Semiconductors energy band gap

On a somewhat larger scale, there has been considerable activity in the area of nanocrystals, quantum dots, and systems in the tens of nanometers scale. Interesting questions have arisen regarding electronic properties such as the semiconductor energy band gap dependence on nanocrystal size and the nature of the electronic states in these small systems. Application [31] of the approaches described here, with the appropriate boundary conditions [32] to assure that electron confinement effects are properly addressed, have been successful. Questions regarding excitations, such as exdtons and vibrational properties, are among the many that will require considerable scrutiny. It is likely that there will be important input from quantum chemistry as well as condensed matter physics. 

Entry number Oxide semiconductor Energy band gap, eV Comments Reference (s)... 

Figure Al.3.8. Schematic energy bands illustrating an insulator (large band gap), a semiconductor (small band gap), a metal (no gap) and a semimetal. In a semimetal, one band is almost filled and another band is almost empty.

Fig. 1. Photoexcitation modes iu a semiconductor having band gap energy, E, and impurity states, E. The photon energy must be sufficient to release an electron (° ) iato the conduction band (CB) or a hole (o) iato the valence band (VB) (a) an intrinsic detector (b) and (c) extrinsic donor and acceptor...

The III-V semiconductors can all be made by direct reaction of the elements at high temperature and under high pressure when necessary. Some properties of the Al compounds are in Table 7.11 from which it is clear that there are trends to lower mp and energy band-gap Eg with increasing atomic number. 

Hamada, N. and Ohnishi, S. (1986) Self-interaction correction to the local-density approximation in the calculation of the energy band gaps of semiconductors based on the full-potential linearized augmented-plane-wave method, Phys. Rev., B34,9042-9044. 

Strongly depends on the chemical composition and systematically increases as the Si fraction in the walls increases. Analogous blue-shift in energy band gap of a nanoporous Ge/Si alloy semiconductor relative to pure mesoporous germanium has been observed by Sun et al. [44],... 

FIGURE 21.10 Bands of MO energy levels for (a) a metallic conductor, (b) an electrical insulator, and (c) a semiconductor. A metallic conductor has a partially filled band. An electrical insulator has a completely filled valence band and a completely empty conduction band, which are separated in energy by a large band gap. In a semiconductor, the band gap is smaller. As a result, the conduction band is partially occupied with a few electrons, and the valence band is partially empty. Electrical conductivity in metals and semiconductors results from the presence of partially filled bands. 

Energy band gaps for selected semiconductors are summarized in Table I. On the basis of the nature of the transition from the valence band to the conduction band, semiconductors are classified as direct or indirect. In a direct semiconductor, the transition does not require a change in electron momentum, whereas in an indirect semiconductor, a change in momentum is required for the transition to occur. This difference is important for optical devices such as lasers, which require direct-band-gap materials for efficient radiation emission (7, 8). As indicated in Figure 7, Si is an indirect semiconductor, whereas GaAs is a direct semiconductor. 

Entry number Oxide semiconductor (s) Energy band gap(s)a, eV Comments Refer- ence... 

Detailed statistical analysis (e.g. see Kittel, 1996) shows that for intrinsic band conduction in a semiconductor, where the concentration of conduction electrons n always equals the concentration of holes p, and where the energy band gap Eg is wide (i.e. Eg > kT),... 

---