Selectivity semiconductor sensors

Additionally to meeting the requirement of high adsorption sensitivity the semiconductor sensors should be response-selective to a specific gas as well as exhibit high signal stability (i.e. reproducibility) over a long operation time. 

Generally speaking, (and this coincides with an opinion of Morrison [8]) today there are four most general approaches to solve the problem regarding selectivity of semiconductor sensors. They entail a) the use of catalysts and promoters, b) the application of the method of temperature control, c) the control of specific surface additives ensuring development of specific adsorption, and, finally, d) the implementation of different filters. 

In several cases application of various additives to the surface of a semiconductor adsorbent, specifically adsorbing or reacting with particles to be detected enables one to improve selectivity. As an example we can mention the use of hygroscopic salts to bind water in humidity sensors, the application of particles of sulfanilic acid to the surface of hhO to detect NO2 [10]. However, the high operational temperature in majority of semiconductor sensors deprives the method of specific surface additives of its general character. 

The use of catalysts and promotors of various reactions applied as a fine dispersion phase to the surface of semiconductor adsorbent became most popular in providing a required selectivity of sensors with respect to a given gas. As it has been established in experiments (see for instance [8] and the reference list therein), apart from obtaining required selectivity application of such additives results in increase of sensitivity of the sensor with respect to a given gas. However, as of today there is no clarity with regard to understanding the mechanism of effect of cata-l)rtic additives on the sensor effect nor in optimization of the choice of catalysts applied. 

It is evident that the multitude of plausible effects of application of catalysts on sensitivity and selectivity of semiconductor sensors cannot be only reduced to above two mechanisms. One should keep in mind the possible influence of contact field spread over substantial area of the adsorbent surface and situated close to metal additives on reaction capacity of adparticles [19] as well as plausible direct catal d ic effect of additives accompanied by creation of electrically active products of reaction from non-active reagents. 

The influence of other active components, such as 1, OH, H on a semiconductor sensor, with other conditions being the same, is comparable with the influence of atomic oxygen [50]. Contribution of N and OH is proportional to their relative contents (compared to that of atomic oxygen) in the atmosphere and may become essential at altitudes lower than 60 - 70 km. The use of selective detectors excludes the influence of atomic hydrogen. Studies of adsorption of water vapours on ZnO films [50] show that their influence is negligibly small at the film temperatures below 100°C. Variations of electric conductivity of the films under the influence of water vapours and of an atomic oxygen are comparable at the ratio of their concentrations [H20]/[0] = 10" . 

From the above-given condensed review of the EEP detection methods one can infer that none of these methods can independently satisfy all the requirements specified for the study of heterogeneous processes involving the EEP participation. To our opinion, the application of semiconductor sensors for detection of EEPs can be provided by a combination of required qualities. The sensors are highly sensitive, miniature, can be operated within wide ranges of gas temperatures and pressures, and are made of simple devices. At the same time, a series of problems arise connected with the preliminary preparation of sensors and improving their selectivity. These and other questions of general nature will be considered in the section that follows. 

Design of Selective Gas Sensors Using Combinatorial Solution Deposition of Oxide Semiconductor Films... 

Noble metal additives such as Pt, Pd, or Ag, are often added to metal oxide semiconductor sensor materials to enhance sensor response to a particular gas or class of gases [35]. Other metals, namely Be, Mg, Ca, Sr, Ba, have been used with tin oxide to enhance performance of an alcohol selective sensor [36], for example. Commercially produced metal oxide sensors may customarily use one or more of the above catalysts. 

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO semiconductor sensors operated under periodic temperature cycle[85-87], a electrochemical sensor using nafion membrane[88], a catalytic combustion sensor composed of catalysts and hydrophobic pol uner[89], a SnOj diode sensor doped with Pd[90] and an optical fiber catalytic sensor with Au/CogO as combustion catalyst[91]. 

Morrison SR (1986) Selectivity in semiconductor sensors. In Proceedings ofthe 2nd International Meeting on Chemical Sensors. Bourdeaux, France, pp 39-48... 

A number of physical devices with chemical sensitivity have been developed previously, including the quartz crystal microbalance (QCM) and other acoustic wave devices, semiconductor gas sensors, and various chemically sensitive field effect transistors. However, based on their intrinsic detection principles, most of the known solid state chemical sensors are not selective, i.e., they respond to more than one or a few chemical species. There is an urgent demand for new families of selective, microscope sensors that can eventually be integrated into microelectronic circuits. We have embarked on a program aimed at the design of conceptually new microporous thin films with molecular recognition capabilities. On the surface of chemical sensors, these membranes will serve as "molecular sieves that control access of selected target molecules to the sensor surface. 

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