P semiconductor

The heart of a light-emitting diode is a junction between a p-type semiconductor and an n-tyqje semiconductor. The different semiconductor types have different electron populations in their bands. The lower-energy band of a p semiconductor is deficient in electrons, while the upper-energy band of an n semiconductor has a small population of electrons. The band structure in the junction region is shown schematically in the figure below. 

Fig. 5.60 The semiconductor/electrolyte interface (a) before equilibration with the electrolyte, (b) after equilibration with the electrolyte in the dark, and (c) after illumination. The upper part depicts the n-semiconductor and the lower the p-semiconductor...

The changes in catalytic activity and conductivity on the acceptor branch of the curve (Fig. 8a) are directly related in the case of an n-semi-conductor and inversely related in the case of a p-semiconductor. It is this correlation that has been found in many experimental works, as noted in Section III. A. 

A phototransistor or photodiode may also be used to detect visible fight. Both devices have p-n junctions. In the photodiode the photon ejects an electron from the p semiconductor to the n semiconductor. The electron cannot cross back across the p-n junction and must travel through the circuitry, an ammeter to return to the p material. In a phototransistor, usually an npn type, the base (p-type semiconductor) is enlarged and photosensitive. Photons dislodge electrons that act as if a potential was applied to the base. This results in an amplified flow of electrons proportional to the number of photons striking the base (Fig. 5.11). 

Ahvisatos, A.P., Semiconductor clusters, nanocrystals, and quantum dots, Science, 271, 933, 1996. 

Maximization of the available energy for electrolysis with a given semiconductor seems to require use of the most heavily doped material together with a minimal band bending consistent with efficient charge separation so that in the n-semiconductor, Ep(n) is as high as possible while E (n, surface) is as low as possible and in the p-semiconductor 2 (p) is as low as possible... 

Photoelectrolysis cells of the p-n variety can be constructed using the same semiconductor material, one doped p-type and the other n-type or with two different (n-and p-) semiconductors. A photoelectrolysis cell reported by Leygraf et al. (1982),... 

Fig. 7.27. Changes in the potential energy of the electron in the p semiconductor and the potential drop at the semicon-ductor/soiution interface when there are surface states at the semiconductor electrode surface.

The n-p junction was discussed in Section 7.4.1.2. In the original concept, this junction resulted from the transfer of electrons from one semiconductor to another. In the figures in Sec. 7.4, potential-distance relations for the junction of n and p semiconductors are shown. It is clear that here the transfer of one charge carrier from one semiconductor to the next in an uphill direction can be thought of as being opposed by the electrical potential hill shown. Such potential hills are termed Schottky barriers. ... 

Current Yield of Phenyl Acetic Acid in Various p-Semiconductor Synthetic... 

Studies by Steinberg et al. (58,121) involved a variety of surfaces between the source of spillover and a reacting surface (OH-OD exchange). They found that n- and p-semiconductors and insulators equally promoted the transport from the source to the reacting surface, although the rate of transport was much less on stainless steel. They concluded that the species spilling over was uncharged and that its transport did not depend on the semiconductor properties of the oxide. However, the oxide (or hydroxide) surface was involved. 

If the density of holes Ps at the surface - or equivalently the quasi-Fermi level Ep p — are equal at the surface of an n- and p-semiconductor electrode, then the same reaction with identical rates, i.e. equal currents, takes place at both types of electrodes (Fig. 15). Since holes are majority carriers in a p-type semiconductor, the position of the quasi-Fermi level Ep,p is identical to the electrode potential (see right side of Fig. 15), and therefore-with respect to the reference electrode - directly measurable. The density of p can easily be calculated, provided that the positions of the energy bands at the surface are known. The measurements of a current-potential curve also yields automatically the relationship between current and quasi-Fermi level of holes. The basic concept implies that the position of the quasi-Fermi level Ep,p at the surface of an n-type semiconductor and the corresponding hole density Ps can be derived for a given photocurrent, since the same relationship between current and the quasi-Fermi level of holes holds. 

P4. Parker, R. P., Semiconductor nuclear radiation detectors. Phys. Med. Biol. 15, 605-620 (1970). 

In a light-emitting diode (LED), which is used in displays on electronic equipment, watches, and clocks, a voltage is imposed across an n-p semiconductor junction. The electrons on the n side combine with the holes on the p side and emit light at the frequency of the hand gap. This process can also be described as the emission of light as electrons fall from levels in the conduction hand to empty levels in the valence band. It is the reverse of the production of electric current by illumination of a semiconductor. 

A water-splitting device has been invented [4], where photo-semiconductor and platinum are used as the cathode and the anode, respectively, instead of setting both the solar cell and the electrolyzer, separately. This method is called photoelectrochemical (PEC) water-splitting or photo semiconductor electrode method . The key phenomenon of PEC watersplitting is the steep rise (fall) of the potential at the interface between the n-(p-) semiconductor and the liquid electrolyte (e.g., KOH). If photons irradiate onto the interface, both the electrons (e ) and positive holes (IT) are excited to their conductive energy bands where they can move freely, so that e and h+ are separated by the interface potential difference. The h+ react with water by the equation ... 

Alivisatos, A.P. Semiconductor nanocrystals as fluorescent biological labels. Science 1998, 281, 75. 

Alivisatos, A. P, Semiconductor Clusters, Nanocrystals, and Quantum Dots, Science 1996, 271, 933 937. 

The compound must have a large affinity for electrons, i.e.-be a "hole-conductor - a p-semiconductor. 

Attempts have been made to solidify the liquid using polymer gels or p-semiconductors, and a high efficiency of more than 7% has been reported. 

Figure 2.32 Schematic hand sketch of intrinsic (i), n-doped (n), and p-doped (p) semiconductor.

Conductivity can also be achieved by doping. By adding small amounts of substances with excess electrons, these electrons can be inserted into the conduction band (n-doping, n-semiconductor). Adding substances with electron deficiency attracts the electrons from the valence band and creates holes in the valence band (p-doping, p-semiconductor). The properties of the semiconductor are then determined by the new majority careers, either by the electrons in the conduction band or the electron holes in the valence band. 

Doping shifts the Fermi energy. For an n-semiconductor the Fermi energy moves to a position near to the lower edge of the conduction band. For a p-semiconductor the Fermi energy moves to a position near to the upper edge of the valence band. This was shown in Figure 2.32. 

Figure 9.2 Potential gradient and band bending in the double layer of a semiconductor-electrolyte contact. (A) n-Semiconductor and (B) p-semiconductor.

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