Semiconductor p-type semiconductor

Describe semiconductors, p-type semiconductors, and n-type semiconductors. 

Define the following terms conductor, insulator, semiconducting elements, donor impurities, acceptor impurities, w-type semiconductors, p-type semiconductors. 

Metal oxide catalysts can be generally divided into three groups insofar as catalytic oxidation reactions are concerned. These are n-type semiconductors, p-type semiconductors, and insulators. The basis for this classification is electrical conductivity (which is related to their catalytic properties). 

N-type semiconductor (p-type semiconductor) A semiconductor containing impurity atoms in which the electron (hole) is the primary charge carrier. 

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.

The deposition of amoriDhous hydrogenated silicon (a-Si H) from a silane plasma doped witli diborane (B2 Hg) or phosphine (PH ) to produce p-type or n-type silicon is important in tlie semiconductor industry. The plasma process produces films witli a much lower defect density in comparison witli deposition by sputtering or evaporation. 

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

Instead of plotting tire electron distribution function in tire energy band diagram, it is convenient to indicate tire position of tire Fenni level. In a semiconductor of high purity, tire Fenni level is close to mid-gap. In p type (n type) semiconductors, it lies near tire VB (CB). In very heavily doped semiconductors tire Fenni level can move into eitlier tire CB or VB, depending on tire doping type. 

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. 

In order to obtain appreciable conductivities, semiconductors must be doped witli small amounts of selected impurities. It is possible to switch tire doping type from n to p type, or vice versa, eitlier during tire growtli of a crystal or by tire selective introduction of impurities after tire growtli. The boundary region between tire p type and n type regions is... 

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... 

A semiconductor laser takes advantage of the properties of a junction between a p-type and an n-type semiconductor made from the same host material. Such an n-p combination is called a semiconductor diode. Doping concentrations are quite high and, as a result, the conduction and valence band energies of the host are shifted in the two semiconductors, as shown in Figure 9.10(a). Bands are filled up to the Fermi level with energy E. ... 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

A photovoltaic cell (often called a solar cell) consists of layers of semiconductor materials with different electronic properties. In most of today s solar cells the semiconductor is silicon, an abundant element in the earth s crust. By doping (i.e., chemically introducing impurity elements) most of the silicon with boron to give it a positive or p-type electrical character, and doping a thin layer on the front of the cell with phosphorus to give it a negative or n-type character, a transition region between the two types... 

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 ... 

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