Semiconductors p-type

In general, then, anion-forming adsorbates should find p-type semiconductors (such as NiO) more active than insulating materials and these, in turn, more active than n-type semiconductors (such as ZnO). It is not necessary that the semiconductor type be determined by an excess or deficiency of a native ion impurities, often deliberately added, can play the same role. Thus if Lr ions are present in NiO, in lattice positions, additional Ni ions must also be present to maintain electroneutrality these now compete for electrons with oxygen and reduce the activity toward oxygen adsorption. 

Figure Bl.28.10. Schematic representation of an illuminated (a) n-type and (b) p-type semiconductor in the presence of a depletion layer fonned at the semiconductor-electrolyte interface.

In n type semiconductors, electrons are tire majority carriers. Holes will also be present tlirough accidental incoriioration of acceptor impurities or, more importantly, tlirough tlie intentional creation of electron-hole pairs. Holes in n type and electrons in p type semiconductors are minority carriers. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

Figure 9.9 Impurity levels I in (a) an n-type and (b) a p-type semiconductor C is the conduction band and V the valence band...

Alternatively, as in Figure 9.9(b), a dopant with one valence electron fewer than the host contributes an impurity band 1 which is empty but more accessible to electrons from the valence band. An example of such a p-type semiconductor is silicon doped with aluminium KL3s 3p ) in which the band gap is about 0.08 eY... 

Eq. (14.1) is known as the Mott-Schotlky equation. We note llial for a given n-lype semiconductor, the harrier height increases as the work function of the metal increases. It is therefore expected that high work function metals will give a rectifying junction, and low work function metals an ohmic contact (it is the reverse for a p-type semiconductor). 

The Ni(OH)2/NiOOH reaction is a topo-chemical type of reaction that does not involve soluble intermediates. Many aspects of the reaction are controlled by the electrochemical conductivity of the reactants and products. Photoelectrochemical measurements [86, 871 indicate that the discharged material is a p-type semiconductor with a bandgap of about 3.7eV. The charged material is an n-type semiconductor with a bandgap of about 1.75eV. The bandgaps are estimates from absorption spectra [87]. 

Bearing in mind the semiconductive properties of PCSs one might expect that these substances, being p-type semiconductors in air, possess photosensitizing activity. We, indeed, have demonstrated40 that PCSs, such as poly(schiff base)s, salts of poly(propynoic acid), or polyquinoline, are active photosensitizers of... 

Solid-state electronic devices such as diodes, transistors, and integrated circuits contain p-n junctions in which a p-type semiconductor is in contact with an n-type semiconductor (Fig. 3.47). The structure of a p-n junction allows an electric current to flow in only one direction. When the electrode attached to the p-type semiconductor has a negative charge, the holes in the p-type semiconductor are attracted to it, the electrons in the n-type semiconductor are attracted to the other (positive) electrode, and current does not flow. When the polarity is reversed, with the negative electrode attached to the n-type semiconductor, electrons flow from the n-type semiconductor through the p-type semiconductor toward the positive electrode. 

FIGURE 3.46 In a p-type semiconductor, the electron-poor dopant atoms effectively remove electrons from the valence band, and the "holes" that result (blue band at the top of the valence band) enable the remaining electrons to become mobile and conduct electricity through the valence band. 

FIGURE 3.47 The structure of a p-n junction allows an electric current to flow in only one direction, (a) Reverse bias the negative electrode is attached to the p-type semiconductor and current does not flow, (b) Forward bias the electrodes are reversed to allow charge carriers to be regenerated. 

Germanium is a semiconductor. If small amounts of the elements In, P, Sb, and Ga are present as impurities, which of them will make germanium into (a) a p-type semiconductor ... 

The YBft, 5 and YbBf,ft single crystals are p-type semiconductors with a band gap... 

Conjugated polymers doped with C60 become p-type semiconductors [305,306] some LB films of two polyalkylthiophenes mixed with arachidic acid and doped with C60 have been prepared [307]. The films of polyalkylthiophene + arachidic acid -l- C60 (spread from mixtures of 1.0 0.33 0.1 ratio) on ITO glass had a well-defined layer structure, as confirmed by x-ray diffraction. The bilayer distance obtained from the Bragg equation was 5.6 nm, the same as for arachidic acid LB films. Since the films were spread on subphases containing... 

Figure 10-53 shows band-gap diagrams of n-type and p-type semiconductors. Electrical current flows in a doped semiconductor in the same way as current flows in a metal (see Figure 10-501. Only a small energy difference exists between the top of the filled band and the next available orbital, so the slightest applied potential tilts the bands enough to allow electrons to move and current to flow.

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. 

Various polymorphs have been reported for SnS with band gap widths in the range 1.0-1.5 eV, depending on the preparation method. The a-SnS (herzenbergite) is the most frequently occurring phase and is a p-type semiconductor with a direct optical transition at 1.3 eV and a high absorption coefficient (> 10" cm ). The orthorhombic S-SnS phase possesses a direct gap between 1.05 and 1.09 eV. 

Interesting systems, mainly with respect to solid-state optoelectronics and chalco-genide glass sensors (due to ionic conductivity effects) are found among the Group IIIB (13) and IVB (14) chalcogenides, such as the p-type semiconductors MSe (M = Ga, In, Sn), SnS, and GeX (X = S, Se, Te). Some of the IIIB compounds. 

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