Acceptor-donor type impurities

In case of anisotropic materials, the conductivity is also anisotropic, such as the dielectric and diamagnetic coefficients however, its anisotropy is related to the dopants, and usually <7n I a n / e . For acceptor-donor type impurities, typically t7 /t7i 1.05-1.3 (increasing slightly with concentration), whereas for ionic impurities the anisotropy is nearly 2. 

Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

If we suppose that there is, in the semiconductor, an acceptor-type impurity A as well as a donor-type impurity D, that their concentrations at charged state are respectively N and N+, and that the charge density p(x) is written ... 

For a semiconductor containing both a singly-ionized donor-type impurity D, of concentration Nd, as well as a singly-ionized acceptor-type impurity A, of concentration Na, the density p(V) can be expressed using relation [4.7] ... 

Diffusion of the neutral impurity into silicon in the presence or absence of acceptor-or donor-type impurities, such as B or P, was investigated using neutron-activation analysis. The diffusivity of Sn alone in near-intrinsic material was described by ... 

By varying the impurity concentration in the semiconductor, one may regulate not only the activity of the catalyst but its selectivity as well. Indeed, if the reaction proceeds along two parallel paths, one of which is of the acceptor type and the other of the donor type, then upon the monotonic displacement of the Fermi level (i.e., upon the monotonic change of Z) the reaction will be accelerated on one path and retarded on the other, as appears, e.g., from a comparison of Figs. 19a and 19b. Doping of the crystal may accelerate the reaction on one path and retard it on the other. 

For every atom of n-type or p-type impurity, an electron or hole is located at the donor or acceptor state, respectively. The material is still neutral, but when conductivity appears. 

Lithium is an n-type impurity (donor atom) with high mobility in silicon (and germanium see next section). When the diffusion begins, the acceptor concentration iNp) is constant throughout the silicon crystal (Fig. 7.22a), while the donor concentration iN ) is high on the surface and zero everywhere else. As the diffusion proceeds, the donor concentration changes with depth, as shown in Fig. 7.22a. At the depth Xj where... 

When a potential is applied over such a crystal, with the positive terminal at the high lithium side, three volumes are created, one of p-type, a middle "intrinsic" one, and an n-type one (p-i-n detectors). In the intrinsic volume the lithium donor electrons neutralize any original impurities, which are of acceptor p-type. The intrinsic volume becomes depleted and thereby sensitive to nuclear radiation, and detectors with depleted volumes up to more than 100 cm are commercially available. Figure 8.14 shows the arrangement of the Dewar vessel with liquid N2, cold-finger, detector, and preamplifier. 

The properties of perovskite materials are heavily dominated by their oxygen content, as weU as by donor- and acceptor-type impurities. An essential contribution to the knowledge of the structural and electronic properties of point defects in these materials comes from theoretical approaches. The results of large-scale computer semiempirical and first-principles modeling of point defects, polarons and perovskite sohd solutions can be found in [722], focusing mostly on KNbOa and KTaOa. 

The creation of electrons or holes in polymers can be achieved by introducing electron-acceptor or electron-donor materials into the polymer matrix to form charge-transfer complexes. The electrical properties can be controlled in a way similar to that for doped semiconductors with n- or p-type impurities. 

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. 

Extrinsic Semiconductors are materials that contain donor or acceptor species (called doping substances) that provide electrons to the conduction band or holes to the valence band. If donor impurities (donating electrons) are present in minerals, the conduction is mainly by way of electrons, and the material is called an n-type semiconductor. If acceptors are the major impurities present, conduction is mainly by way of holes and the material is called a p-type semiconductor. For instance in a silicon semiconductor elements from a vertical row to the right of Si of... 

As shown in Figs. 2-17 and 2-18, semiconductors containing impurities are classified into the following two types n-type semiconductors with localized donor levels close to the conduction band, and p-type semiconductors with localized acceptor levels close to the valence band in the band gap. The liberation of electrons from the donor levels into the conduction band and the liberation of holes from the acceptor levels into the valence band are represented by the ionization processes of donor D and acceptor A, respectively, as shown in Eqns. 2-16 and 2-17 ... 

Two types of impurities should be distinguished namely, acceptor and donor impurities which play the part of traps (i.e., localization centers) for the free electrons and the free holes, respectively. It should be especially stressed that foreign particles dissolved in the crystal may act as acceptors or donors depending not only on their nature, but also on whether they enter the lattice (interstitially or substitutionally). For example, the interstitial Li atoms in the NiO lattice are donors, but the same Li atoms when replacing the Ni atoms act as acceptors. In the case of a substitutional solution, the foreign atoms of a given type may be either acceptors or donors depending on the lattice in which they are dissolved. For example, Ga atoms are donors in the ZnO lattice and acceptors in the Ge lattice. Thus, if the adsorbed particles are, say, acceptors, the same particles when dissolved in the volume of the crystal may act as donors, and vice versa. 

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