**4. Thin film based sensor**

A thin film-based sensor is a type of transducer which converts a physical or chemical quantity into equivalent electrical for measurement. It is used to detect the presence of stimulus to very low concentrations of toxic or harmful target environment (gases) of importance, such as ammonia [77], carbon monoxide [78], carbon dioxide [79], nitrogen dioxide [80], sulfur dioxide [81], propane [82], liquefied petroleum gas [83], hydrogen sulfide [84], and volatile organic compounds. Worldwide thin film gas sensing technology is playing a major role in protecting the environment and improving homeland security. Sensors are also critical in improving the reliability and efficiency of manufacturing operations by providing faster and more accurate feedback regarding product quality. In the area of environmental health and safety, lowering the limits of detection can improve the quality of life through precise information regarding the pollutants in air, water and soil. High-performance thin-film sensors and systems are essential to monitoring various kinds and quantities of analysts.

The typically thin film-based sensors are described using the main characteristics such as sensing response, stability, repeatability, reproducibility, linearity, response time, and recovery time. An efficient thin-film sensor;


The thin film-based gas sensors are used for equally the identification and quantification of gases, and hence should be both selective and sensitive to a required target gas in a mixture of gases. Sensitivity defines the smallest concentration of gas/vapour that can be fruitfully and repeatedly sensed by a thin film sensor.

Thin film-based semiconductor is commonly used materials as sensor application as indicated in **Table 1**. This is because of its versatile advantages like high sensitivity and low manufacturing is metal oxide which contain the elements having one oxidation state because it requires more energy to form more than one oxidation states.


#### **Table 1.**

*Literature survey of various gas sensing characteristics of different metal oxidebased nanomaterials with different modifiers.*

Semiconductor metal oxide films have been exploited for the sensing of various toxic and harmful gases in the form of ceramics, thick films, thin films or nanostructures. Sensors based on ceramics have shown advantages in terms of their mechanical strength, large resistance to chemical attack and good thermal and physical stability and most of the available commercial sensors are based on ceramics only. One of the additional attractive features associated with low temperature operated semiconductor thin film sensor is that it can lead to a complete integration with well-established Si based micro-electronics technology.

The limitation of thin film based sensor was described. A number of thin film sensors might be recognized from sensor arrays which yield slightly different responses to various target gases. The availability of thin film gas sensor potentially creates a complicated selection problem, and is more important in view of cost and technology limitations. Many researchers have self-sufficiently confirmed practical limitations to thin film gas detection at low temperature and have attributed it to the requirement of high activation energy which can be attained only at elevated temperatures. A reduction in the number of sensors to be involved in E-Nose is advantageous due to several reasons as discussed. Sensors which exhibit an insignificant response to

### *Recent Developments on the Properties of Chalcogenide Thin Films DOI: http://dx.doi.org/10.5772/intechopen.102429*

target gases, increase variance (noise) in E-Nose and do not assist pattern recognition process. Furthermore, sensors exhibiting similar responses to the target gases provide no additional information and are redundant.

In future, low temperature operation of the thin film sensors is an attractive proposition for the industry since it not only holds a promise to cut down the costs but also overcome technological limitations of miniature heaters of high wattage. In order to identify the target gases other classification technique such as artificial neural network approach is required where the selected features/variables obtained from principal component analysis (PCA) could be used as input features, and will be carried out in future. Therefore, a new methodology or novel design approach is essentially required in order to fulfil the essential requirements of future sensor in the market.
