1、半导体外文翻译附 录附录A:外文资料翻译原文部分SemiconductorA semiconductor is a solid material that has electrical conductivity between those of a conductor and an insulator; it can vary over that wide range either permanently or dynamically.1Semiconductors are important in electronic technology. Semiconductor devices, e
2、lectronic components made of semiconductor materials, are essential in modern consumer electronics, including computers, mobile phones, and digital audio players. Silicon is used to create most semiconductors commercially, but dozens of other materials are used.Bragg reflection in a diffuse latticeA
3、 second way starts with free electrons waves. When fading in an electrostatic potential due to the cores, due to Bragg reflection some waves are reflected and cannot penetrate the bulk, that is a band gap opens. In this description it is not clear, while the number of electrons fills up exactly all
4、states below the gap.Energy level splitting due to spin state Pauli exclusionA third description starts with two atoms. The split states form a covalent bond where two electrons with spin up and spin down are mostly in between the two atoms. Adding more atoms now is supposed not to lead to splitting
5、, but to more bonds. This is the way silicon is typically drawn. The band gap is now formed by lifting one electron from the lower electron level into the upper level. This level is known to be anti-bonding, but bulk silicon has not been seen to lose atoms as easy as electrons are wandering through
6、it. Also this model is most unsuitable to explain how in graded hetero-junction the band gap can vary smoothly.Energy bands and electrical conductionLike in other solids, the electrons in semiconductors can have energies only within certain bands (ie. ranges of levels of energy) between the energy o
7、f the ground state, corresponding to electrons tightly bound to the atomic nuclei of the material, and the free electron energy, which is the energy required for an electron to escape entirely from the material. The energy bands each correspond to a large number of discrete quantum states of the ele
8、ctrons, and most of the states with low energy (closer to the nucleus) are full, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in the semiconductor materials is very nearly full under usual operating conditions,
9、thus causing more electrons to be available in the conduction band.The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands, and it is the size of this energy bandgap that serves as an arbitrary dividing li
10、ne (roughly 4 eV) between semiconductors and insulators.In the picture of covalent bonds, an electron moves by hopping to a neighboring bond. Because of the Pauli exclusion principle it has to be lifted into the higher anti-bonding state of that bond. In the picture of delocalized states, for exampl
11、e in one dimension that is in a wire, for every energy there is a state with electrons flowing in one direction and one state for the electrons flowing in the other. For a net current to flow some more states for one direction than for the other direction have to be occupied and for this energy is n
12、eeded. For a metal this can be a very small energy in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to the electrical conductivity. However, as the temperature of a semiconductor rises above absolute zero, there is more energy
13、in the semiconductor to spend on lattice vibration and more importantly for us on lifting some electrons into an energy states of the conduction band, which is the band immediately above the valence band. The current-carrying electrons in the conduction band are known as free electrons, although the
14、y are often simply called electrons if context allows this usage to be clear.Electrons excited to the conduction band also leave behind electron holes, or unoccupied states in the valence band. Both the conduction band electrons and the valence band holes contribute to electrical conductivity. The h
15、oles themselves dont actually move, but a neighboring electron can move to fill the hole, leaving a hole at the place it has just come from, and in this way the holes appear to move, and the holes behave as if they were actual positively charged particles.One covalent bond between neighboring atoms
16、in the solid is ten times stronger than the binding of the single electron to the atom, so freeing the electron does not imply destruction of the crystal structure.Holes: electron absence as a charge carrierThe notion of holes, which was introduced for semiconductors, can also be applied to metals,
17、where the Fermi level lies within the conduction band. With most metals the Hall effect reveals electrons to be the charge carriers, but some metals have a mostly filled conduction band, and the Hall effect reveals positive charge carriers, which are not the ion-cores, but holes. Contrast this to so
18、me conductors like solutions of salts, or plasma. In the case of a metal, only a small amount of energy is needed for the electrons to find other unoccupied states to move into, and hence for current to flow. Sometimes even in this case it may be said that a hole was left behind, to explain why the
19、electron does not fall back to lower energies: It cannot find a hole. In the end in both materials electron-phonon scattering and defects are the dominant causes for resistance.Fermi-Dirac distribution. States with energy below the Fermi energy, here , have higher probability n to be occupied, and t
20、hose above are less likely to be occupied. Smearing of the distribution increases with temperature.The energy distribution of the electrons determines which of the states are filled and which are empty. This distribution is described by Fermi-Dirac statistics. The distribution is characterized by th
21、e temperature of the electrons, and the Fermi energy or Fermi level. Under absolute zero conditions the Fermi energy can be thought of as the energy up to which available electron states are occupied. At higher temperatures, the Fermi energy is the energy at which the probability of a state being oc
22、cupied has fallen to 0.5.The dependence of the electron energy distribution on temperature also explains why the conductivity of a semiconductor has a strong temperature dependency, as a semiconductor operating at lower temperatures will have fewer available free electrons and holes able to do the w
23、ork.Energymomentum dispersionIn the preceding description an important fact is ignored for the sake of simplicity: the dispersion of the energy. The reason that the energies of the states are broadened into a band is that the energy depends on the value of the wave vector, or k-vector, of the electr
24、on. The k-vector, in quantum mechanics, is the representation of the momentum of a particle.The dispersion relationship determines the effective mass, m * , of electrons or holes in the semiconductor, according to the formula:The effective mass is important as it affects many of the electrical prope
25、rties of the semiconductor, such as the electron or hole mobility, which in turn influences the diffusivity of the charge carriers and the electrical conductivity of the semiconductor.Typically the effective mass of electrons and holes are different. This affects the relative performance of p-channe
26、l and n-channel IGFETs, for example (Muller & Kamins 1986:427).The top of the valence band and the bottom of the conduction band might not occur at that same value of k. Materials with this situation, such as silicon and germanium, are known as indirect bandgap materials. Materials in which the band
27、 extrema are aligned in k, for example gallium arsenide, are called direct bandgap semiconductors. Direct gap semiconductors are particularly important in optoelectronics because they are much more efficient as light emitters than indirect gap materials.Carrier generation and recombinationWhen ioniz
28、ing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as electronhole pair generation. Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source.Electr
29、on-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than the band gap, be accompanied by the emission of thermal energy (in the form of phonons) or radiation (in the form of photons).In some st
30、ates, the generation and recombination of electronhole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics. The precise quantum mechanical mechanisms of generation and recombination are governed by conser
31、vation of energy and conservation of momentum.As the probability that electrons and holes meet together is proportional to the product of their amounts, the product is in steady state nearly constant at a given temperature, providing that there is no significant electric field (which might flush car
32、riers of both types, or move them from neighbour regions containing more of them to meet together) or externally driven pair generation. The product is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately
33、 1exp(EG / kT), where k is Boltzmanns constant, T is absolute temperature and EG is band gap.The probability of meeting is increased by carrier traps impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce
