Dopant acceptor-type

Extrinsic semiconductors ate those in which the carrier concentration, either holes or electrons, are controlled by intentionally added impurities called dopants. The dopants are termed shallow impurities because their energy levels lie within the band gap close to one or other of the bands. Because of thermal excitation, -type dopants (donors) are able to donate electrons to the conduction band and p-type dopants (acceptors) can accept electrons from the valence band, the result of which is equivalent to the introduction of holes in the valence band. Band gap widening/narrowingmay occur if the doping changes the band dispersion. At low temperamres, a special type of electrical transport known as impurity conduction proceeds. This topic is discussed in Section 7.3. 

Moderate doping with rare earth cations such as or 80 + stabilizes cubic zirconia at relatively low temperatures and increases conductivity, the maximum of this quantity being reached when the concentration of acceptor-type dopants is close to the minimum necessary for stabilizing the cubic phase (Kharton et al., 2004). For YSZ,... 

The maximum ionic conductivity in ZrO2-based systems is observed when the content of acceptor-type dopant cations with the smallest radii (Sc, Yb, Y) is close to the minimum necessary to completely stabilize the cubic fiuorite-type phase in the operating temperature range [9,11,16, 32-35]. This concentration (often referred to as the low stabilization limit) and the conductivity of the ceramic electrolytes are dependent, to a finite extent, on the pre-history and various micro structural features. In addition to the metastable states discussed above, critical microstructural factors... 

In the simplest case, a p-n junction is formed between two layers of the same semiconductor with different types of dopants, creating a p-n homojunction. One semiconductor contains dopants that are acceptor-type (p-type), making it a semiconductor with a Fermi level lower than the intrinsic, or undoped, semiconductor. The other contains donor-type (n-type) dopants, making it a semiconductor with a Fermi level higher than that of the intrinsic material. When the n-type and p-type semiconductor layers are brought into contact, there is a discontinuity in the Fermi level at the interface between the two materials, and charge will flow across this interface until... 

Table 2.5 The photoconduction performance of poly(methyl phenyl silylene) containing electron-acceptor-type dopants [55].

Co-doping can be easily achieved by having a small excess of Bi over V during the synthesis of the material, so that [Bi] -1- [W] = [Vj. This is a well known and powerful trick to increase the solubility of binary oxides in ternary oxides in order to incorporate donor- or acceptor-type dopants. 

The maximum ionic conductivity in Zr02-based systems is achieved when the concentration of acceptor-type dopant(s) is cJose to the minimum necessary for complete stabilization of the cubic fluorite structure [227, 228]. For example, the highest conductivity in Zri, Y 02 x/2 and Zri, SC < 02 x/2 ceramics is achieved at x = 0.08-0.11 and 0.09-0.11, respectively. Further adchtions decrease the ionic conductivity due to an increasing association of the oxygen vacancies and dopant cations into complex defects of low mobUity [229]. This effect is more pronounced... 

Loop annealing was used to study self diffusion for a wide range of dopant contents. It was found that the diffusion coefficient decreased as the n-type dopant content was decreased or the p-type dopant concentration was increased. At a given temperature, the diffusion coefficient depended linearly upon the electron concentration. This behavior was in agreement with an acceptor-type behavior of the point defects which were responsible for self diffusion. At 970 to 1070C, the results could be described by ... 

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

When a sibcon crystal is doped with atoms of elements having a valence of less than four, eg, boron or gallium (valence = 3), only three of the four covalent bonds of the adjacent sibcon atoms are occupied. The vacancy at an unoccupied covalent bond constitutes a hole. Dopants that contribute holes, which in turn act like positive charge carriers, are acceptor dopants and the resulting crystal is -type (positive) sibcon (Fig. Id). 

Mg2+ ion. 49 has been used to deposit MgO by atomic layer epitaxy,222 and is commonly employed as a />-type dopant for semiconductors, particularly GaAs,223 GaN,224,225 and AlGaN.226 In GaN, Mg doping induces a blue 2.8 eV photoluminescence band arising from donor-acceptor (D-A) pair recombination.227 It is likely that isolated Mg... 

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