Band gap materials

Semiconductors can be divided into two groups direct and indirect band gap materials. In direct semiconductors the minimum energy in the conduction band and the maximum in the valence band occur for the same value of the electron momentum. This is not the case in indirect materials. The difference has profound consequences for the transitions of electrons across the band gap in which light is emitted, the radiative transitions, of interest here. 

A more effective carrier confinement is offered by a double heterostmcture in which a thin layer of a low band gap material (the active layer) is sandwiched between larger band gap layers. The physical junction between two materials of different band gaps, and chemical compositions, is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in Figure 4. Electrons injected under forward bias across the p—N junction into the lower band gap material encounter a potential barrier, AE at thep—P junction which inhibits their motion away from the junction. The holes see a potential barrier of AE at the N—p heterointerface which prevents their injection into the N region. The result is that the injected minority... 

Fig. 17. The absorption of light in a band-gap material (left) and the variation of color with the size of the band gap (right) (5).

CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. 

As mentioned earlier, CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. In addition, electron-beam excitation of solids may produce much greater carrier generation rates than typical optical excitation. In such cases, CL microscopy and spectroscopy are valuable methods in identifying various impurities, defects, and their complexes, and in providing a powerful means for the analysis of their distribution, with spatial resolution on the order of 1 pm and less. ... 

Extensions in wavelength, into both the infrared and the ultraviolet ranges will continue, motivated by increasing interest in narrow band-gap semiconductors and wide band-gap materials. 

A Stable Class of Low-Band-Gap Materials 24 Organic Field Effect Transistors (FETs) 25 Synthesis 26 Aldol Route 27... 

As an example, GulnSe2 is a known low band-gap material (1.0 eV) that shows promise for use in solar energy conversion [106]. We can imagine preparing rare-earth based materials using the formulation shown in Table 14.5. 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

Singh K, Mishra SS (2002) Photoelectrochemical studies on galvanostaticaUy formed multiple band gap materials based on CdSe and ZnSe. Sol Energy Mater Sol Cells 71 115-129... 

The hard carbon produced by this method has a range of different properties from those of plasma produced films (Table V). Note that the maximum band gap achievable with ICBD is 1.2eV at maximum hydrogenation (35 atomic %) while values up to 4eV can be obtained by plasma deposition. These wide band-gap materials are soft and easily scratched though they are more optically transparent. 

Sankapal, B. R. Sartale, S. D. Lokhande, C. D. Ennaoui, A. 2004. Chemical synthesis of Cd-free wide band gap materials for solar cells. Sol. Energy Mater. Sol. Cells 83 447-458. 

In Chapter 5.4, optical ultraviolet radiation sensors are described, including UV-enhanced silicon-based pn diodes, detectors made from other wide band gap materials in crystalline or polycrystalline form, the latter being a new, less costly alternative. Other domestic applications are personal UV exposure dosimetry, surveillance of sun beds, flame scanning in gas and oil burners, fire alarm monitors and water sterilization equipment surveillance. 

To make a breakthrough in household appliances and other consumer product markets UV sensors have to become significantly cheaper while spectral selectivity as a major key feature must be guaranteed. Most of today s UV photodiodes are made from crystalline semiconductor materials. The cheaper materials (Si) lack spectral selectivity, and the wide band gap materials are very expensive. What they all have in common their top performance regarding sensitivity and speed. Crystalline photodiodes have risetimes of often below 1 s. However, the described processes to be sensed here are not faster than some milliseconds or even much slower. In order to obtain a reasonably-priced SiC or GaN photodiode, the photoactive area is often reduced to below 1 mm2 and barely fills the sensor housing. So far, the top sensitivity offered by the semiconductor has been sacrificed for a competitive... 

---