In 1938, Wagner and Hauffe discovered that the interaction of certain atoms and molecules with semiconductor materials caused the material’s surface properties (e.g., conductivity and voltage potential) to change. In 1953, Brattain and Bardeen studied the phenomenon of large changes in the electrical resistance of metal oxide semiconductors due to the adsorption of gases into their surfaces. In 1962, the first chemoresistive semiconductor gas sensor was developed by Seiyama et al.
When a target gas makes contact with a chemoresistive sensor, it interacts with the sensor both physically and chemically, causing charge exchanges between the adsorbate layer of the target gas and the sensor material, resulting in a variation in conduction or resistance.
There are two main types of chemoresistive sensors: conductometric sensors (operate based on conductivity change) and potentiometric sensors (operate based on voltage potential change). This section primarily focuses on conductometric sensors due to their broad application in gas sensing, relatively simple structure, and low cost. Conductometric sensors are further divided into bulk conductance and surface conductance sensors. In a bulk conductance sensor, the entire volume of the material is involved in the chemical reaction; thus, the bulk chemistry defect of the sensor is most important. Bulk sensors are often used in combustion process control (e.g., to measure oxygen partial pressure). In a surface conductance sensor, only the surface of the material is involved in the reaction. Surface conductivity changes are primarily due to changes in free electron concentration caused by charge exchanges between the sensing material and the adsorbed target during the chemosorption and heterogeneous reactions occurred on the sensor surface. Most chemoresistive sensors are surface conductance sensors. The following kinetic scheme describes the complex, temperature-dependent process of oxygen adsorption that involves charge (electron) exchange:
Where “ads” means adsorb, e is the electron, and X represents reducing gas. The law of mass action applied to each step of the aforementioned scheme yields the steady-state occupancy for the different oxygen surface states, which contributes to the variation in the surface resistance. By reacting with oxygen ion species at the surface, the reducing gas X generates conduction electrons, resulting in a decrease in the surface resistance. Thus, the electric resistance is directly related to the surface state’s (oxygen) occupancy. Using an n-type semiconductor as an example, the two main processes are involved:
In this reaction, the surface conductivity decreases (resistance increases) because two electrons are taken away during the oxidation process.
In this reaction, the surface conductivity increases (resistance decreases) because one electron is returned during the reduction process.
If the oxide is a p-type semiconductor, oxidation will reduce the resistance, while reduction will increase the resistance. The reaction between the gas and the oxide surface depends on the sensor temperature, the gases involved, and the sensor material.
The following generic power law describes the variation of sensor resistance, R, in the presence of a gas. This model is suitable for carbon black polymer composite films as well as other film materials:
Where R0 is the sensor’s base resistance measured in a reference gas (e.g., clean air); kG is a sensitivity coefficient for a target gas, and it can be positive or negative depending on the nature of the gas, the sensing material used, an increase or decrease in sensor resistance when the gas is introduced; CG is the gas concentration in ppm (per million); γ is a low power exponent; T is the temperature in kelvin; and KT is the temperature coefficient of the gas. If KT the positive, an increase in temperature will reduce the resistance of the sensor.
The aforementioned resistance model can be applied to a mixture of gases by adding the effect of each individual gas as a separate input assuming no interaction between them.This assumption is valid for low concentrations of volatile organic compounds. The above resistance model can also be expanded to include the independent additive effect of humidity:
where kH is the sensitivity coefficient for humidity (water vapor), CH is the water vapor concentration in ppm, γH is the low power exponent, and KTH is the temperature coefficient of the water vapor.