Electron conducting devices

In all the preceding examples of conductance or resistance measurement, it was assumed that the probe leads and the contacts to the measured conductor were ideal. An ideal lead and contact have zero resistance and no thermally generated voltages or uncompensated contact potentials. In a very wide range of measurements of electron-conducting devices, the ideal conditions are met within the desirable or practical error limits. However, the measurement of very low resistances can pose a problem in that the lead and contact resistance must be negligible compared to the resistance measured. Normal lead and contact resistance can be several tenths of an ohm, which limits 1 % accuracy measurements to values greater than about 50 fi. 

Apart from transistor-like devices, single-electron junctions can also be useful for sensor applications. The simplest one might be the monitoring of H2S. Since the formation of CdS nanogranules takes place when an initial cadmium arachidate layer is exposed to this gas, we can expect the appearance of single-electron conductivity only when it is present in the atmosphere. 

Wang W, Lee T, Reed MA (2003) Mechanism of electron conduction in self-assembled alkanethiol monolayer devices. Phys Rev B 68(3) 035416... 

Ionic conductivity, in the context of solids, refers to the passage of ions across a solid under the influence of an externally applied electric field. High ionic conductivity is vital to the operation of batteries and related devices. As outlined earlier (Section 2.4), in essence a battery consists of an electrode (the anode), where electrons are moved out of the cell, and an electrode (the cathode), where electrons move into the cell. The electrons are thus generated in the region of the anode and then move around an external circuit, carrying out a useful function, before entering the cathode. The anode and cathode are separated inside the battery by an electrolyte. The circuit is completed inside the battery by ions moving across an electrolyte. A key component in battery construction is an electrolyte that can support ionic conduction but not electronic conduction. 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

The second type of solar cell is based on a /m-heterojunction in analogy to semiconductor devices [274]. Excitons generated by light, diffuse and dissociate at the interface between a hole and an electron-conducting material. The optimum layer thickness was calculated to be 1.5 times the exciton diffusion length [275]. 

Electronically conducting polymers also have an important role to play as electrodes in electrochromic devices. This is described in Chapter 9. 

---