Two Semiconductors

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. ... 

Junctions involving a bilayer (two-semiconductor) electrode and a liquid have been investigated as to their photoelectrochemical properties primarily in works which... 

A comparative study of oxides which were closely related, but had different electrical properties, showed that both n- and p-type semiconduction promoted the oxidation reaction, forming CO as the major carbon-containing product. In a gas mixture which was 30% methane, 5% oxygen, and 65% helium, reacted at 1168 K the coupling reactions were best achieved with the electrolyte Lao.9Sro.1YO 1.5 and the /i-lype semiconductor Lao.sSro MntL A and the lily pe semiconductor LaFeo.sNbo.2O1 a produced CO as the major oxidation product (Alcock et al., 1993). The two semiconductors are non-stoichiometric, and the subscript 3 — x varies in value with the oxygen pressure and temperature. Again, it is quite probable that the surface reactions involve the formation of methyl radicals and O- ions. 

An alternative approach involves using two semiconductor electrodes, with the oxidation reaction occurring at an n-type semiconductor (Ti02) and the reduction reaction occurring at a p-type semiconductor (GaP) (Figure 11.10). 

Photoelectrochemical semiconductor cells are used to convert photon energy into chemical substances or into electricity, the former is a photodectrolytic cell and the latter is a photovoltaic cell. A photoelectrochemical semiconductor cell consists of either a pair of metal and semiconductor electrodes or a pair of two semiconductor electrodes. 

The photoelectrolytic cell consisting of n-lype and p-lype semiconductor electrodes provides an advantage over the cell consisting of semiconductor and metal electrodes a cell consisting of two semiconductor electrodes with their small band gaps adsorb the energy of solar photons more efficiently than the cell consisting of semiconductor and metal electrodes, in which the semiconductor electrode requires a relatively wide band gap for the decomposition of water. 

Figure 8. Schematic showing energy correlations at equilibrium for cell with two semiconductor electrodes in contact with aqueous solution and through an external circuit with each other. An n-type semiconductor anode and a p-type cathode are shown to left and right, respectively. In each case the minimum light energy to give rise to a photocurrent is indicated by hvmin (n) and hvmin (p), respectively. The energies available for oxidation and reduction are also indicated. and Ev(n) are conduction and valence band edges for the n-type material and Ec(p) and E fp) are those for the p-type material. Other symbols as in Figure 7.

The application of Langmuir-Blodgett films as rectifiers and/or switches have been also proposed. Peterson [106] investigated two semiconductors polyparaphenylene 178 and polyphenylenevinylene 179 with these purposes in mind. These systems with the chain lengths of at least 20 units could also be used as photovoltaic devices since their electroluminescence should be readily detectable. The doping of such materials may be necessary but at present it is not clear whether they will form Langmuir-Blodgett films when doped. 

The semiconductor-electrolyte solution interface is a contact of two conducting media, so that some of its properties are similar to those of contacts between a semiconductor and a metal or between two semiconductors. At the same time, the interface considered is a contact of two media with essentially different types of conductivity—electronic and ionic moreover, these media are in different states—solid and liquid. Therefore, such an interface possesses a number of unique features. 

In a few cases (70) catalysts have been studied that consist of mixtures of two semiconductors of the same chemical composition but with different levels of doping. The considerations here are analogous to those presented above. Of course, the ultimate proof as to whether electronic factors are indeed responsible for the catalytic effects discussed above will have to come through physical measurements of the electronic properties of catalyst/support systems. 

In solid-state devices semiconductors are interfaced with other materials and junctions are formed. A junction between two semiconductors of opposite polarity is called a pn-junction. If the concentration of one type of dopant is much higher than that of the other (e.g., N > > Afo) it is called an abrupt junction (Fig. C.3). For the approximately equal doping levels we talk about, we use the term two-sided junction. A gradient of dopants is found in graded junctions. The concentrations of carriers on the two sides of the junction (subscripted) are... 

The different behavior (direct versus indirect band gap) of Si and Ge with respect to the film orientation can be explained in term of confinement effects on the conduction band minima (CBM) of the two semiconductors.Whereas the six equivalent ellipsoidal CBM of bulk Si occur in the (100) directions about 80% of the way to the zone boundary, in bulk Ge there are eight symmetry-related ellipsoids with long axes along the (111) directions centered on the midpoints of the hexagonal zone faces. Also the different confinement energy shifts with respect to the orientation of the layer can be interpreted in terms of the different highly anisotropic behavior of the effective masses for bulk Ge and Si [170,171]. 

In order to suppress the recombination of the photogenerated electron-hole pairs, some researchers [6, 15] have described the photocatalytic activity of composite photocatalysts consisting of two semiconductors. In these configurations, after absorption of a photon, the transfer of the electrons from the conduction band of the photoexcited component to that of the unexcited component occurs, leading to stable semiconductor particles with separated charges that do not... 

The dark currents of photovoltaic detectors, which are exponentially dependent on the bandgap of the semiconductor materials, are reduced in US-A-4791467 through the use of two semiconductor layers having different bandgaps. 

Composite semiconductor nanoclusters can be classified into two categories, namely, capped- and coupled-type heterostructures. The capped nanoclusters essentially have a core-shell geometry while in a coupled system two semiconductor nanoclusters are in contact with each other. The principle of charge separation in capped and coupled semiconductor systems is illustrated in Fig. 12. 

Hot electron spin transistors are hybrid metal/semiconductor devices that rely on spin-dependent transport of hot (nonthermalized) electrons rather than electrons near the Fermi level. The spin-valve transistor (SVT) was the first example of this new class of spintronic devices [128, 129], It has a three-terminal structure consisting of a metallic spin-valve base that is sandwiched between two semiconductor substrates, serving as the emitter and the collector, respectively. The electrons in this device are transported perpendicular to the spin-valve layers at energies just above the collector Schottky barrier height. 

In bulk heterojunction solar cells, the metal/semiconductor interface is even more complex. Now the metal comes into contact with two semiconductors, one p-type (typically the polymer) and one n-type (typically the fullerene) semiconductor. A classical electrical characterization technique for studying the occurrence of charged states in the bulk or at the interface of a solar cell is admittance spectroscopy. If a solar cell is considered as a capacitor with capacitance C, the complex admittance Y is given by... 

In order to get a fast characterization of the degradation processes, the individual semiconductors (MDMO PPV and Ceo) and a blend of the two semiconductors as used in bulk heterojunction solar cells are studied under illumination in pure oxygen. ATR-FTIR spectra before and after an 8 h degradation process and difference spectra showing only the spectral changes... 

Schematic of a semiconducting pn junction distributed Bragg59 reflector laser diode (or injection laser), such as used in 1-mW laser pointers (HeNe, 633 nm (obsolete) deep red AIGaAs 650 to 670 nm red-orange 635 nm frequency-doubled green 532 nm blue, 473 nm) and CD and DVD players (670 nm "Blue-Ray" InGaN 405 nm). The optically flat front surface (shown) is partially reflective ( 95%) the optically flat back surface (not shown here) is highly reflective ( 100%) and parallel to the front surface. The top and bottom electrical contacts provide the necessary external bias across the two semiconductor regions. The wavelength is determined by the distance between the two optically flat surfaces. The angular divergence of the output beam is considerable.

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