Resistive semiconductor type

Fig. 8.5 Time courses of output signals of semiconductor-type oxygenate sensors, potentiometric CO sensor and ND-IR C02 sensors. The delay of CO gas sensor was due to the resistance of active carbon filter which removes oxygenate compounds (reproduced by permission of Elsevier from [19]).

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

Figure 20-24 shows the resistance of Zr02-MgO as a function of water vapor content (ppmw). The resistance decreases rapidly with an increase in water vapor from 10 to 10 ppmw. Compared with the ionic-type humidity sensor, the response of the semiconductor-type is rather slow because of the slow rate of chemisorption or the subsequent electron transfer process on the oxide surface. The microstructure of the elements as defined by surface area and average particle size, has a less pronounced effect on sensing characteristics than is the case in the ionic-type humidity sensors [31]. 

The relationship between sensor resistance of semiconductor type and gas concentration can be expressed by the exponential function... 

Temperature can be measured with resistive temperature sensors, thermocouple temperature sensors, and radiation pyrometers. There are two types of resistive temperature sensors the conductive type and the semiconductor type. Both operate on the principle that the resistance of sensor material changes with temperature. 

The semiconductor type sensor utilizes the fact that the resistance of a semiconductor decreases with temperature. The most common type of semiconductor temperature sensor is the thermistor shown in Fig. 4.9. 

Another focus for research is on which type of semiconductor has more corrosion resistance. Nanocrystallization can reverse the semiconductor type of a material from p-type to n-type or fromn-type to p-type. Afterthe nanocrystallization, the corrosion resistance of passive materials appears to have been enhanced. However, according to a series of investigations, there is at present no distinct evidence to support which type has more resistance. 

There have been no reported resistivity data for the hexagonal trihydrides although Wallace et al. (1963) reported that the resistivities of dysprosium and holmium hydrides increase by five orders of magnitude when these hydrides are fully hydrogenated. Singh et al. (1976) found semiconductor-type behavior for erbium trihydride. It is generally believed that the hexagonal rare earth hydrides are semiconductors. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Data-based (DDC) or programmable (PLC) controllers with universal inputs and outputs can be used. It is essential that they are configured before use. In some cases the input may be used only for temperature measurement from special types of thermistors. (Thermistors are constructed from semiconductor materials where the resistance changes reversibly proportional to the temperature, i.e., a negative temperature coefficient.)... 

Recent developments are leading toward other materials like silica gel or polymers. Certain types of semiconductors are also used as resistive probes. The measurement range of resistive sensors varies depending on materials used. It can be as wide as 0-99% RH. The dynamics are fast enough for normal ventilation applications and the stability of good resistive sensors is high. This does not reduce the need for calibration, but the intervals of successive calibrations can be extended. 

Modern techniques use thin-film resistors deposited directly on the area and semiconductor units are available which are considerably more sensitive than the resistive type. Dynamic measurements can also be made. The change in resistance unbalances the bridge, causing a voltage to appear across the detector terminals. This voltage is then amplified and applied to a CRO or the information can be stored digitally for future retrieval. 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

Figure 29.4 shows an example, the energy diagram of a cell where n-type cadmium sulfide CdS is used as a photoanode, a metal that is corrosion resistant and catalytically active is used as the (dark) cathode, and an alkaline solution with S and S2 ions between which the redox equilibrium S + 2e 2S exists is used as the electrolyte. In this system, equilibrium is practically established, not only at the metal-solution interface but also at the semiconductor-solution interface. Hence, in the dark, the electrochemical potentials of the electrons in all three phases are identical. 

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