Conduction mechanisms Semiconductors

Semiconductors are materials that are characterized by resistivities iatermediate between those of metals and of iasulators. The study of organic semiconductors has grown from research on conductivity mechanisms and stmcture—property relationships ia soHds to iaclude appHcations-based research on working semiconductor junction devices. Organic materials are now used ia transistors, photochromic devices, and commercially viable light-emitting diodes, and the utility of organic semiconductors continues to iacrease. 

The fair agreement of expressions (2.67) and (2.71) with experimental data as well as agreement of independently obtained experimental data concerning kinetics of the change of a with the data on equilibrium enabled the author of paper [89] to conclude that the proposed mechanism of effect of hydrogen on electric conductivity of semiconductors can be one of active mechanisms. The heat of total reaction (2.63) calculated from the values found was about 4.6 kcal. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

A comparison of the relevant equations for metals, band theory semiconductors, and hopping semiconductors is given in Table 7.1. These equations can be used in a diagnostic fashion to separate one material type from another. In practice, it is not quite so easy to distinguish between the different conductivity mechanisms. 

Although these materials are often best described as hopping semiconductors in which the mobility is proportional to the exponential of the energy required to liberate the charge carriers, the conduction mechanism in devices is often complex. For application, mechanisms ate sometimes neglected in favor of empirical relationships. 

Solid state materials that can conduct electricity, are electrochemically of interest with a view to (a) the conduction mechanism, (b) the properties of the electrical double layer inside a solid electrolyte or semiconductor, adjacent to an interface with a metallic conductor or a liquid electrolyte, (c) charge-transfer processes at such interfaces, (d) their possible application in systems of practical interest, e.g. batteries, fuel cells, electrolysis cells, and (e) improvement of their operation in these applications by modifications of the electrode surface, etc. 

Analysis of Hall-effect data has been one of the most widely used techniques for studying conduction mechanisms in solids, especially semiconductors. For the single-carrier case, one readily obtains carrier concentrations and mobilities, and it is usually of interest to study these as functions of temperature. This can supply information on the predominant charge-carrier scattering mechanisms and on activation energies, i.e., the energies necessary to excite carriers from impurity levels into the conduction band. Where two or more carriers are present, the analysis becomes more complex, but much more information can be obtained from sludies of the temperature and magnetic held dependencies. 

Some examples of intrinsic semiconductors are silicon and germanium. These materials do not contain any impurities. The conduction mechanism can be represented in a simplified way by means of a projection diagram of a silicon or germanium crystal (fig. 11.4.6) By supplying energy to the material, the electrons are torn free from their atoms and positively charged electron holes (+) arise. Placing the material in an electric field will result in the transport of holes ... 

Finally, any doping leading to a finite conductivity cr will lead to a loss contribution according to tan5 = conduction mechanism like hopping conductivity may give rise to complicated temperature and frequency dependences, which cannot be discussed in detail here. 

Fig. 1.11. Illustration of the three main conduction mechanisms expected in an amorphous semiconductor.

So far no amorphous semiconductors have been made with a Fermi energy in the extended states beyond the mobility edge. The Fermi energy of doped a-Si H moves into the band tails, but is never closer than about 0.1 eV from the mobility edge. There is no metallic conduction, but instead there are several other possible conduction mechanisms, which are illustrated in Fig. 1.11. 

Photovoltaic devices are based on the concept of charge separation at an interface of two materials having different conduction mechanisms, normally between solid-state materials, either n- and p-type regions with electron and hole majority carriers in a single semiconductor material, heterojunctions between different semiconductors, or semiconductor-metal (Schottky) junctions. In photoelectrochemical cells, the junctions are semiconductor-electrolyte interfaces. In recent years, despite prolonged effort, disillusion has grown about the prospects of electrochemical photo-... 

Properties of representative conducting polymers. Doped conjugated polymers have generated substantial interest in view of possible applications such as lightweight batteries, antistatic equipment, and microelectronics to speculative concepts such as molecular electronic devices.37-38 These polymers include doped polyacetylene, polyaniline, polypyrrole, and other polyheterocycles (Figure 5). While the conduction mechanism of metals and inorganic semiconductors is well understood and utilized in microelectronics, this is not true to the same... 

The transduction mechanisms of these sensors are based on the conduction of semiconductors such as tin oxide [16], or polymers such as polypyrrole [17]. More sensitive are sensors that weigh impinging molecules [18] and more sensitive still is the biological nose. Recently there has been a renewal of interest in optical sensors incorporating fluorescent molecules [19]. Typically a device will have 3 to 30 sensors, the output of each being a voltage. This may be measured at the steady state, or the time development of the voltages may be monitored. Humidity and temperature control is important for many sensors. 

Attempts have recently been made to determine the dominat electric conductivity mechanism using the results of measurements of the current flow across asymmetric systems, such as metal -polymer-metalo (Me -P-Mc2) and metal-polymer-semiconductor (M-P-Sj such studies involved plasma-polymerized styrene (2 ), silo-xane and silazane ( ). The possibility of tunnel-... 

The perylene derivatives are n-t3rpe organic semiconductors. They are of great interest as components for organic electronics. In particular, films of perylenetetracarboxylic diimide derivative (PTCDI) are used as n-layers in heterojunctions of organic solar cells [1]. The industrial application of these materials is now limited by insufficient knowledge about conductivity mechanisms and their correlation with structural features of the films. 

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