Gas concentration transducers with frequency output signal based on reactive properties of semiconductor structures with negative differential resistance

Abstract

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, 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 paper considers the mechanism of formation of the impedance of semiconductor gas sensors, in which the active component is determined by the surface resistance, and the reactive component is determined by the surface capacitance. The dependence of the magnitude of the impedance on changes in the concentration of active gases characterizes the essence of the gas-reactive effect of the sensors, which unambiguously changes the output frequency of autogenerating gas transducers. 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 ed, 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 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 ing a constant voltage of the power supply. The conversion functions and the sensitivity equations of an autogenerating gas transducer have been determined, with the device sensitivity ranging 300 ppm to 1000 ppm being 270 Hz/ppm, and ranging 1000 ppm to 5000 ppm – 120 Hz/ppm.