Sensor semiconductor

Construction In tin dioxide semiconductor sensors, the sensing material is small sintered particles. For the sensor current flow, particle boundaries form potential energy barriers, which act as a random barrier netw ork. Different types t)f semiconductor gas sensors are shown in Fig. 13..54. 

This book is the first attempt to provide a detailed description of scientific basis of the method of semiconductor chemical sensors which are presently widely applied in various domains of industry and in everyday life. The major feature of this book (which distinguishes it from the literature published up to date) is that it mainly examines the use of the method of semiconductor sensors in fine physical and chemical studies. 

Arbitrary the book can be divided into two complementary parts. The first one describes the physical and chemical basics leading to description of the method of semiconductor sensors. The mechanisms of underlying processes are given. These processes involve interaction of gas with the surface of semiconductor adsorbent which brings about tiie change of electric and physics characteristics of the latter. Various models of absorption-induced response of electric and physics characteristics of semiconductor adsorbent are considered. Results of numerous physical and chemical experiments carried out by the authors of this book and by other scientists underlying the method of semiconductor sensors are scrupulously discussed. The possibility of qualitative measurements of ultra-small concentrations of molecules, atoms, radicals as well as excited particles in gases, liquids and on surfaces of solids (adsorbents and catalysts) is demonstrated. 

We also pay attention impact of preparation technique of polycrys-tal adsorbents and applicability domains of obtained semiconductor sensors. 

In Chapter 3 we briefly outline the methods of manufacturing of sensitive elements of semiconductor sensors in order to proceed with the studies of several physical and chemical processes in gases, liquids as well as on the surface of solids. Here we show the peculiarity of preparation of these elements depending on objective pursued and operation conditions. We outline the detection methods (kinetic and stationary), their peculiarities and advantages of their application in various physical and chemical systems. 

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

The combined use of the method of semiconductor sensors and that of molecular beams enabled us to investigate adsorption of atom, molecular and cluster particles of metals on metal oxides. 

We indicate a possible use of semiconductor sensors in the studies of interaction of hydrogen ions and electrons with various energy with a... 

The text of the book is provided with numerous figures and tables exemplifying the applicability of the method of semiconductor sensors to studies of above processes. This illustration material enables one to compare the resulte obtained with those obtained by similar measurements using different techniques. 

Drawing the bottom line, this book is very useful for scientists in various disciplines and experts in domain of interface physical chemistry interested in development and application of the method of semiconductor sensors as well as for post-graduate and graduate students specialized in above domain of science. 

Semiconductor Sensors in Hiysico-Cheniical Studies L.Yu. Kupriyanov (Editor)... 

Physical and chemical basics of the method of 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. 

Even listing all above problems and requirements leading to their solution indicates that development of the method of semiconductor chemical sensors opens a wide research domain. In order to resolve this problems and implement all capabilities of the method of semiconductor sensors there are two ways now the old trial and error approach and approach related to further studies of physical and chemical properties of surface phenomena, reactions and processes underlying this method. It is quite clear that the second approach is more promising in order to obtain semiconductor sensors designed for the use in accurate scientific studies and for practical gas analysis. 

Not pursuing the objective to describe the adsorption phenomenon in detail, we would like, however, to dwell briefly on its principle ideas as well as on main theoretical and experimental results necessary for further understanding of the concept of the method of semiconductor sensors. 

The idea, to a certain extent opposite to above one, deals with an option to control the content of ambient gas atmosphere analyzing the changes in its electrophysical properties. This idea, as far as we are aware, was initially and practically simultaneously put forward by Heiland [82] and Myasnikov [83]. It is this idea which provide the basis for presently widely spread method of semiconductor sensors. 

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. 

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. 

A detailed description of analytical techniques is given in a number of original articles and books [3]. We will focus our interest on comparison of capacities of the mentioned physical and chemical methods with those of semiconductor detectors (SCD) or semiconductor sensors (SCS). These detectors are growing popular in experimental studies. They are unique from the stand-point of their application in various branches of chemistry, physics, and biology. They are capable of solving numerous engineering, environmental and other problems. 

Of all existing methods to monitor electrical properties while using semiconductor sensors, only two [5] have become widely implemented both in experimental practice and in industrial conditions. These are kinetic method, i.e. measurement of various electrical parameters under kinetic conditions, and stationary (equilibrium) method based on the measurement of steady-state parameters (conductivity, work function. Hall s electromotive force, etc.). 

Preliminary activation may be performed not only by means of dissociation of the components being analyzed, but also by electronic and vibrational excitation, either in the gaseous phase, or even better, directly on the film of semiconductor sensor. It should be also noted that this method is applicable to dissociation in the adsorbed layer. Excitation of the molecules in adsorbed layer (we are referring to physically adsorbed particles) can be performed optically, by an electron (ion) beam, or by an electronically excited atom beam, by Hg, for example [10, 11]. 

Fig. 3.8. Experimental set-up to examine interaction of atom particles with the surface of a solid body by means of atom beam reflection. I - Chamber with atom particles source installed II, III - Intermediate and main chambers / -Pyrolysis filament 2 - Collimation channel 3 - Beam chopper 4 - Titanium atomizer 5 - Collimation slot 6 - Target 7 - Deflector 8 - To vacuum pump pipe 9 - Filament 10 - ZnO semiconductor sensor...

Above reasoning can be confirmed by a number of experimental results which showed that although with some peculiarities irrelevant to the properties of semiconductor sensors the correlation between the amount of the atoms in the flux incident on the target, or their surface concentration, and the variation (increase, if we are dealing with semiconductor of n-type) of the target conductivity takes place [28]. Based on the relations cited in Chapter 1, one can estimate concentrations (i. e., flow intensities) of these particles in vacuum or in gaseous medium if these values are quite small, using the values of conductivity variation of the semiconductor film. 

Here it is relevant to mention results of some experiments. They were carried out specifically to substantiate the applicability of semiconductor sensors to solve numerous problems dealing with metal atoms and clusters (both in vacuum and on the surface) in cases when the use of other techniques does not yield sound results. 

Fig. 3.10. Reaction cell. 1 - Atom gun 2 - Thermostate 3 - Metal evaporator 4 - Pt/Pt-Rh thermocouple 5/7 - Collimation holes (diameter 3 mm) 8 -Shutter 9 - ZnO semiconductor sensor 10 - Mobile quartz weight 11 - Platinum contacts terminals 12 - Vitrificated iron bars controlled by a magnet 13 -Quartz guides...

We feel it is interesting and important to dwell on other experiments which characterize the relationship between the number of atoms of different metals and the variation of conductivity in a film of a semiconductor sensor as well as the dependence of the signal value on the degree of surface occupation. 

Fig. 3.12. Reaction cell. 1 - the evaporator 2 - thermocouple 3 - beam splitter 4 - semiconductor sensor (ZnO) 5 - collimating holes 6 - atom beam shutter...

Table 3.3 shows the data obtained while measuring intensities of a Ag beam by isotope and semiconductor sensor method using a ZnO oxide film as a sensitive element. The data reveals that in this case there is also a satisfactory linear dependence between the number of silver... 

Evaporation temperature, c Ag atoms flow intensity, isotope method, atoms s l Current carrier concentration variation rate in the film, semiconductor sensor method, Vg- 10 , electrons s Ve... 

In this part we dwell on the properties of the simplest radicals and atoms in the adsorbed layer of oxide semiconductors as well as analyse the quantitative relationships between concentrations of these particles both in gaseous and liquid phases and on oxide surfaces (mostly for ZnO), and effect of former parameters on electrophysical parameters. Note that describing these properties we pursue only one principal objective, i. e. to prove the existence of a reliable physical and physical-chemical basis for a further development and application of semiconductor sensors in systems and processes which involve active particles emerging on the surface either as short-lived intermediate formations, or are emitted as free particles from the surface into the environment (heterogeno-homogeneous processes). 

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