N-region

AE at the n-p heterointerface which prevents their injection into the n region. The result is that the injected minority carriers are confined to the thin narrow-bandgap region. If this region is thinner than the average diffusion... 

A more effective carrier confinement is offered by a double heterostmcture in which a thin layer of a low band gap material (the active layer) is sandwiched between larger band gap layers. The physical junction between two materials of different band gaps, and chemical compositions, is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in Figure 4. Electrons injected under forward bias across the p—N junction into the lower band gap material encounter a potential barrier, AE at thep—P junction which inhibits their motion away from the junction. The holes see a potential barrier of AE at the N—p heterointerface which prevents their injection into the N region. The result is that the injected minority... 

FIG. 22 (a) Log-log plot of the scaling function vs C = upper curves refer to the mean-square end-to-end distance, R, while the lower ones show results for i g. (b) The same vs scaling variable N "Region I— free chains, II—crossover, and III—renormaUzed free chains. The slope 0.6 corresponds to l cross = 0.78. [20]... 

Longer ice-core records show that methane concentrations have varied on a variety of time scales over the past 220 000 years (Fig. 18-15) Qouzel et al, 1993 Brook et al, 1996). Wetlands in tropical (30° S to 30° N) and boreal (50° N to 70° N) regions are the dominant natural methane source. As a result, ice-core records for preanthropogenic times have been interpreted as records of changes in methane emissions from wetlands. Studies of modem wetlands indicate that methane emissions are positively correlated with temperature, precipitation, and net ecosystem productivity (Schlesinger, 1996). 

The photovoltage is esentially determined by the ratio of the photo- and saturation current. Since io oomrs as a pre-exponential factor in Eq. 1 it determines also the dark current. Actually this is the main reason that it limits the photovoltage via Eq. 2, The value of io depends on the mechanism of charge transfer at the interface under forward bias and is normally different for a pn-junction and a metal-semiconductor contact. In the first case electrons are injected into the p-region and holes into the n-region. These minority carriers recombine somewhere in the bulk as illustrated in Fig. 1 c. In such a minority carrier device the forward current is essentially determined... 

In the FETs [Fig. 2.16(b) and (c)], however, the p region is relatively broad in comparison with the n regions moreover, the p surface has been completely covered by an insulating layer preferably of Si02, so that the transistor steering can only be achieved by a field effect. In FET (b) the insulating layer is in touch with a metal plate, the so-called "gate , and the area between both n... 

Figure 6.14. The n-n region of the ultraviolet absorption spectrum of (a) compound (7), n = 1 (solid line) and (b) equal molar mixture of acetophenone and trans-/3-methylstyrene.<89) Reprinted by permission of the American Chemical Society.

Fig. 1. Interstellar 3He/H abundances as a function of source metallicity [2], The [3He/H] abundances by number derived for the H n region sample are given with respect to the solar ratio. Also shown is the abundance derived for the planetary nebula NGC3242 (triangle). We note that there is no trend in the 3He/H abundance with source metallicity...

Hu region abundances in gas-rich dwarfs. Richer, McCall, Stasinska (1998) compared dlrr H n region O abundances with O abundances of planetary nebulae (PNe) in dSphs. While the offset persisted, PNe have only been detected in the two most luminous dSphs and trace primarily intermediate-age populations as opposed to the present-day abundances in Hu regions. 

Fig. 3. The 3He abundances (by number relative to hydrogen), j/3 = 10B(3He/H), derived from Galactic H n regions [4], as a function of galactocentric distance (filled circles). Also shown for comparison is the solar system abundance (solar symbol). The open circles are the oxygen abundances for the same H n regions (and for the Sun).

The Aviram-Ratner D-ct-A molecule is analogous to a pn junction rectifier the electron-rich donor region D would be similar to the electron-rich semiconducting n region, while the electron-poor A region would be similar to a semiconductor s p region [79]. However, note that under forward bias the preferred direction of Aviram-Ratner electron flow is from A to D, while in a pn junction rectifier the preferred direction is from n to p. 

Fig. 3.22. Spectrum of an extragalactic H n region with low abundances, SBS 0335-0520 from the Second Byurakan Survey. Adapted from Melnick et al. (1992).

Fig. 3.25. Trends of nebular line strengths in H n regions with oxygen abundance. This figure shows oxygen abundance in H n regions of the Milky Way and spiral and irregular galaxies (determined using measured electron temperatures) vs. log R23, after Pilyugin (2003) the p parameter is the line ratio [O iii]/([0 11] + [O hi]).

Supernova remnants (SNR) in early stages of expansion display results of advanced (explosive) nucleosynthesis, but in later stages light up the interstellar medium by shock excitation and give information about the ISM in external galaxies complementary to that derived from H n regions. 

Fig. 3.40. Abundances in Galactic stars, H n regions and planetary nebulae, as a function of Galactocentric distance, with the Sun shown for comparison. After Hou, Prantzos and Boissier (2000). The curves show a model calculation by the authors nitrogen is underproduced in the model because only massive stars were considered. 

Type II, moderately C- and N-rich, belonging to an intermediate-age disk population like the Sun. This is the commonest class, and it shows Galactic abundance gradients compatible with those derived from stars and H n regions, although the N/O ratio is 2 or 3 times higher. 

Fig. 4.7. 3He/H in simple Galactic H n regions, i.e. those thought to be reasonably well represented by homogeneous spherical models (Balser et al. 1999), and one planetary nebula, as a function of their oxygen abundance. 3He/H is plotted on a logarithmic scale relative to the proto-solar value of 1.5 x 10-5. After Bania, Rood and Balser (2002). Reprinted by permission from Macmillan Publishers Ltd. Courtesy Tom Bania.

A Y/ A Z or AF/A(0/H) may vary, either systematically as a function of Z or randomly, e.g. if some H n regions are self-polluted with helium and other elements (e.g. N) ejected in winds from massive embedded stars. There is some evidence that this actually happens in a very few cases, such as the nucleus of NGC 5253, where the continuum shows a strong, broad feature at the He+ wavelength X 4686, due to Wolf-Rayet stars. 

Fig. 8.1. Ne/O ratio vs. O/H from (mainly optical) observations of H n regions in spiral and irregular galaxies and the Sun. Filled and open symbols represent results from different authors. After Garnett (2004).

Fig. 8.3. C/O ratio vs. O/H in Galactic and extragalactic H n regions, based on International Ultraviolet Explorer (IUE) and Hubble Space Telescope (HST) observations. After Garnett (2004).

Fig. 8.4. [C/O] vs. [O/H] in stars, after Akerman et at. (2004) and references therein, with an additional point representing a damped Lyman-a system at a high redshift. The broad trend is in fair agreement with the H n region results shown in Fig. 8.3 there is a hint of a possible upturn at the lowest metallicities. Courtesy Max Pettini.

Fig. 11.7. N/O vs. O/H in extragalactic H n regions and damped Lyman-a absorption line systems shown by triangles, with schematic primary and secondary tracks. Open triangles show objects where sulphur was used as a proxy for oxygen. The Sun is shown by the large filled circle. After Pettini et al. (2002a), updated to Dec 2003. Courtesy Max Pettini.

Fig. 11.8. N/O vs. O/H in stars and H n regions, from various sources, fitted with an analytical model, a is the coefficient in a Twarog-type inflow model with inflow rate = aaf. After Henry, Edmunds and Koppen (2000).

Fig. 11.9. N/0 vs. 0/H in stars and H n regions, from various sources. Curves A,...

Longer in length (26-58aa) twin-arginine motif in n-region also Sec-avoidance lysine in c-region not found at ER but found at chloroplasts... 

Forms opposite NexoCcyt orientation few or no charges in n-region longer h-region is favored than in type II anchor... 

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