Semiconductors electron movement

Semiconductors (qv) are materials with resistivities between those of conductors and those of insulators (between 10 and 10 H-cm). The electrical properties of a semiconductor determine the hmctional performance of the device. Important electrical properties of semiconductors are resistivity and dielectric constant. The resistivity of a semiconductor can be varied by introducing small amounts of material impurities or dopants. Through proper material doping, electron movement can be precisely controlled, producing hmctions such as rectification, switching, detection, and modulation. 

Owing to difficulties in electron movement in semiconductors, when a steady state has been achieved almost all the applied potential appears within the semiconductor, creating a region of potential variation close to the surface called the space-charge region (Fig. 3.16). 

Thermoelectricity is one of the methods for electric power generation of energy transformation from heat to electric power. For this purpose, one can use electron-movement in semiconductor by generating temperature difference between two substances or materials. 

When a Schottky junction between a metal and an n-type semiconductor is illuminated and photons having enough energy for the creation of an electron-hole pair are absorbed in the depletion region, the electric field at the junction is able to separate the photogenerated charges, preventing recombination. Electron movement toward the semiconductor and hole... 

Ballistic transport—Movement of a carrier through a semiconductor without collisions, resulting in extraordinary electrical properties. Carriers—Charge-carrying particles in semiconductors, electrons, and holes. 

We have seen that when intrinsic semiconductors are heated or illuminated, electrons are promoted from the valence band to the conduction band. This generates vacancies (which also permit electron movement) in the valence band. In n-type semiconductors, we feed electrons into the conduction band without affecting the valence band, and in p-type semiconductors we introduce vacancies in the valence band but leave the conduction band empty. The three types of semiconductor are shown in Figure 8.17. 

At temperatures above absolute zero, some electrons will have enough thermal energy to move from the valance band to the conduction band. Each electronic transition leaves a hole in the valence band. These holes have an effective positive charge, are mobile, and contribute to the conduction of electrical charge, along with the electrons. Insulators differ from semiconductors in that their band gap is wider (4-5 eV compared to 2-3 eV for semiconducting oxides). In insulators the valence electrons are no longer able to access the conduction band and, as a result, electron movement is not induced by the application of an electric field. 

Electric current is conducted either by these excited electrons in the conduction band or by holes remaining in place of excited electrons in the original valence energy band. These holes have a positive effective charge. If an electron from a neighbouring atom jumps over into a free site (hole), then this process is equivalent to movement of the hole in the opposite direction. In the valence band, the electric current is thus conducted by these positive charge carriers. Semiconductors are divided into intrinsic semiconductors, where electrons are thermally excited to the conduction band, and semiconductors with intentionally introduced impurities, called doped semiconductors, where the traces of impurities account for most of the conductivity. 

Flat single-layer graphene is a zero band-gap semiconductor [50], in which every direction for electron transport is possible. However, when the graphene sheet is rolled up to form a SWCNT, the number of allowed states is limited by quantum confinement in the radial direction [17], i.e. the movement of electrons is confined by the periodic boundary condition [51] ... 

Technically this means rather more than bad conductor . Metals conduct electricity because some of their electrons come free of their parent atoms and are at liberty to roam through the material. Their motion corresponds to an electrical current. A semiconductor also has wandering electrons, but only a few. They are not intrinsically free, but can be shaken loose from their atoms by mild heat some are liberated at room temperature. So a semiconductor becomes a better conductor the hotter it is. Metals, in contrast, become poorer conductors when hot, because they gain no more mobile electrons from a rise in temperature and the dominant effect is simply that hot, vibrating atoms obstruct the movement of the free electrons. 

Figure 3 Schematic of a nanoporous 2 film in the dark showing the movement of compensating positive ions (circles with + ) through the film that screens a negative potential (electrons shown as - ) applied to the Sn02 substrate electrode, (a) The electric field is screened close to the substrate when the potential is positive of the conduction band, but (b) extends further into the semiconductor for more negative potentials. The potential distribution also depends on the relative rates of interfacial versus interparticle charge transfer (Fig. 2).

The effect of an external electric field is to produce an acceleration of the electrons in the direction of the field, and this causes a shift of the Fermi surface. It is a necessary condition for the movement of electrons in the fc-space that there are allowed empty states at the Fermi surface hence electrical conductivity is dependent on partially filled bands. An insulating crystal is one in which the electron bands are either completely full or completely empty. If the energy gap between a completely filled band and an empty band is small, it is possible that thermal excitation of electrons from the filled to the empty band will result in a conducting crystal. Such substances are usually referred to as intrinsic semiconductors. A much larger class of semiconductors arises from impurities... 

Fig. 4. Simplified version of Digby s semiconductor theory of biomineralization. In the arthropod (top) ions are continually diffusing out of the animal across the cuticle at different rates setting up a potential with the outer surface positive. This causes a flow of electrons leaving the inner surface rich in proteins and the outer surface with hydroxyl ions. The alkaline outer surface favors CaC03 formation. In molluscs (bottom) muscular movements cause salt flow through the periostracum followed by an alkaline reaction on the inside inducing CaC03 deposition. (After Simkiss445 )...

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