Frequency Transducers of Gas Concentration Based on Transistor Structures with Negative Differential Resistance
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The paper presents the results of studies on the creation of frequency microelectronic transducers of gas concentration based on autogenerator transistor devices.
In the proposed devices, the gas concentration is converted into a frequency output signal. Such a conversion allows to remote all disadvantages of analog gas sensors: low output voltage (millivolts), low sensitivity, parasitic influence of measuring channels on each other, instability in operation, need for amplifying devices and analog-digital transducers by subsequent processing of measurable information.
The analysis of operation of an autogenerator in the quasilinear mode was carried out. This autogenerator is the main element of frequency transducers. The conditions for self-excitation and its main characteristics were determined. The physical mechanism of operation of resistive gas sensors is considered. It is shown that oxide semiconductors under the action of a gas change the surface electric conductivity. This effect underlies the construction of gas-resistive sensors, which are used as primary transducers of gas concentration in frequency devices. Elements of the theory of near-surface charge in semiconductors of the type ZnO2, SnO2 are also considered. On the basis of the solution of the Poisson equation, analytical expressions for the surface resistance of hole and electronic semiconductors are obtained. The theoretical dependence of the surface resistance for metal oxides on the methane concentration is calculated.
Schemes of frequency microelectronic transducers of gas concentration based on transistor structures with negative resistance are proposed and studied. In the falling section of the current-voltage characteristics, the operating point of the device is selected, which ensures the self-excitation of the transducer self-oscillator. The energy losses in the oscillatory circuit of the autogenerator are replenished by negative resistance. A method for calculating current-voltage characteristics, output impedance, transformation functions, and sensitivity of devices are proposed. The calculations are based on mathematical models of transducers, which follow from the Kirchhoff equations. Equations are compiled on the basis of nonlinear equivalent circuits of frequency devices.
The microelectronic frequency transducer of gas concentration works in the oscillation mode of the oscillator because of proper selecting a constant voltage of the power supply. The gas sensitive resistor is included in the positive feedback loop of the autogenerator. When the concentration of gas affected on the gas sensitive resistor, so. its resistance decreases, which leads to a change in the equivalent capacitance of the oscillator circuit of the self-oscillator, and this in turn changes the generation frequency. The sensitivity of the frequency transducers was 10–800 Hz/ppm in the range of gas concentration changes from 0 to 5∙103 ppm.
KeywordsRadiomeasuring transducer gas concentration MEMS sensor Frequency output signal Negative resistance
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