Semiconductors density of states

FIGURE 12.13 Gerischer-type diagram for interfacial electron transfer. The rate constants for interfacial electron transfer are dependent on the overlap of the sensitizer and the semiconductor density of states. Note that the density of states of the semiconductor is not a singular parameter and can shift with a change in environment, that is, pH, ionic strength, solvent, and so on. 

Paasch G, Scheinert S (2007) Space charge layers in organic field-effect transistors with Gaussian or exponential semiconductor density of states. J Appl Phys 101 024514 Sze SM (1981) Physics of semiconductor devices, 2nd edn. WHey, New York Lin Y-Y, Gundlach DJ, Nelson SF, Jackson ThN (1997) Pentacene-based organic thin-film transistors. IEEE Trans Election Dev 44 1325-1331... 

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

In an intrinsic semiconductor, charge conservation gives n = p = where is the intrinsic carrier concentration as shown in Table 1. Ai, and are the effective densities of states per unit volume for the conduction and valence bands. In terms of these densities of states, n andp are given in equations 4 and... 

Why is the d-band of a metal narrower at the surface than in the interior Draw a simple version of the density of states for the electron bands of a metal (a good conductor), a semiconductor and a perfect insulator. 

B a proportionality constant, h Planck s constant, and co the frequency. Tauc has argued that the density of states near the band edges has a square-root dependence on energy, as is the case for crystalline semiconductors [75]. This results in p = = 1/2. Thus, extrapolating... 

Figure 4.3 Density of states for (a) metal and (b) semiconductor nanocrystals. The HOMO-LUMO gap increases in semiconductor nanocrystals of smaller sizes. Adapted from [19], reproduced with permission from Wiley-VCH Verlag GmbH.

If the tunneling current is from the surface to the tip, the STM images the density of occupied states. If the potential is reversed, the current flows in the other direction, and one images the unoccupied density of states, as the reader can easily understand from Fig. 7.19. This figure also illustrates a necessary condition for STM there must be levels within an energy e-V from the Fermi level on both sides of the tunneling gap, from and to which electrons can tunnel In metals, such levels are practically always available, but when dealing with semiconductors or with adsorbed molecules, this condition may be a limitation. A second condition is that the sample possesses conductivity perfect electrical insulators cannot be measured with STM. 

STM has also been shown to provide surface electronic information (1). The most common application to date in this area is use of STM to probe the density of states in metals and semiconductors. Materials of interest to electrochemists that have been investigated include Si (17,48), GaAs (16,49), graphite (18),... 

Fig. 1 Schematic drawing to show the concept of system dimensionality (a) bulk semiconductors, 3D (b) thin film, layer structure, quantum well, 2D (c) linear chain structure, quantum wire, ID (d) cluster, colloid, nanocrystal, quantum dot, OD. In the bottom, it is shown the corresponding density of states [A( )] versus energy (E) diagram (for ideal cases).

Fig. 19 The effect of doping on the density of states distribution in a disordered organic semiconductor at variable concentration of charged dopants. The energy scale is normalized to the width of the DOS, expressed through a, of the undoped sample. The parameters are the intrinsic site concentration V and the dopant concentration N. From [125] with permission. Copyright (2005) by the American Institute of Physics...

Arkhipov VI, Heremans P, Emelianova EV, Adriaenssens GJ, Bassler H (2002) Weak-field carrier hopping in disordered organic semiconductors the effects of deep traps and partly filled density-of-states distribution. J Phys Condens Matter 14 9899... 

The STM images of large superstructures on metal surfaces exhibit a very simple form. As shown first time by Tersoff and Hamann (1983, 1985), at the low-bias limit, the STM images of large superstructures on metal surfaces are independent of tip electronic states, and an STM image is simply a contour of an important quantity of the sample surface only the Fermi-level local density of states (LDOS), taken at the center of curvature of the tip. An attempt was also made to interpret the observed atom-resolved images of semiconductors... 

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