Semiconductors donor type

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. 

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. 

Figure 6.1 Schematic band structures of solids (a) insulator (kT ,) (b) intrinsic semiconductor (kT ,) (c) and (d) extrinsic semiconductors donor and acceptor levels in n-type and p-type semiconductors respectively are shown, (e) compensated semiconductor (f) metal (g) semimetal top of the valence band lies above the bottom of the conduction band.

For the case of semiconductors doped with donors ( -type) and acceptors (p-type), three regions can be distinguished. In the low-temperature extrinsic region kT Eg), the conductivity is given by... 

A bulk heteroj unction is by definition a blend of p-type and n-type semiconductors (donor/acceptor). As a prototype bulk heteroj unction, we shall discuss the properties of polymer/fullerene blends. Apart from the poly-... 

Define the following terms conductor, insulator, semiconducting elements, donor impurities, acceptor impurities, w-type semiconductors, p-type semiconductors. 

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... 

Radha and Swamy (278) proposed a possible mechanism for the dehydrogenation of 2-propanol over La2MnM06 (M = Co, Ni, Cu). These authors found that admission of H2, together with the alcohol, does not have any influence on the reaction rate however, admission of acetone with 2-propanol decreases the reaction rate at all partial pressures. It can be inferred that H2 acts as a mere diluent whereas acetone has an inhibiting effect that may be due to its slow desorption. They also measured the conductivity changes of the catalyst in the presence of the reactants or products of the dehydrogenation. As a result of these studies it was concluded that the catalyst surface is covered predominantly with acetone under reaction conditions. Because acetone adsorbs by a donor-type mechanism, as shown by the decrease of the conductivity on its adsorption, its desorption involving electron transfer from the p-type semiconductor catalyst to the adsorbed species can be expected to be the slow process. 

Figure 4.1. Semiconductor with surface states of acceptor-type (a, b) or donor-type (c, d)...

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 ... 

Figure 4.6 shows the energy diagrams of p-type semiconductor in the presence of donor-type molecules. 

Within the framework of the boundary layer theory, Aigrain and Dugas" solved the Poisson equation for an n-type semiconductor with fully-ionized donor-type impurities, the boundary conditions being ... 

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] ... 

With the development of organic semicondnctors which support electron or hole transport (in analogy of p- and n-type inorganic semiconductors) bilayer type p-n heterojunction devices have been constructed. In the case when organic p- and n-type materials are deposited consecutively in two layers we get lateral heterojunction devices which mimic classical p-n junction junction solar cells based on silicon. The first such cell was designed by Tang and pnb-lished in 1986 (Fignre 3)." Copper phthalocyanine was utilized as electron donor (p-type) material, whereas pery-lene derivative was nsed as electron acceptor connterpart (n-type material). 

A structural correlation of the AR rectifier to a normal silicon junction diode shows that, acceptor part of the molecule can be mimicked with p-type semiconductor, donor part can be mimicked with n-type semiconductor and o-bond can mimic the pn-j unction barrier. With these favorable structural features of an organic molecule, it can be expected to result in similar characteristics like that of a semiconductor rectifier. 

If donor-type surface states are located below the Fermi level of an n-type doped semiconductor, they do not cause band bending as shown in Figure 9.53a. The same holds for acceptor-type surface states located above the bottom of the conduction band as shown in Figure 9.53b. 

Figure 13.43 Surface electronic states and induced band bending of an n-doped semiconductor. (a) Filled surface states (donor type) are found below the valence band maximum, empty surface states (acceptor type) above the conduction band minimum. In this case no charge transfer between surface and bulk states occurs, corresponding to flat-band condition (i.e. no band bending), (b)...

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

Fig. 1. Band-edge energy diagram where the energy of electrons is higher in the conduction band than in the valence band (a) an undoped semiconductor having a thermally excited carrier (b) n-ty e doped semiconductor having shallow donors and (c) a -type doped semiconductor having shallow acceptors.

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. 

For lightly doped n-type semiconductors at normal operating temperatures there is complete donor dissociation (donor saturation). 

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