Semiconductors sandwich-type

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

Fig. 7.2 Sandwich (or stacked) configurations placed in an electrolyte solution (a) p-n type, (b) semiconductor-metal type photochemical diodes. Both p-type and n-type semiconductors are provided with ohmic contacts. In p-n type light is incident from both directions and ohmic contacts are connected through metal contacts.

Van Mensfoort SLM, Coehoom R (2008) Effect of Gaussian disorder on the voltage dependence of the current density in sandwich-type devices based on organic semiconductors. Phys Rev B 78 085207... 

Formation of composite and sandwich-type semiconductors constitutes an interesting development (Fig. 100) [576,605-609]. A composite semiconductor, like a cherry and its stone, contains one material as its core and a second material as its shell. CdS particles coated by Cd(OH)2, CdSe coated by ZnS, and HgS coated by CdS are examples of recently prepared composite semiconductors... 

A series of LEDs with different active-layer thicknesses was grown by low-pressure MOCVD, with the structure given in the schematic in Fig. 6. The LEDs used were double-heterostructure, edge-emitting devices wherein p-type and n-type semiconductors sandwich an undoped, low-band-gap energy semiconductor (active layer). The p-n junction double-heterostructure is more efficient at trapping electrons and holes within the active layer for recombination, enhancing EL efficiency. 

Fig. 2.21 n-type semiconductor sandwiched between two metal electrodes. 

If the vacuum tube is the grandfather, then the transistor is the parent of today s electronic components. Transistors (Figure 1.2) work in basically the same manner as vacuum tubes. A small voltage applied to one pole controls a larger voltage on the other poles. The difference between a transistor and a vacuum tube is that a transistor uses a sandwich of silicon instead of tube filled with gas to do the switching. Silicon falls into a family of elements that are neither conductor nor insulator they re called semiconductors. This type of element will be either a conductor or an insulator depending on some condition. In the case of transistors, an electrical current will cause the silicon to be a conductor. 

Quantum films are usually produced in a periodic sequence of sandwich type of two semiconductors, one with a small, the other with a large bandgap, as illustrated in Fig. 9.7a. They can be produced with much higher precision than quantum dots of a narrow... 

Eq. (4) shows the dependence of the tunneling probability, F(E), on the parameters Fj, A s, the distance between two electrodes d and frequency m, which are characterized the interface and the type of a semiconductor sandwiched between two superconductor electrodes. However, the current density, /], for the transmitted electrons through the barrier will be obtained after substituting eq. (4) into eq.(l) and performing the integration, we get ... 

Many solar cells contain two types of semiconductors sandwiched together. These types are called and types. 

An intrinsic semiconductor is characterized by a small band gap and a low density of highly mobile intrinsic charge carriers. Electrons as well as holes contribute to the conductivity which increases with temperature. Phthalo-cyanine radicals such as the sandwich type PC2LU or PcLi carry intrinsic charges. Their facile oxidation and reduction suggests that intrinsic conductivity should be possible. The electrical properties of these materials, especially as thin films incorporated in various devices, have been studied [32]. 

Four different types of junctions can be used to separate the charge carriers in solar cebs (/) a homojunction joins semiconductor materials of the same substance, eg, the homojunction of a p—n sibcon solar ceb separates two oppositely doped layers of sibcon 2) a heterojunction is formed between two dissimbar semiconductor substances, eg, copper sulfide, Cu S, and cadmium sulfide, CdS, in Cu S—CdS solar cebs (J) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal—insulator—semiconductor junction (MIS), a thin insulator layer, generaby less than 0.003-p.m thick, is sandwiched between a metal and semiconductor material. 

Fig. 5 Molecular structures of sandwich bis(phthalocyaninato) rare earth double-decker complexes used as semiconductors of p-type OFET...

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

Solid state detectors consist of three layers, a layer of pure silicon sandwiched between a p-type and an n-type conductor. We recall that an example of an n-type conductor is germanium to which is added P or As, an impurity. The extra electron in the phosphorus or arsenic atoms is thought of as being in an energy level close to the conduction band. These electrons are readily thermally excited into the conduction band increasing the conductivity. A p-type semiconductor may be silicon to which a trivalent element such as boron or aluminum is added as an impurity. This creates holes close to the valence band. Electrons are readily promoted to these holes leaving positive holes in the valence band that provide for a conduction pathway. 

This behavior becomes useful when a third bit of semiconductor material is added, making a pnp or npn sandwich. Current cannot flow in either direction in a pnp or npn sandwich—unless some other source injects or pulls out electrons from the middle layer of the sandwich, making ppp- or nnn-type materials, which can conduct. 

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