MEM semiconductor

Figure 33. Dye (D) sensitized electrochemical photovoltaic cell based on a wide bandgap semiconductor. Reprinted from R. Memming, Semiconductor Electrochemistry, Wiley-VCH Verlag GmbH, Weinheim (2001). Copyright 2001 with permission from Wiley-VCH Verlag GmbH.

Reprinted from R. Memming, Semiconductor Electrochemistry, Wiley-VCH Verlag GmbH, Weinheim (2001). 

Bard, A. J., R. Memming, and B. Miller, Terminology in semiconductor electrochemistry and photoelectrochemical energy conversion, Pure Appl. Chem., 63, 569 (1991). 

Memming, R. Photoinduced Charge Transfer Processes at Semiconductor Electrodes and Particles. 169, 105-182 (1994). 

A wide variety of solid-state sensors based on hydrogen-specific palladium, metal oxide semiconductor (MOS), CB, electrochemical, and surface acoustic wave (SAW) technology are used in the industry for several years. Microelectromechanical systems (MEMS), and nanotechnology-based devices for the measurement of hydrogen are the recent developments. These developments are mainly driven by the demands of the fuel cell industry. Solid-state approaches are gaining rapid popularity within the industry due to their low cost, low maintenance, replacements, and flexibility of multiple installations with minimal labor. 

Prior to the 1970 s, electrochemical kinetic studies were largely directed towards faradaic reactions occurring at metal electrodes. While certain questions remain unanswered, a combination of theoretical and experimental studies has produced a relatively mature picture of electron transfer at the metal-solution interface f1-41. Recent interest in photoelectrochemical processes has extended the interest in electrochemical kinetics to semiconductor electrodes f5-151. Despite the pioneering work of Gerischer (11-141 and Memming (15), many aspects of electron transfer kinetics at the semiconductor-solution interface remain controversial or unexplained. 

The high selectivity of wet etchants for different materials, e.g. Al, Si, SiOz and Si3N4, is indispensable in semiconductor manufacturing today. The combination of photolithographic patterning and anisotropic as well as isotropic etching of silicon led to a multitude of applications in the fabrication of microelectromechanical systems (MEMS). 

Further, the thicknesses of the diffuse and space charge layers depend on the potentials Ma and i sc across the respective layers for the space charge layer the thickness, dgc, is expressed, to a first approximation, by dsc = 2Lox (eA /kT)- [Memming, 1983]. The Debye length, Ld, is about 100 nm in usual semiconductors with impurity concentrations in the order of 10 cm and is about 10 nm in dilute 0.01 M ionic solutions. 

In the same way as described in Sec. 5.2 for a diifiise layer in aqueous solution, the differential electric capacity, Csc, of a space charge layer of semiconductors can be derived from the Poisson s equation and the Fermi distribution function (or approximated by the Boltzmann distribution) to obtain Eqn. 5-69 for intrinsic semiconductor electrodes [(Serischer, 1961 Myamlin-Pleskov, 1967 Memming, 1983] ... 

The energy barrier of a depletion layer (the potential across a depletion layer I I) is called the Schottky barrier in semiconductor physics. Assuming that all the impurity donors or acceptors are ionized to form a fixed space charge in the depletion layer, we obtain the following approximate equation, Eqn. 5—75, for the thickness of depletion layer, dx, [Memming, 1983] ... 

Fig. 5-64. Band edge levels of compound semiconductor electrodes in aqueous solutions at different pH values hydrated redox partides and their standard redox potentials are on the right hand side. [From Gleria-Memming, 1975.]...

TABLE 8-1. Preference for the conduction band mechanism (CB) and the valence band mechanism (VB) in outer sphere electron transfer reactions of hydrated redox particles at semiconductor electrodes (SC) Eo = standard redox potential referred to NHE c, = band gap of semiconductors. [From Memming, 1983.]... 

For n-type semiconductor electrodes in which a redox reaction of cathodic hole iiyection reaches its quasi-equilibrium state at the electrode interface, the recombination current of iiqected holes (minority charge carriers) with electrons (minority charge carriers), w, is given by Eqn. 8-70 [Reineke-Memming, 1992] ... 

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

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