Basic Aspects and Challenges of Semiconductor Gas Sensors
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Basic Sensing Mechanisms Semiconductor sensors detect gases via variations in their resistances (or conductances). The most widely accepted explanation for this is that negatively 18
charged oxygen adsorbates play an im portant role in detecting inflammable gases such as H 2 and CO. Actually, several kinds of oxygen adsorbates, such as 0 2 ~, O', and O 2 - , are known to cover the surface of semiconductive metal oxides in air.9 Of these, O " is the most reactive with inflammable gases in the temperature ränge of 300-500°C, in which most semiconductor gas sensors are operated, and eventually the Variation in surface coverage of O - dominates the sensor resistance. In the case of n-type semicon ductive metal oxides, the formation of these oxygen adsorbates builds a space-charge region on the surfaces of the metal-oxide grains, resulting in an electron-depleted surface layer due to electron transfer f rom the grain surfaces to the adsorbates as follows: l/20 2 (g) + 2 e - = 2 0 - ( a d ) .
tance. This resistance change is used as the measurement parameter of a semi conductor gas sensor, with sensitivity k defined as the ratio of the resistance in air to that in a sample gas containing an inflammable component. The reactivity of the oxygen adsorbates is, of course, a function of both the kind of inflammable gas and the sensor temperature. There fore, the temperature TM at which the maximum sensitivity kM is observed is dependent upon the particular inflam mable gas used. Since semiconductor gas sensors respond more or less to any in flammable gas due to this mechanism, the sensors usually suffer from crosssensitivity, that is, lack of selectivity to a specific gas. Occasionally, inflammable gases interact directly with sensor materials; for example, hydrogen chemisorbs negatively on Sn0 2 under certain conditions.11'12 Such phenomena are usually observed at low temperatures, at which the reaction with oxygen adsorbates is limited. In addition to the reaction with oxygen adsorbates, electrical interactions with intermediate products of inflammable gases lead to more complicated sensor responses. 1314 The response of «-type semiconductor gas sensors to oxidizing gases such as 0 3 or N 0 2 is relatively simple—the sensor resistance increases upon exposure to these gases as a result of their negatively charged chemisorption on the grain sur face. Therefore, sensitivity is a function of the amount of chemisorption, provided that the surface coverage of oxygen adsorbates remains constant.
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The depth of this space-charge layer (L) is a function of the surface coverage of oxygen adsorbates and intrinsic electron concentration in the bulk. The resistance of an «-type semiconductor gas sensor in air is therefore high, due to the develop ment of a potential barrier to electronic conduction at each grain boundary, as shown in Figure l.10 When the sensor is exposed to an atmosphere containing inflammable gases at elevated tempera tures, the oxygen adsorbates are consumed by the subsequent reactions, so that a lower st
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