Grain conductance sensors

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

In conduction models of semiconductor gas sensors, surface barriers of intergranular contacts dominate the resistance. Electrons must overcome this energy barrier, eV., in order to cross from one grain to another. For these... 

Figure 2.3 shows a schematic view of the nano crystalline sensor material. It consists of single-crystalline tin-oxide grains with a typical size of 10 nm and a narrow size distribution [68]. The grains are in loose contact. The lower graph in Fig. 2.3 schematically represents the conduction band of the layer. 

Thin metal Hlms (Pt, Pd) have been used for the adsorption and detection of gases such as H2 and NH3 [138,139]. While the interaction mechanisms for these sensors were not specified, it is well known that H2 dissolves to a significant extent in Pd, with concomitant changes in the density, electrical conductivity, and mechanical properties of the fllm. The H2/Pt interaction as well as the interaction of NHa with both Pd and Pt undoubtedly involves chemisorption on surface sites. Metal thin films deposited by nearly all techniques are polycrystalline chemisorption along grain boundaries can often lead to a response that is considerably larger than predicted from the properties of metal single crystals. 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

Fig. 4 Schematic mechanism of the differing effect of electron donor analytes on the conductivity of hole conducting (green) and electron conducting (yellow) channel materials in an OTFT sensor where adsorption of analyte is directed by specific metal-ligand type coordination at the grain surface...

Several studies have examined the influence of film morphology on sensor behavior [109, 168-178]. Two main areas of interest are the influence of film thickness and grain size on the time scale for sensor response as well as the magnitude (sensitivity) of sensor response. Other factors that may influence the conductivity response are the nature of the semiconductor-metal device contacts, as well as charge accumulation at other device interfaces, such as the gate oxide in field effect sensor devices. 

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 ... 

The theory of gas response can be applied or extended to the analyses of other related phenomena of gas sensors, though such work is still in its early stages. For example, the rates of response and recovery have been formulated theoretically. It has also been derived that a type of sensitization takes place when semiconductor grains are dispersed with an additive that deprives them of conduction electrons and thus affects the reduction of the effective radius of the grains. ... 

---