Germanium electron conductivity

To determine forbidden energy gaps in a semiconductor, one can investigate how its electronic conductivity (a) varies with temperature. Semiconductor theory indicates that the logarithm of the conductivity varies linearly with the inverse temperature. The value of the energy gap, Eg, is given by the slope of the In a Vs. 1/T line. Table 5A.4 contains the values of nine runs performed in duplicate with a germanium intrinsic semiconductor. The data were obtained at the Modern Physics Laboratory of the Physics Department of the Londrina State University, under the supervision of Prof. J. Scarminio. 

The deliberate addition of carefully chosen impurities to silicon, germanium and other semiconductors is called doping. It is carried out so as to modify the electronic conductivity, and the dopants are chosen so as to add either electrons or holes to the material. It is possible to gain a good idea of how this is achieved very simply. Suppose that an atom such as phosphoms, P, ends up in a silicon crystal. This can occur, for example, if a small amount of phosphorus impurity is added to molten silicon before the solid is crystallised. Experimentally the impurity atom is found to occupy a position in the crystal that would normally be occupied by a silicon atom, and so forms a substitutional defect. 

The major disadvantage associated with the use of germanium thermometers is the lack of a simple analytical representation. This is because conduction (in the -type) changes gradually from impurity conduction at low temperatures to free electron conduction at higher temperatures, leading to a... 

Germanium single crystals intended for electronic apphcations are usuaHy specified according to conductivity type, dopant, resistivity, orientation, and maximum dislocation density. They may be specified to be lineage-free unless the specified resistivity is below about 0.05 H-cm. Minority carrier lifetime and majority carrier mobHity are occasionaHy specified. 

Analysis of refined germanium products is done in a wide variety of ways, including several methods that have become ASTM standards (47). Electronic-grade Ge02 is analyzed using an emission spectrograph to determine its spectrographic purity. Its volatile content is measured in accord with ASTM F5 and its bulk density with F6. Other ASTM standards cover the preparation of a metal biHet from a sample of the oxide (F27), and the determination of the conductivity type (F42) and resistivity (F43) of the biHet. 

The short-range order in a material is important in determining optoelectronic properties. For instance, x-ray and electron diffraction experiments performed on amorphous siHcon (i -Si) and germanium (a-Ge) have revealed that the nearest neighbor environments are approximately the same as those found in their crystalline counterparts (6) photoemission experiments performed on i -Si show that the DOS in valence and conduction bands are virtually identical to the corresponding crystal with the exception that the singularities (associated with periodicity) present in the latter are smeared out in the former. 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

Materials in which there is a substantial difference in energy between occupied and vacant MOs are poor electron conductors. Diamond, where the gap between the filled valence band and the empty conduction band is 500 kj/mol, is an insulator. Silicon and germanium, where the gaps are 100 kj/mol and 60 kj/mol respectively, are semiconductors. 

The relatively large band gaps of silicon and germanium limit their usefulness in electrical devices. Fortunately, adding tiny amounts of other elements that have different numbers of valence electrons alters the conductive properties of these solid elements. When a specific impurity is added deliberately to a pure substance, the resulting material is said to be doped. A doped semiconductor has almost the same band stmeture as the pure material, but it has different electron nonulations in its bands. 

A major and growing use of the minor metalloids is in semiconductor fabrication. Germanium, like silicon, exhibits semiconductor properties. Binary compounds between elements of Groups 13 and 15 also act as semiconductors. These 13-15 compounds, such as GaAs and InSb, have the same number of valence electrons as Si or Ge. The energy gap between the valence band and the conduction band of a 13-15 semiconductor can be varied by changing the relative amounts of the two components. This allows the properties of 13-15 semiconductors to be fine-tuned. 

In substitutional metallic solid solutions and in liquid alloys the experimental data have been described by Epstein and Paskin (1967) in terms of a predominant frictional force which leads to the accumulation of one species towards the anode. The relative movement of metallic ion cores in an alloy phase is related to the scattering cross-section for the conduction electrons, which in turn can be correlated with the relative resistance of the pure metals. Thus iron, which has a higher specific resistance than copper, will accumulate towards the anode in a Cu-Fe alloy. Similarly in a germanium-lithium alloy, the solute lithium atoms accumulate towards the cathode. In liquid alloys the same qualitative effect is observed, thus magnesium accumulates near the cathode in solution in bismuth, while uranium, which is in a higher Group of the Periodic Table than bismuth, accumulated near the anode in the same solvent. 

We discuss the dissolution of surface atoms from elemental semiconductor electrodes, which are covalent, such as silicon and germanium in aqueous solution. Generally, in covalent semiconductors, the bonding orbitals constitute the valence band and the antibonbing orbitals constitute the conduction band. The accumulation of holes in the valence band or the accumulation of electrons in the conduction band at the electrode interface, hence, partially breaks the covalent bonding of the surface atom, S, (subscript s denotes the surface site). 

In general, the activation energy for the release of electrons from surface atoms into the conduction band increases with increasing band gap of the semiconductor electrode with this increase the capture of holes by the surface atoms and radicals predominates. Except for germanium, most covalent semiconductors have been found to dissolve anodically through this valence band mechanism [Memming, 1983]. 

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