Metal p-type semiconductor

The foregoing discussion has shown that, in case of an n-type semiconductor, a metal-semiconductor contact is rectifying if > Oj and is nonrectifying if < Oj. The opposite is true for a metal p-type semiconductor contact. The work functions of various metals and semiconductors are given in Table 3.1. ... 

Combining the two expressions for the metal/n-type and metal/p-type semiconductor contacts, we obtain... 

Figure 3. Energy-band diagram of a gold-protein (metal-p-type semiconductor contact)Ec conduction band Ey. valence band Ef Fermi level Ea Acceptors level E ac- Vacuum level work function of...

Figure8.2 A metal p-type semiconductor pair before (a) and after (b) contact with no surface/interface states. The metal work function is greater than that for the semiconductor (( )s>< ), ).

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

Eq. (14.1) is known as the Mott-Schotlky equation. We note llial for a given n-lype semiconductor, the harrier height increases as the work function of the metal increases. It is therefore expected that high work function metals will give a rectifying junction, and low work function metals an ohmic contact (it is the reverse for a p-type semiconductor). 

Figure 10-53 shows band-gap diagrams of n-type and p-type semiconductors. Electrical current flows in a doped semiconductor in the same way as current flows in a metal (see Figure 10-501. Only a small energy difference exists between the top of the filled band and the next available orbital, so the slightest applied potential tilts the bands enough to allow electrons to move and current to flow.

The change in the electronic properties of Ru particles upon modification with Se was investigated recently by electrochemical nuclear magnetic resonance (EC-NMR) and XPS [28]. In this work, it was established for the first time that Se, which is a p-type semiconductor in elemental form, becomes metallic when interacting with Ru, due to charge transfer from Ru to Se. On the basis of this and previous results, the authors emphasized that the combination of two or more elements to induce electronic alterations on a major catalytic component, as exemplified by Se addition on Ru, is quite a promising method to design stable and potent fuel cell electrocatalysts. 

In order to have more infoimation on the nature of the oxygen species active in partial and total oxidation we investigated the interaction of the hydrocarbons with the pre-oxidized surfaces of oxides where different types of surface oxygen species are formed. In particular we investigated p-type semiconductors like chromia, chromites and cobalt oxide C03O4. Moreover, we studied n-type metal oxides like FezOs, metal ferrites and CuObased catalysts. 

Fig. 3.2 Fermi level of isolated phases, metal, w-type semiconductor and p-type semiconductor, in a vacuum...

Although the role of rare earth ions on the surface of TiC>2 or close to them is important from the point of electron exchange, still more important is the number of f-electrons present in the valence shell of a particular rare earth. As in case of transition metal doped semiconductor catalysts, which produce n-type WO3 semiconductor [133] or p-type NiO semiconductor [134] catalysts and affect the overall kinetics of the reaction, the rare earth ions with just less than half filled (f5 6) shell produce p-type semiconductor catalysts and with slightly more than half filled electronic configuration (f8 10) would act as n-type of semiconductor catalyst. Since the half filled (f7) state is most stable, ions with f5 6 electrons would accept electrons from the surface of TiC>2 and get reduced and rare earth ions with f8-9 electrons would tend to lose electrons to go to stabler electronic configuration of f7. The tendency of rare earths with f1 3 electrons would be to lose electrons and thus behave as n-type of semiconductor catalyst to attain completely vacant f°- shell state [135]. The valence electrons of rare earths are rather embedded deep into their inner shells (n-2), hence not available easily for chemical reactions, but the cavitational energy of ultrasound activates them to participate in the chemical reactions, therefore some of the unknown oxidation states (as Dy+4) may also be seen [136,137]. 

Thus, although the potential required for polarization would be much larger at n-type semiconductors than at illuminated p-type semiconductors and despite the fact that not all n-type semiconductors can be used because of corrosion (or reduction) of semiconductor materials themselves, the use of n-type semiconductors to examine C02 reduction seems to be indicated because the cathodic current is much larger (the electron is the major carrier for n-type semiconductors), approaching that of metal electrodes, compared to the photocurrent obtained at illuminated p-type semiconductors,... 

Nakato, Y., Tonomura, S., and Tsubomura, H., The catalytic effect of electrodeposited metals on the photo-reduction of water at p-type semiconductors, Ber. Bunsen., Phys. Chem., 80, 1289, 1976. 

For Ere(jox sufficiently negative for n-, or sufficiently positive for p-type, semiconductors the electrode behaves as a metallic electrode, not blocking the flow of electrons in either direction. This situation is analogous to the criteria for forming an ohmic contact to an n- or p-type semiconductor.(14)... 

The Schottky-Mott theory predicts a current / = (4 7t e m kB2/h3) T2 exp (—e A/kB 7) exp (e n V/kB T)— 1], where e is the electronic charge, m is the effective mass of the carrier, kB is Boltzmann s constant, T is the absolute temperature, n is a filling factor, A is the Schottky barrier height (see Fig. 1), and V is the applied voltage [31]. In Schottky-Mott theory, A should be the difference between the Fermi level of the metal and the conduction band minimum (for an n-type semiconductor-to-metal interface) or the valence band maximum (for a p-type semiconductor-metal interface) [32, 33]. Certain experimentally observed variations of A were for decades ascribed to pinning of states, but can now be attributed to local inhomogeneities of the interface, so the Schottky-Mott theory is secure. The opposite of a Schottky barrier is an ohmic contact, where there is only an added electrical resistance at the junction, typically between two metals. 

If the holes are able to gain enough energy to move from a cation when illuminated, the materials are photoconducting. Thermal energy may also be able to liberate the holes and the solids are p-type semiconductors. The transition-metal monoxides NiO and CoO represent this behavior (Sections 1.11.4 and 4.3.2). 

The high-pressure region is associated with the electroneutrality equation [h ] = 2[V ]. Holes predominate, so that the material is a p-type semiconductor in this regime. In addition, the conductivity will increase as the g power of the partial pressure of the gaseous X2 component increases. The number of metal vacancies (and nonmetal excess) will increase as the partial pressure of the gaseous X2 component increases and the phase will be distinctly nonstoichiometric. There is a high concentration of cation vacancies that would be expected to enhance cation diffusion. 

A cation vacancy will be opposite to this in behavior. Removal of a neutral metal atom from a material will involve removal of a cation plus the correct number of electrons, which are taken from the valence band. Cation vacancies will therefore be represented as acceptor levels situated near to the valence band together with an equivalent number of holes in the band. These materials are p-type semiconductors. 

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

But even in a homogeneously doped material an etch stop layer can be generated by an inhomogeneous charge carrier distribution. If a positive bias is applied to the metal electrode of an MOS structure, an inversion layer is formed in the p-type semiconductor. The inversion layer passivates in alkaline solutions if it is kept at the PP using a second bias [Sm5], as shown in Fig. 4.16b. This method is used to reduce the thickness variations of SOI wafers [Og2]. Illuminated regions... 

Fig. 10-lS. Ehietgy levels and polarization curves of cathodic hydrogen reaction at a metal electrode and at a photoexdted p-type semiconductor electrode = cathodic current ... 

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