P-type semi-conductivity

The amount of information available in the literature concerning the use of perovskite catalysts for the water gas shift reaction is very minimal. However it has been shown that some ABO3 perovskites have exhibited significant activity for the reaction. GdFeOs is such an example whose activity is attributed to their p-type semi-conductivity due to a limited number of Fe cations, existing in their crystal lattice and associated with the presence of some cation vacancies [10]. 

Another characteristic of VPP is related to its p-type semi-conductivity associated with defects by Eq. 20.5." -Butane would be activated by means of... 

Within the different temperature intervals, any of the elementary processes (oxygen adsorption, disintegration of the O2 molecule, surface processes at the Pb/PbO and PbO/air interfaces, transport of 0 vacancies and electrons through the PbO layer) may become rate limiting and define a different kinetic equation for the oxygen process. In the presence of alloying additives in lead, the oxidation process may be further complicated, since the dopants affect not only the semi-conductor properties n-, or p-type PbO conductivity), but also the mobility of ionic defects. 

The inteicoimection is ensured by a conductive band, 9 nun wide and -85 jun thick, deposited along the length of the tube by plasma technology. The stresses are particularly severe. The selected material is doped lanthanum chromite (Lai.xSrxCr03), a p-type semi-conductor in a large domain of Pq2 (air-10 bar). 

The fact that pure silicon is not a conductor but is "doped to make it a good semi-conductor might seem odd, particularly since considerable effort goes into obtciining very pure silicon. Nonetheless, the undoped silicon is not conductive. This allows one to form discrete areas of n-type and p-type silicon on the same wafer and to position these in conjunction with each other in a manner so that a current amplifying device results. An example of such a device is shown in the following diagram ... 

Electrical conductivity of metals is very high and is of the order of 106 108 ohm-1 cm-1 while that of insulators is of the order of 10-12 ohm-1 cm-1. Semi-conductors have intermediate conductivity which lies in the range 102 10-9 ohm-1 cm1. Electrical conductivity of solids may arise through the motion of electrons and positive holes (electronic conductivity) or through the motion of ions (ionic conductivity). The conduction through electrons is called n-type conduction and through positive holes is called p-type conduction. Pure ionic solids where conduction can take place only through motion of ions are insulators. However, the presence of defects in the crystal structure increases their conductivity. 

Other Applications. Despite the ability of TED spectroscopy to detect surface states, comparatively little work has been done with semi-conductors. In a careful study of germanium Shepherd identified emission from the valence band and also from a band of surface states. Conduction band emission is negligible in the [100] direction but has been detected in emission from Ge (111). TED spectra from both field-evaporated and annealed 200 ohm-cm p-type silicon tips show electrons to be emitted from occupied surface states within the band gap which lie close to, and overlap, the valence band edge. TED spectra from CdS, PbTe, and GaP have proved to be broad and to contain tittle information. Weak unidentified features appear in the TED from TiC crystals and much stronger field-dependent peaks... 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

Point defects are particularly important in ceramics because of the role they can play in determining the properties of a material. The entire semiconductor industry is possible because of minute concentrations of point defects that are added to Si the dopants determine if the Si is n-type, p-type, or semi-insulating they determine the electrical properties. Solid-oxide fuel cells work because of the large concentrations of oxygen vacancies present the vacancies provide fast ion conduction pathways. CZ is cubic because of the presence of point defects that stabilize the cubic structure. 

ABO3 perovskite-type oxides with transition-metal ions at the B-site have high ionic and electronic transport in the form of p or n semi-conductivity (mixed ionic and electronic conductivity), caused by different oxidation states of the transition-metal cation. For dense ceramic membranes, perovskite-type oxides with the following cations are preferred A = Ln (lanthanide ion), Ca, Sr, Ba B = Cr, Mn, Fe, Co, Ni, Cu. 

Semi-conducting GaAs of n- and p-type is produced by adding shallow acceptors (mainly Zn, Be, C) and shallow donors (mainly Si, Te) with concentrations of 0(10 cm ) directly into the melt. Then, the Fermi level is pinned near the valence and conduction band, respectively. Silicon, which is most frequently used as a n-type dopant, shows amphoteric behavior in GaAs, i.e. it can be substituted for an As, as well as a Ga atom with roughly [Sica] 0.9 [Si]. 

As mentioned earlier, photochemical diodes489 can be either of the Schottky type, involving a metal and a semiconductor, or a p n junction type, involving two semi conductors (which can be the same, i.e., a homojunction or different, a heterojunc tion). Only the latter type is considered in this Section involving two irradiated semi conductor/ electrolyte interfaces. Thus n Ti02 and p-GaP crystal wafers were bonded together (through the rear Ohmic contacts) with conductive Ag epoxy cement.489 The resultant heterotype p n photochemical diode was suspended in an acidic aqueous... 

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