Sensing Mechanisms and OFET Models

As shown in Fig. 7.26, when the sensor is exposed to vapor, individual molecules can diffuse into the semiconductor thin film and be adsorbed mostly at the grain boundaries [13], If the adsorbed analytes have large dipole moment, such as H2O ( 2 debye) and DMMP ( 3 debye), the adsorption of those analyte molecules at the grain boundaries close to or at the semiconductor-dielectric interface can locally perturb the electrical profile around the conduction channel, and hence change the trap density in the active layer. We can interpret the trapping effects by a simple electrostatic model discussed briefly in Sect. 7.2. The electric field induced by a dipole with dipole moment of p (magnitude qL in Fig. 7.4) is  

Where /to is the trap free mobility and a and op are the total surface charge density and free surface charge density respectively. 

In the context of the multiple trapping and release (MTR) model, the decrease in transistor current upon introduction of vapor results from an increase in the density of traps due to the electrostatic properties of the analyte molecules, which introduce additional trap states in the material. As a result, the ratio of mobile charges to total charges decreases, causing a decrease in the measured mobility. Physically, the analyte molecule acts as an electrostatic 

Alternatively, a model which considers granular film morphology defines the mobility as 

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