屬，而到達把金屬和氧化層之界 面層，氫原子受到閘極80之内建電場的影響會被極化，造成一 偶極矩作用A而在界面層形成一偶極矩層，此一偶極矩層的作用 方向與内建電場相反，因而產生抵消作用，使内建電場變小、空 乏區的電荷重新分配並減小，因此使蕭特基能障高度降低B，進 而j生内部的二維電子雲調變及通道電流調變c，造成通道電阻 的變小，藉由觀察上述電阻值的變化，我們可以得知氫氣的存 在，並且依其電阻值的變化大小來評估氫氣濃度量的多寡。 、第五圖係為本發明感測器於30T時在不同氫氣含量所測量之 汲極電流對汲一源極電壓的輸出特性曲線圖；該感測器1〇〇在 30f，於 air 及氫氣含量為 4· 3、48、191、980 以及 9970 ppmH2/air 之氣體中，分別外加閘-源極電壓為〇、—〇· 3以及—〇· 6 v的環境 下分別感測，即可得到如第五圖所示之特性曲線圖，如圖所示可 知該感測器100在空氣中及在不同氫氣含量的環境下，所有電流 一電壓特性曲線都呈現良好的線性度，並且隨著氬氣濃度的增 加，其電流變化也隨之上升許多，因此可得知當通道電阻值之下 降變化量會隨氫氣濃度的增加而增加。Gallium (GaAs) _ layer 2G 'the thickness is above the surface A buffer layer 2G; - Schottky contact layer 4G, which grows in the == base contact layer 4G, the more the thickness is divided into 2xl () lw and fine a ; 'The system grows on the semi-conductive contact layer 4G, which is an undoped gallium arsenide (GaAs) cap layer 5G having a thickness of 5qA.曰', Shishi Shen tiirrtt field uses the method of clear paste to engrave to semi-insulating 氓 ί _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ The vapor-plated gold-锗-to-metal layer, which is used as the drain and source electrode 6 (), is connected to the 7 200809189 atmosphere, and the money is dry and oxygen is connected to the genus, ^ίί '- channel The layer 3i has a thickness of 150 A, and its package ^=(=Ga_As) is an undoped bismuth gallium (Α1 ^ λ layer, - layer 32 'the layer 50 A, and i5M^_^2f :76As) spacer layer % 'its thickness is 曰, carrier rib layer 33, the layer is a 矽 (Si) miscellaneous carrier providing layer 33' with a concentration of ww, and the structure of the Wei 2 active layer 3G Alternatively, the three layers may be arranged in different ways, and have a (L+M+N)! arrangement, and the 排 τ Λ >1 χ '隹 排 排 数值 数值 数值 数值 数值Between 5, feet, if L = 1 = ΜΝ - layer channel layer, two layers of isolation layer, two layers of carrier providing layer 7, thus, the active layer of the hai has (1 + 2 + 2 (four) kinds of arrangement, that is, it will have 12排列 kinds of arrangement and electricity 4 expensive trs number = ~ three c diagram:, description The equivalent of this system operating process v J,, and; decay D 糸 and 亟 resistance 'Rs source resistance ' Rch system channel resistance, vDS system and - source voltage, and vGS system gate _ source voltage. H, i=, f is the equivalent circuit diagram of the gate bias grounding of the system operation mode. When the gate bias of the sensor is grounded, it can be regarded as __ sensor plus positive closed bias In time, it can be regarded as a three-terminal type of active-type 主动-type active sensing component. When the above-mentioned sense = sex zone, the channel resistance of the component can be == two depending on the applied hydrogen concentration and additional The gates are different in polarity and produce different 8200809189 resistance value changes. The fourth figure is the energy band diagram of the sensor of the present invention when sensing argon gas, wherein the horizontal axis dotted line represents Fermi energy. As shown in the figure, when there is a hydrogen containing knives in the gas, since the gate 8 is a catalytic metal-palladium metal, it has a good catalytic effect on the gas, so the hydrogen molecules are adsorbed. When it is palladium metal, it will dissociate into hydrogen atoms and penetrate the metal through diffusion, and reach the metal and oxide layer. In the surface layer, the hydrogen atom is polarized by the built-in electric field of the gate 80, causing a dipole moment A to form a dipole moment layer in the interface layer, and the action direction and built-in of the dipole layer The opposite of the electric field produces a counteracting effect, which reduces the built-in electric field and redistributes and reduces the charge in the depletion region, thus reducing the Schottky barrier height by B, and then the internal two-dimensional electron cloud modulation and channel current. By changing c, the channel resistance becomes smaller. By observing the change of the above resistance value, we can know the existence of hydrogen, and evaluate the amount of hydrogen concentration according to the change of the resistance value. The fifth graph is a graph showing the output characteristics of the 电流-source voltage measured by different sensors in the hydrogen content of the present invention at 30T; the sensor 1 is at 30f, in air and hydrogen In the gas with the contents of 4·3, 48, 191, 980 and 9970 ppmH2/air, respectively, the gate-source voltages are respectively sensed in the environment of 〇, -〇·3, and -〇·6 v, respectively. As shown in the fifth graph, as shown in the figure, it can be seen that the current-voltage characteristic curve of the sensor 100 in the air and in different hydrogen content environments exhibits good linearity, and with argon. As the gas concentration increases, the current change also increases a lot. Therefore, it can be known that the amount of change in the channel resistance value increases as the hydrogen concentration increases.
第六圖係為本發明感測器於3 0〇c時在不同的閘極偏壓值所測 量到靈敏度對不同的氫氣濃度之關係圖，靈敏度的定義為 酬祕，其中&為靈敏度，^為感=在空= 的電阻值，Rm為感測元件在含有氳氣環境中的電阻值。從圖可 知其莖破度會隨著虱氣濃度的增加而增加，以閘_源極電壓為〇V 9 200809189 ί iiff農度從4.3增加到彻鹏驗，其靈敏度也 =不^ 2，隨料加負叫源極賴值愈大時， 9970 tJ 的莖敏度也得到有效的改善；以氳氣含量為 ppm H2/air為例，當外加的間_源極電壓從〇變化至_〇 6 v 度^ 24 7提升至82.8%。同時在最低氫氣探測濃度 it ί ί環釘，其錄度也從3.6財至33.3%。對於 产处=^而s ’靈敏度的提升代表著所能探測的氫氣濃 達到更低的漢度範圍。因此，該感測器應操作在三端空乏 比操作在兩端勸式赋之感曝性更㈣有效改善並 于提升。 第七圖為本發明感·於30。以口 7(rc時對不同氮氣含量之氣 -，閘—源極電壓為〇 v時所測量之暫態響應圖。其中D代表The sixth figure is a graph showing the sensitivity versus sensitivity for different hydrogen concentrations measured at different gate bias values of the sensor at 30 ° C. The sensitivity is defined as the reward, where & ^ Sense = resistance value at empty =, Rm is the resistance value of the sensing element in a helium-containing environment. It can be seen from the figure that the stem breakage will increase with the increase of the concentration of helium, and the gate_source voltage is 〇V 9 200809189 ί iiff agronomy increases from 4.3 to the Pei Peng test, and its sensitivity is also not ^ 2, with The larger the feedstock source and the source value are, the better the stem sensitivity of 9970 tJ is improved. Taking the helium content as ppm H2/air as an example, when the applied inter-source voltage changes from 〇 to _〇 6 v degrees ^ 24 7 increased to 82.8%. At the same time, the lowest hydrogen detection concentration of it ί ί ring nails, its recording also from 3.6 to 33.3%. The increase in sensitivity for the yield = ^ and s ' represents a lower concentration of hydrogen that can be detected. Therefore, the sensor should operate at the three-end depletion ratio and operate at both ends to encourage the exposure (4) to effectively improve and improve. The seventh figure is the invention of 30. The transient response diagram measured by port 7 (the gas with different nitrogen content - rc, the gate-source voltage is 〇 v), where D represents
48ppm H2/air、E 代表 98ppm m/air、F 代表 191ppm H2/air、G ，表 494_ H2/air、H 代表 980_ H2/air 以及 j 代表 5_ _ H2/air如圖中所示圓形和矩形符號分別代表氫氣導入以及氣氣 關3閉之瞬間操作點，流入測試腔中之氣體流速控制在5〇〇 cm/min，測试的條件為維持在線性區一固定的汲—源極電壓 VwO· 3 V的環境下。當氫氣導人時，由於解離之氳原子形成偶極 矩層，造成_麟基能障下降，電流猶上升，因此電阻值也 Ik之下降。而著導人氫氣濃度的不同，電阻值也產生相對應的 下降，以30°C為例，其在空氣中的電阻值為38Ω，當導入氫氣濃 度為48 ppm Hz/air時，其對應的電阻值降至33· 7Ω，且隨著濃 當氫氣，關閉時，其電阻值也能回復到原來在空氣中的電阻值 38Ω。最後再導入5040 ppm HVair來測試再現性及穩定性的表 現。，顯然，感測元件具備良好的再現性與穩定性。另外從3〇和 70 C的暫態響應圖相對比較下，我們可以得知溫度造成電阻值變 化量的下降，同時也減少其反應時間和回復時間，意即感測元件 度一直增加至5040 ppm Hz/air，其電阻值也一直降至28· 8Ω。 200809189 在咼溫下反應速率加快，造成響應時間縮短，然而溫度的提 也限制了感測訊號變化量的表現。 綜上所述，本發明之優點包括··多種操作模式，可做為三端主 動式兀件，具備多工的範疇，使得感測元件有更寬廣的應用，同 時發揮電晶龍積似及可積體化之伽，藉由成熟料積 體電路製造及微機電技術，將可以開發出具有多重應用功能之低 功率智慧型感測晶片，該系統實具備更實用的經濟效益，帶來更 廣泛的應贿圍，因此已充份符合了發明專利之法定要件，爱依 j文申請之。為此，騎f審麵貞詳料查， 準專利，至感德便。 以上已將本發明做—詳細說明，惟以上所述者，僅為本發明 之較佳實_而已，當不能限定本發财施之細，本發明當可 =應，到許多半導體裝置，並且可_許多不_半導體材料 1作，在不脫離本發明之精神和範圍内之均等變化與修飾等，皆 應仍屬本發明之專利涵蓋範圍意圖保護之範疇。 【圖式簡單說明】 圖； 圖’係為本發辭導體電阻錢祕測n之結構圖； 一圖’係為本發解導體雜式氫氣《it之主動層結構 m本發0胖導體電阻式氫氣感測器操作方式之 閘極偏壓接地之等效電路圖； “第圖’係為本發明半導體電阻式氬氣感測11操作方式之 外加正閘極偏壓之等效電路圖； 冰圖’係為本發明半導體電阻式氫氣·11操作方式之 外加負閘極偏壓之等效電路圖； 第四圖’係為本發料導體電阻錢祕測ϋ於感測到氫氣 11 200809189 時之能帶圖； 第五圖，係為本發明半導體電阻式氫氣感測器於30°C時在不 同氫氣含量所測量之汲極電流對汲—源極電壓的輸出特性曲線 圖；48ppm H2/air, E stands for 98ppm m/air, F stands for 191ppm H2/air, G, Table 494_ H2/air, H stands for 980_ H2/air and j stands for 5_ _ H2/air as shown in the figure. The symbols represent the instantaneous operation point of hydrogen introduction and gas-gas shut-off, and the gas flow rate into the test chamber is controlled at 5〇〇cm/min. The test condition is to maintain a fixed 汲-source voltage VwO in the linear region. · 3 V environment. When the hydrogen is introduced, the dislocation of the helium atom forms a dipole moment layer, causing the lining energy barrier to decrease and the current to rise, so that the resistance value also decreases. And the difference in the hydrogen concentration of the conductor, the resistance value also produces a corresponding decline, taking 30 ° C as an example, its resistance value in air is 38 Ω, when the introduction of hydrogen concentration is 48 ppm Hz / air, its corresponding The resistance value drops to 33·7Ω, and with the concentration of hydrogen, when turned off, its resistance value can also return to the original resistance value of 38Ω in air. Finally, 5040 ppm HVair was introduced to test the performance of reproducibility and stability. Obviously, the sensing element has good reproducibility and stability. In addition, from the comparison of the transient response diagrams of 3〇 and 70C, we can know that the temperature changes the resistance value, and also reduces the reaction time and recovery time, which means that the sensing component has been increased to 5040 ppm. Hz/air, its resistance has also been reduced to 28·8Ω. 200809189 The reaction rate is accelerated at the temperature of the temperature, which causes the response time to be shortened. However, the temperature increase limits the performance of the sensing signal. In summary, the advantages of the present invention include a plurality of operation modes, which can be used as a three-terminal active component, and have a multiplexed category, so that the sensing component has a wider application, and at the same time, the electro-optic crystal is formed. With the integration of mature bulk circuit and MEMS technology, it is possible to develop a low-power smart sensing chip with multiple application functions, which has more practical economic benefits and brings more A wide range of bribes, so it has fully complied with the statutory requirements of the invention patent, Aiyi application. To this end, riding the f-reviewed face to look at the details, quasi-patent, to the sense of virtue. The present invention has been described above in detail, but the above is only a preferred embodiment of the present invention. When the present invention is not limited, the present invention can be applied to many semiconductor devices, and It is to be understood that the invention is not intended to be limited to the scope of the invention. [Simple diagram of the diagram] Figure; Figure ' is the structure diagram of the conductor resistance of the speech conductor n; a picture 'is the solution of the conductor hybrid hydrogen'it active layer structure m this hair 0 fat conductor resistance The equivalent circuit diagram of the gate bias grounding of the hydrogen sensor operation mode; the "figure diagram" is the equivalent circuit diagram of the semiconductor resistor type argon gas sensing 11 operation mode plus the positive gate bias voltage; 'This is the equivalent circuit diagram of the semiconductor resistive hydrogen ·11 operation mode plus negative gate bias; the fourth figure ' is the hair conductor resistance of the material, and the energy is sensed when the hydrogen 11 200809189 is sensed. The fifth figure is a graph showing the output characteristic of the 电流-source voltage measured by the semiconductor current resistance hydrogen sensor at 30 ° C at different hydrogen contents;
第六圖，係為本發明半導體電阻式氫氣感測器於3〇°C時在不 同的閘極偏壓值所測量到靈敏度對不同的氫氣濃度之關係圖；與 第七圖’為本發明半導體電阻式氫氣感測器於30°C和70QC 時對不同氳氣含量之氣體，在閘—源極電壓為〇 V時所測量之暫 態響應圖。 【主要元件符號說明】 100半導體電阻式氳氣感測器 10 基板 20 緩衝層 30 主動層 31 通道層 32 隔離層 33 载子提供層 40蕭特基接觸層 50 帽層 60 汲極與源極電極 7〇 絕緣層 80 閘極電極 A 偶極矩作用 B 蕭特基能障高度降低 C ^部的二維電子雲調變及通道電流調變 D 氣體浪度48ppm Hz/air 12 200809189 E 氣體濃度98ppm H2/air F 氣體濃度 191ppm H2/air G 氣體濃度 494ppm H2/air H 氣體濃度 980ppm Hz/air I 氣體濃度 5040 ppm Hz/air 13The sixth figure is a graph showing the sensitivity versus the hydrogen concentration measured at different gate bias values of the semiconductor resistive hydrogen sensor of the present invention at 3 ° C; The transient response of a semiconductor resistive hydrogen sensor measured at 30 ° C and 70 QC for gases with different helium contents when the gate-source voltage is 〇V. [Main component symbol description] 100 semiconductor resistive helium sensor 10 substrate 20 buffer layer 30 active layer 31 channel layer 32 isolation layer 33 carrier supply layer 40 Schottky contact layer 50 cap layer 60 drain and source electrode 7〇Insulation layer 80 Gate electrode A Dipole moment action B Schottky barrier height reduction C ^ 2D electron cloud modulation and channel current modulation D Gas wave 48ppm Hz/air 12 200809189 E Gas concentration 98ppm H2/air F gas concentration 191ppm H2/air G gas concentration 494ppm H2/air H gas concentration 980ppm Hz/air I gas concentration 5040 ppm Hz/air 13