Band model intrinsic

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

Figure 2.13 Schematic band model of the region from source to drain through the intrinsic gate with an applied drain voltage, Vp. (From [98], 2003 IEEE. Reprinted with permission.)...

This situation can be expressed in terms of the band model as shown in Fig. 1.24. Stoichiometric NiO is an intrinsic semiconductor, having an energy gap of Eq (=Eq—E ). Non-stoichiometric Nij O, which has metal vacancies or electronic defects, has an acceptor level A between the valence... 

Fig. 3.15 Band model for an intrinsic semiconductor. The valence band is totally filled and the conduction band empty. Conduction occurs via promotion of electrons from Ey to Ecy the conductivity increasing with increase in temperature, (a) Definition of energy levels (b) Variation of density of available states with...

Eg between the valence band and the conduction band. The band structure of a direct II-VI intrinsic semiconductor like CdSe can be represented reasonably well by a parabolic band model like that shown schematically in Fig. 2. Here, k = 7r/ris the wave vector and r is the radial distance from an arbitrary origin in the center of the crystal. The kinetic energy of the electron is proportional to E- and the energy minimum of the conduction band and the maxima of the valence bands occur at k = 0 (corresponding to r = co in a bulk sample). 

The schematic band models of solids i.e. insulator, metal, intrinsic semiconductor and impurity semiconductor which are classified according to electronic properties, drawn ... 

However, solids do not always behave according to the idealised band model. When photoabsorption spectra are studied in the X-ray range, it can happen that the observations reveal quasiatomic properties only slightly modified by solid state effects. These atomic effects in solids must be distinguished from purely solid state properties, but are also of intrinsic interest once properly understood, they provide an alternative route to the study of atoms in condensed matter. 

Additional experiments on the same Pt(NH3)4 and PtCb " solutions demonstrated that certain solutions consistently gave crystals of higher conductivity, which suggests that differences in the composition of these solutions were responsible for the observed conductivity variations. On the basis of these findings and the previously mentioned arguments against the intrinsic band model, it was suggested that the conductivity in MGS was impurity dominated (24, 31). 

We have examined (8), as did Brady and coworkers (3), the electronic absorption spectrum of the Saltman- piro ball. The spectrum (Figure 6) (8) shows a pattern of four weak bands with the lowest band at about 900 nm, in very good agreement with [Fe(III)06]oct coordination. The derived LF parameters are A<>ct = 11,260 cm" C/B = 3, and B = 815 cm" We can say with considerable confidence that most of the Fe(III) ions occupy octahedral coordination sites. There is no hint of bands attributable to [Fe(III)04]tet, and as these bands are intrinsically more intense than are those assigned to [Fe(III)06]oct> we can eflFectively rule out tetrahedral coordination in this synthetic model compound. 

Figure 3. Band model for semiconductors. Energy levels shown are Ecj the conduction band edge the valence band edge Eg, the band gap the Fermi energy for an intrinsic semiconductor E, electron donors in n-type semiconductors electron acceptors in p-type semiconductors.

The electronic properties of solids were described in Chapter 2 using the band model. A characteristic feature of semiconductors is the separation of the electron energy levels into two bands, the valence band with occupied energy levels and the conduction band with unoccupied energy levels. Both bands are separated by an energy gap. The band gap energy determines the intrinsic conductivity because electricity can only be transported through the semiconductor if some electrons are excited from the valence band to the conduction band. Then either holes in the valence band or electrons in the conduction band become mobile. The mobility of valence band holes and conduction band electrons... 

Figure 9.1 Band model of an intrinsic semiconductor and definition of energies. is the electron affinity, the work function, and the gap energy.

Gallium Antimonide (GaSb). Transport in n-t)fpe GaSb is cort5)licated by the contribution of three sets of conduction bands with minima located at F, L, and X. The data on transport coefficients can be consistently explained by a three-band model, the X bands contributing to transport above 180 °C. Intrinsic carrier concentrations i of the order of 10 cm have been estimated for a tert5)erature T = 365 K. 

Band model of (a) a metal, (b) intrinsic, (c) n-type, and (d) p-type semiconductor with Fermi energy Ep valence band VB, conduction band CB, and donor and acceptor levels. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

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