Dynamic properties and sensitivity of semiconductor metal-oxide thick-film sensors to various gases in air gaseous medium

Abstract

A technique has been developed for formation of square pulses of the concentration of the gases under analysis in a measuring chamber with a sensing element. The pulse edge duration is of the order of fractions of a second. The methods of investigation and experimental data processing are discussed. A method of time constant calculation at the level of 90% of the maximum value in a conductivity pulse ( σ) of semiconductor metal-oxide sensors is graphically illustrated both on the gas concentration increase τ 0.9 ↑, and decrease τ 0.9 ↓. Investigations have been carried out of dynamic parameters and sensitivity ( S= σ/ σ 0) of gas sensors in dry gaseous mixtures of air with methane, hexane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide and ethanol in the temperature range 300–600 °C with a 50 °C step at two values of gas concentrations, i.e., 20 ppm and 1 vol.% for methane, 10 and 2000 ppm for hexane, 100 ppb and 1000 ppm for hydrogen and ethanol, 5 and 98 ppm for carbon monoxide, 10 and 100 ppm for ammonia, and 3 and 30 ppm for hydrogen sulphide. Proceeding from a combination of dynamic parameters, sensitivity and sensor drift in a gas mixture, structures of gas-sensitive layers are defined for sensors most suitable for detection of the gases under analysis in air. Numerical values of parameters, the values of exponent, n, in the formula for sensitivity S−1=( σ− σ 0)/ σ 0= AC n and sensor sensitivity thresholds have been defined. It has been found, that sensor sensitivity to the gases under analysis in most cases is of extremal character, and its temperature interval is practically independent of the gas concentration value. The time constants of sensors are shown to be of sharply drop-down character with the value stabilization at the minimum level τ 0.9≈1–2 s at temperatures above 500 °C and are independent of the gas concentration either. Besides, it has been established, that covering a chip with a metal cap reduces the sensor quick-action approximately by 2–4 times. The results of measurements and calculations are presented in the graphic and tabulated forms.