Band diagrams

This model fits well to all data which have been measured and reported here. Special optical effects which have been observed need a more detailed description of interface recombination phenomena and of light interaction with deep levels. 

---

Figure C2.15.7. Generic band diagrams insulator, metal, semimetal, and semiconductor.

Calculated plots of energy bands as a function of wavevector k, known as band diagrams, are shown in figure C2.16.5 for Si and GaAs. Semiconductors can be divided into materials witli indirect and direct gaps. In direct-gap... 

Instead of plotting tire electron distribution function in tire energy band diagram, it is convenient to indicate tire position of tire Fenni level. In a semiconductor of high purity, tire Fenni level is close to mid-gap. In p type (n type) semiconductors, it lies near tire VB (CB). In very heavily doped semiconductors tire Fenni level can move into eitlier tire CB or VB, depending on tire doping type. 

Figure C2.16.7. A schematic energy band diagram of a p-n junction witliout external bias (a) and under forward bias (b). Electrons and holes are indicated witli - and + signs, respectively. It should be remembered tliat tlie energy of electrons increases by moving up, holes by moving down. Electrons injected into tlie p side of tlie junction become minority carriers. Approximate positions of donor and acceptor levels and tlie Feniii level, are indicated.

The distributions of excess, or injected, carriers are indicated in band diagrams by so-called quasi-Fenni levels for electrons or holes (Afp). These functions describe steady state concentrations of excess carriers in the same fonn as the equilibrium concentration. In equilibrium we have... 

A band diagram of a biased n-p-n BIT is shown in figure C2.16.8. Under forward bias, electrons are injected from tlie n type emitter, giving rise to tlie current 7. flowing into tlie p type base. Some of tlie carriers injected into tlie base recombine in tlie base or at tlie surface. This results in a reduction of tlie base current by 7, tlie lost recombination current, and tlie base current becomes 7g = At tlie same time, holes are injected from tlie... 

Figure C2.16.8. Schematic energy band diagram for an n-p-n bipolar junction transistor. Positions of quasi-Fenni levels and bias voltages are indicated.

A more effective carrier confinement is offered by a double heterostructure in which a thin layer of a low-gap material is sandwiched between larger-gap layers. The physical junction between two materials of different gaps is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in figure C2.l6.l0. The electrons, injected under forward bias across the p-n junction into the lower-bandgap material, encounter a potential barrier AE at the p-p junction which inliibits their motion away from the junction. The holes see a potential barrier of... 

Figure C2.16.10. Band diagram of a doubie p-p-n doubie heterostmcture witiiout tire extemai bias. The gap is smaiier tiian 2 ...

Extended-zone and reduced-zone representations of band diagram for ID lattice with no external potential. 

Fig. 1. Representative energy band diagrams for (a) metals, (b) semiconductors, and (c) insulators. The dashed line represents the Fermi Level, and the shaded areas represent filled states of the bands. denotes the band gap of the material.

Fig. 4. Schematic cross section and the band diagram of a double heterostmcture showing the band-edge discontinuities, AE and AE used to confine carriers to the smaller band gap active layer, (a) Without and (b) with forward bias. See text.

Fig. 2. (a) A schematic diagram of a n—p junction, including the charge distribution around the junction, where 0 represents the donor ion 0, acceptor ion , electron °, hole, (b) A simplified electron energy band diagram for a n—p junction cell in the dark and in thermal equilibrium under short-circuit... 

Fig. 9. Schottky barrier band diagrams (a) a rare situation where the metal work function is less than the semiconductor electron work affinity resulting in an ohmic contact (b) normal Schottky barrier with barrier height When the depletion width Wis <10 nm, an ohmic contact forms.

Fig. 8. (a) Structure of a typical resonant tunneling diode (RTD) (b) conduction band diagram for the barrier stmcture where (-------) represents the... 

Fig. 4. Band diagram of a nondegenerate ground state conducting polymer.

Figure 5-7. Band diagram of solilons wilh posiiively charged, neutral, and negatively charged systems, from left to riglu.

Figure 5-9. Band diagram of negatively charged bipolaiuns (BP ) in PPV.

The current-voltage profile of rectifying junctions is strongly asymmetrical. The reason for this can be explained with the aid of a simple band diagram shown in Figure 14-2. 

Figure 15-10. Schematic band diagrams for single-layer conjugated polymer devices at various values of forward bias. Forward bias is defined with respect lo ITO.

Figure 15-16. Schematic flat band diagram of a MDMO-PPV/Cfcu system (a) and under short circuit conditions (b).

Conjugated polymers are generally poor conductors unless they have been doped (oxidized or reduced) to generate mobile charge carriers. This can be explained by the schematic band diagrams shown in Fig. I.23 Polymerization causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the monomer to split into n and n bands. In solid-state terminology these are the valence and conduction bands, respectively. In the neutral forms shown in Structures 1-4, the valence band is filled, the conduction band is empty, and the band gap (Eg) is typically 2-3 eV.24 There is therefore little intrinsic conductivity. 

Fig. 3.18 Schematic outline and ideal band diagram of an extremely thin absorber solar cell. The n-Ti02 crystallites are clustered together to form a relatively open, network-like morphology, accommodating a thin layer of CdTe absorber, with p-ZnTe at the back contact. (Reprinted from [270], Copyright 2009, with permission from Elsevier)...

FIG. 73. Schematic cross seclion of a iriple-junclion u-Si H subsirale solar cell on stainless steel (a), and the corresponding schematic band diagram (b). (From R. E. I. Schropp and M. Zeman. "Amorphous and Microcrystalline Silicon Solar Cells—Modeling. Materials and Device Technology, Kluwer Academic Publishers. Boston. 1998, with permission.)... 

The theoretical models of effects of recharging of the surface on the band diagram in the surface-adjacent domain of semiconductor adsorbent accompanying adsorption have been developed. The effect of the surface band bending in semiconductor adsorbent on its electrophysical characteristics caused by transition phenomena have been studied. The theories of adsorption-caused response of above characteristics were derived for both ideal monocrystalline adsorbent [4] and monocrystal with... 

Fig. 5.23. Band diagram of the Schottky barrier at the gold - zinc oxide interface...

Scheme 6.2. Diagram and table of tentative assignment of transitions for the different PL bands. Diagram is not drawn to scale.

A point defect in an insulator or semiconductor is represented on band diagrams as an energy level. These energy levels can lie within the conduction or valence bands, but those of most consequence for electronic and optical properties are those that lie in the band gap. The effects of these impurities on the electronic properties of the solid will... 

Figure 4. Band diagram showing the relative positions of the conduction and valence bands in WSe2 with respect to the reduction potentials in aqueous solutions for the redox couples shown in Figure 3.

---