Rectifiers Schottky barrier

Both ohmic and rectifying behavior are possible, depending on the sign of Unlike the p—n junction the current in a rectifying Schottky barrier... 

Finally, surface analysis has been used in the investigation of metal silicides used to form rectifying Schottky barrier contacts to semiconductors. These silicides are formed by thermal or laser sintering of the metal after deposition onto the substrate. Excess unreacted metal is removed by chemical etching. XPS has been used to show that the metal has been oxidized if the excess metal cannot be removed (52). 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

Inorganic Schottky barrier rectifiers are commercially available. Of course, for a monolayer, one cannot speak of band bending as in a bulk semiconductor, but the ideas of a barrier, and of dipoles across it, are valid. 

Between 1982 and 1997, one of us (Metzger) studied many D-cr-A molecules as potential rectifiers, but could not measure their IV properties reliably [11, 12, 100]. Due to difficulties in interpreting how electron transport occurs between adjacent layers in a multilayer, Metzger decided to focus on monolayers, and to avoid difficulties with asymmetric Schottky barriers, decided to use the same metal on both sides of the monolayer (first A1 for 36a, later Au). 

Hetero junctions, forming a Schottky barrier like a metal-semiconductor junction, normally change the energy levels of conduction and valence bands. When the Fermi level of the semiconductor equilibrates with the energy level of the redox couple in the solution, the electric energy level at the surface is pinned and a depletion layer is formed. This is postulated since the rectified current can be observed at semiconductor plate electrodes. The bending of the band in the semiconductor at the surface can be described as a solution of the one-dimensional Poisson-Boltzmann equation... 

Tetraphenylporphine (TPP) and other metal porphyrine derivatives coated on platinum (87,88,89) or gold (89,90) electrodes have been investigated in photoelectrochemical modes. Photocurrents reported are cathodic or anodic, depending on the pH as well as the composition of the electrolyte employed. Photocurrent quantum efficiencies of 2% (89) to 7% (87) were reported in systems using water itself or methylviologen as the redox species in aqueous electrolyte. Photocurrent generation at Zn-TPP-coated metal cathodes (89) was interpreted in terms of a rectifying effect of the Schottky barrier formed at a metal-p-type... 

The application we have in mind for the metal-polymer interfaces discussed in this book is primarily that where the polymer serves as the electroactive material (semiconductor) in an electronic device and the metal is the electric contact to the device. Metal-semiconductor interfaces, in general, have been the subject of intensive studies since the pioneering work of Schottky, Stromer and Waibel1, who were the first to explain the mechanisms behind the rectifying behaviour in this type of asymmetric electric contact. Today, there still occur developments in the understanding of the basic physics of the barrier formation at the interface, and a complete understanding of all the factors that determine the height of the (Schottky) barrier is still ahead of us2. 

Walter Haus Schottky (1886-1976) received his doctorate in physics under Max Planck from the Humboldt University in Berlin in 1912. Although his thesis was on the special theory of relativity, Schottky spent his life s work in the area of semiconductor physics. He alternated between industrial and academic positions in Germany for several years. He was with Siemens AG until 1919 and the University of Wurzburg from 1920 to 1923. From 1923 to 1927, Schottky was professor of theoretical physics at the University of Rostock. He rejoined Siemens in 1927, where he finished out his career. Schottky s inventions include the ribbon microphone, the superheterodyne radio receiver, and the tetrode vacuum tube. In 1929, he published Thermodynamik, a book on the thermodynamics of solids. Schottky and Wagner studied the statistical thermodynamics of point defect formation. The cation/anion vacancy pair in ionic solids is named the Schottky defect. In 1938, he produced a barrier layer theory to explain the rectifying behavior of metal-semiconductor contacts. Metal-semiconductor diodes are now called Schottky barrier diodes. 

The charge carrier depletion width, W, at the rectifying contact, which forms a Schottky barrier, can be calculated using the following Eq. (99) [47] ... 

Schottky barrier diodes - a diode consisting of a metal-semiconductor contact which has rectifying characteristics similar to a p-n junction differs from a p-n junction diode in that the diode s forward voltage is different (lower for commonly used materials), and there is no charge stored when the diode is forward biased device can therefore be turned off very rapidly by application of reverse bias, as storage time is negligible. 

Rectification the asymmetric current-voltage characteristics of a semiconductor junction, in which charge-carrier flow is impeded in one direction, but not in the other direction Reverse bias the sign of the applied potential opposite to forward bias. In reverse bias, the current through a diode is essentially independent of the applied potential Schottky barrier a rectifying semiconductor/metal junction Surface states energy levels arising from atoms at the semiconductor surface... 

The rectification properties of semiconductor interfaces are the most important electrical characteristic of semiconductor contacts. Certain types of devices, such as transistors, require both ohmic and rectifying contacts on a given semiconductor surface, whereas other devices, such as Schottky barriers, are based on the inherent rectification properties of semiconductor/metal Junctions. Numerous photonic devices, such as photon detectors and photovoltaic cells, require rectification at a semiconductor Junction, and light-emitting diodes require both ohmic contacts and rectifying Junctions in a well-defined geometry. Thus, successful fabrication of a desired device structure depends entirely on the electrical properties of the specific semiconductor contacts that are formed in the process. The principles described above allow the rational fabrication of contacts with the desired properties, and also describe the operation of the resulting devices within a simple, chemically intuitive, kinetic framework. 

Schottky barrier - A potential barrier associated with a metal-semiconductor contact. It forms the basis for the rectifying device known as the Schottly diode. 

Because the Schottky barrier on crystalline Si is not understood in detail, the understanding of the barrier on a-Si H is at a very fundamental level. Most progress has been made by empirical studies, which used techniques well tested on studies of crystalline Si. What has emerged from this is a rudimentary understanding that allows crude applications of rectifying and ohmic contacts to experimental and device configurations. 

Schottky barriers are metal-semiconductor junctions that have the ability to rectify current, because the work fimction of the metal is greater than that of the semiconductor. The junction thus creates a barrier between the semiconductor and the metal that decreases when the junction is forward biased and vice versa. Conduction in Schottky devices is by majority carriers, principally electrons. In conventional p-n devices reverse conduction is predominately via minority carriers. In p- -junction devices, charge is stored in the junction during forward conduction and has to be removed if the jimction is reverse biased before the diode can switch off. The junction capacitance and the capacitive reactance are voltage dependent. 

The main advantage of poly-p-phenylene is that, due to its nonacetylenic composition, it has a much higher thermal stability (450°C in air and 550°C in inert atmosphere). Potential applications of poly-p-phenylene are similar to those envisaged for polyacetylene, such as Schottky barriers in photocells. (A Schottky barrier is a metal semiconductor contact that has rectifying characteristics similar to a p-n junction.)... 

Milestone Four took place when Mark A. Weed developed a "nanopore" technique. Into a thinned-out region of a Si wafer, Au was evaporated to form a 50-nm-wide, few-nm-deep "bowl" of Au II, onto which thiols were self-assembled (Fig. 5). The nanopore was then covered with Ti (and its oxide), then Au. Thus, sandwiches Au thiolate Ti/Ti02 Au could be studied, with maybe a thousand SAM molecules of area 2 nm each, in parallel. A SAM of 4-ethynyl-phenyl-4 -ethynylphenyl-benzene-l-thiolate was a Schottky barrier rectifier." In contrast, a SAM of 2 -amino-4-ethynylphenyl-4 -ethy nylphenyl-5 -nitro-... 

There are three distinct processes for asymmetrical conduction, that is, rectification, in MOM assemblies S, A, and U. S rectifiers are due to Schottky barriers at the metal organic interface(s)." " ... 

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