Semiconductors, crystalline silicon electronic structures

Crystalline silicon is well known as one of the most useful semiconductors for electronic devices. Many theoretical calculations on the electronic structure of crystalline silicon have been done by the band structure calculation or the cluster method. 

Crystalline silicon is the most widely used semiconductor material today, with a maiket share of above 90%. Because of its indirect electronic band structure, however, the material is not able to emit light effectively and therefore carmot be used for key applications like light-emitting diodes or lasers. Selected one- or two-dimensional silicon compounds like linear or branched polysilylenes [1] or layered structures like siloxene [2], however, possess a direct band gap and therefore exhibit intense visible photoluminescence. Siloxene, a solid-state polymer with a sheet-like layered structure and an empirical formula Si H (OH) , in particular, is considered as an alternative material for Si-based liuninescent devices. Detailed studies of stmctural and photophysical properties of the material, however, are strraigly impeded by its insolubility in organic solvents. 

Silicon is by far the most important semiconductor for electronics and photovoltaics since the 1960s and for the foreseeable future. It is used in many forms, with monocrystalline and multicrystalline (MC) self-supporting silicon wafers being dominant but also in the forms of deposited thin-films in amorphous or crystalline form. The structure of one commercially important crystalline silicon cell technology is shown in Fig. 1. 

In an amorphous semiconductor with a low defect density such as hydrogenated amorphous silicon (a-Si H), charge transport takes place in the electronic states in the vicinity of the conduction- and valence-band edges. However, no complete theory of the electronic structure near the band edges in a-Si H or any amorphous semiconductor has yet been devised. The problem appears to be extraordinarily complex. The disorder generates localized states near the band edges that are not present in crystalline material. 

Silicon s tetravalent pyramid crystalline structure, similar to tetravalent carbon, results in a great variety of compounds with many practical uses. Crystals of sihcon that have been contaminated with impurities (arsenic or boron) are used as semiconductors in the computer and electronics industries. Silicon semiconductors made possible the invention of transistors at the Bell Labs in 1947. Transistors use layers of crystals that regulate the flow of electric current. Over the past half-century, transistors have replaced the vacuum tubes in radios, TVs, and other electronic equipment that reduces both the devices size and the heat produced by the electronic devices. 

The specific application of a material generally determines the particular structure desired. For example, hydrogenated amorphous silicon is used for solar cells and some specialized electronic devices (10). Because of their higher carrier mobility (see Carrier Transport, Generation, and Recombination), single-crystalline elemental or compound semiconductors are used in the majority of electronic devices. Polycrystalline metal films and highly doped polycrystalline films of silicon are used for conductors and resistors in device applications. 

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