Semiconductor gas sensors rely on a gas coming into contact with a metal oxide surface and then undergoing either oxidation or reduction. The absorption or desorption of the gas on the metal oxide changes either the conductivity or resistivity from a known baseline value. This change in conductivity or resistivity can be measured with electronic circuitry. Usually the change in conductivity or resistivity is a linear and proportional relationship with gas concentration. Therefore, a simple calibration equation can be established between resistivity/conductivity change and gas concentration.
An example of a calibration equation for a change in resistivity is given by Fine et al (2010) for carbon monoxide:
R/Ro = 1+A[CO]
where R is the resistance following contact with a gas, Ro is the baseline resistance, A is a constant, and CO is carbon monoxide.
The metal oxide surface is usually a thin film of a transition or heavy metal. The exact metal that is used will depend on the application and example metals include tin dioxide (SnO2) or tungsten oxide (WO3). The film overlies a layer of silicon and is heated to a temperature between 200 and 400C, again depending on the application. In this way, the chemical processes is hastened and the effects of fluctuating external temperatures is minimised.
Semiconductor sensors work best when they have a large surface area. Such a sensor can absorb as much of the target gas as possible particularly at low concentrations.

example CO sensor calibration output


The figure above is an example calibration output from a CO sensor taken from the ESCOD-200 Carbon Monoxide Detector Manual. The graph on the left is a sensor with a voltage output and the graph on the right is a sensor with a 4..20mA output. Both graphs show a simple, linear relationship. At 2V, CO level is 0ppm where as at 10V the CO level is 300ppm. Similarly, for the current sensor, at 4mA the CO level is 0ppm and at 20mA the CO level is 300ppm. A simple algebraic extraction will determine what the CO levels are between 0 and 300ppm for a given voltage or mA output.

advantages of semiconductor sensors
Semiconductor sensors are relatively inexpensive to manufacture due to their simplicity and scalability.
Specific sensors can be designed for particular applications. For example, a sensor can be designed for low concentration applications whereas an alternative sensor can be designed for high concentration applications.

limitations of semiconductor sensors
Non-target gases may absorb on the oxide surface and provide false measurements. Important disrupting gases include ozone, water and volatile organic compounds.
One of the advantages of semiconductor sensors is their flexibility in sensor design for specific applications (see above in Advantages section). However, this can also be a limitation. A user will need to purchase multiple sensors depending on the application.
The relationship between R/Ro and CO2 is not necessarily linear leading to issues with consistent calibration across sensors. Individual calibration of sensors may be necessary, leading to increased sensor costs associated with resources and labour time.
Fine et al (2010) state that semiconductor CO2 sensors perform best in the range between 2,000 and 10,000 ppm. This limited range means the semiconductor CO2 sensors will not be suitable for a number of applications. For example, in plant physiology, atmospheric CO2 concentrations are most important in the range between 280ppm and 800 ppm. For indoor air quality applications, an ideal monitoring range is in the order between 500 and 1,000 ppm.
Semiconductor CO2 sensors can also be unreliable at high relative humidity and varying temperatures.