Electronic conductivity charge carriers

Materials are often classified according to their DC conductivity as conductors, semiconductors, (electronic), and insulators. For electronic conductors (charge carriers electrons), this classification is based on energy levels and the Fermi-Dirac statistics of the free electrons. A division between metals and semiconductors is sometimes based on the temperature coefficient of the conductivity, a Semiconductors (as for ionic conductors) have a positive da/dT whereas metals have a negative da/dT. 

A semiconductor that has been doped so that electrons (negative "charge carriers") can conduct is called "N-type" material. The original pure material was "I-type," before being doped. 

Also referred to as total cOTiductivity. This total conductivity has three components electronic n-type (electron charge carriers), electronic p-type (electron hole charge carriers), and ionic conductivity. 

Nb is the most common pentavalent dopant [104—108], As with the trivalent A-site dopants, the additional charge is compensated by electrcmic carriers at I0W/7O2 leading to high electronic conductivity. These carriers are generated by reductitMi of to Ti " ... 

When the temperature is increased, electrons in the valence band are excited across the forbidden energy gap to the conduction band. This is the intrinsic ionisation. The electrons in the conduction band and the unoccupied electron sites in the valence band (electron holes) can move in an electric field. The electron holes behave as though they were positively charged and move in the opposite direction of the electrons. The intrinsic ionisation thus produces pairs of electron + electron hole charge carriers. When the electronic conductivity is due to intrinsic ionisation only, the semiconductor is called an intrinsic semiconductor. According to this model electron and electron hole conductivities increase with increasing concentrations of electrons in the conduction band and electron holes in the valence band. 

The dc asymmetric polarization technique has been used quite extensively to evaluate the partial conductivity of the electronic minority charge carriers in marty solid electrolytes, e.g., doped zirconia, silver halides, copper halides,and solid lithium conductors. ... 

Similar to the dc polarization technique for the determination of the partial electronic conductivity in ionic conductors, the electronic majority charge carriers may be blocked by introducing a pure ionic conductor that is permeable to the ions under consideration. In order to provide defined thermodynamic conditions, reversible reference electrodes, which can act as a source or sink for the ions, are employed at the other side of the sample (Figure 9.16). 

In the above presentation of the band theory, the temperature was implicitly assumed to be 0 K. In case of an insulator, the effect of temperature is to promote the electrons of the valence band into the conduction band. These two energy bands are then neither totally filled nor totally empty respectively, and both contribute to conductivity. If conductivity becomes relatively significant (= lO -lO Q cm ), we refer to semi-conduction. Charge carriers are the electrons present in the conduction band, in concentration n, and also the electrons of the valence band. For the latter, their contribution with respect to the different physical properties, conductivity, specific heat, etc., is the same as positively charged particles called electron holes, with concentration p, the fraction of unoccupied states. The concentrations n and p are fixed by the following relationships ... 

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

For insulators, Z is very small because p is very high, ie, there is Htde electrical conduction for metals, Z is very small because S is very low. Z peaks for semiconductors at - 10 cm charge carrier concentration, which is about three orders of magnitude less than for free electrons in metals. Thus for electrical power production or heat pump operation the optimum materials are heavily doped semiconductors. 

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

Semiconductivity in oxide glasses involves polarons. An electron in a localized state distorts its surroundings to some extent, and this combination of the electron plus its distortion is called a polaron. As the electron moves, the distortion moves with it through the lattice. In oxide glasses the polarons are very localized, because of substantial electrostatic interactions between the electrons and the lattice. Conduction is assisted by electron-phonon coupling, ie, the lattice vibrations help transfer the charge carriers from one site to another. The polarons are said to "hop" between sites. 

The conductivity becomes complex if mote than one type of charge carrier is present and involved in the conduction process. The total conductivity is the sum of all the conduction associated with the net motion of electrons, holes, and ions, ie ... 

The equations generally developed include all forms of the conduction. Eor example, to determine the flux or conductivity of ions in a soHd electrolyte as compared to electrons in a semiconducting ceramic, two terms are of interest the number of charge carriers and the mobiUty. The effects of temperature, composition, and stmeture on each of these terms must also be considered. 

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