A brief introduction to MOS sensors and their applications

What applications are MOS sensors used for

MOS sensors are used in Air quality monitoring of VOCs and toxic gases such as CO, NO2 and NH3.

They are also employed for Safety purposes, residential, commercial and Industrial to detect for leaks of combustible gases and refrigerants and carbon monoxide, Quality control of food and beverages, Automotive exhaust gas emissions for improved engine control and Medical diagnostics.

Breath analysis using MOS gas sensors can help in the early detection of diseases, such as lung cancer, diabetes, kidney disease and asthma, by identifying specific biomarker gases in a patient’s breath. Ketones, aldehydes, ammonia, nitric oxide, hydrogen sulphide and hydrocarbons are all biomarkers for specific diseases, e.g. acetone is a biomarker for diabetes.

The MOS sensor is employed in different ways dependent on the application:

As an alarm …Industrial and residential safety applications

As a monitoring device….providing a measured concentration output

As a diagnostic device….as part of an array of sensors combined with Machine Learning and Artificial Intelligence to analyse a mass of data, develop pattern recognition algorithms and provide information. An electronic nose works in this way – each sensor in the array responds differently to various gases, and the collective response pattern provides a unique “fingerprint” for a given mixture of gas component

What is a MOS sensor and how does it function?

Metal Oxide Semiconductor gas sensors detect the presence of the target gas by recording changes in resistance of the sensing material due to the target gas in contact with the sensor. There are essentially 2 types of MOS sensor , n-type and p-type that differ in their construction and their functionality.  All further references are to the n-type sensor

MOS gas sensors are typically comprised of a metal oxide semiconductor layer, a heater, and a pair of electrodes. The most common metal oxide used is tin dioxide (SnO₂), although zinc, titanium and tungsten oxides are also used. These metal oxides may be doped with noble metals such as Pt, Ag, Au or Pd or other metals such as Al, Ga or Zr to improve the gas sensing response.

To improve the sensitivity and selectivity of the sensor, a heater is incorporated into the device. The heater maintains a constant operating temperature, typically between 200 and 400°C. This elevated temperature accelerates the adsorption and reaction processes increasing the sensor’s speed of response.

Oxygen particles in the atmosphere naturally stick to the surface of a MOS sensor, but reducing gases will seize O2 from the surface of the sensor, releasing electrons and decrease the electrical resistance within the sensor’s circuitry.  The change in resistance is proportional to the concentration of the *target gas and is measured by the sensor’s electrodes.

Performance features of MOS sensors

MOS sensors offer many advantages over other types of gas sensors on the market:

Low cost
Sensitive
Rapid response
Relatively long life of 5-10 years
low power consumption, particularly in the case of mems type sensors
Small footprint

BUT, their limitations need to be understood so that the necessary steps can be taken to ensure a meaningful output is obtained.

*Lack of selectivity as unfortunately MOS sensors can respond to all reducing gases , not only the Target gas. Means of improving selectivity of the actual sensors include use of filters, dopants and heater temperature which is where Figaro’s expertise lies and why Figaro offer such a diverse range of MOS sensors for different applications. In some instances additional sensors can be used to measure and compensate for interferants.

Sensitivity to environmental conditions, such as temperature and humidity.

Baseline drift with time (stability)