4、Semiconductor-2.pptVIP

Specialized English School of Physics Optoelectronic Engineering W. Zhang 1.1.5 Density of States（态密度） When electrons move back and forth along the x-direction in a semiconductor material, the movements can be described by standing wave oscillations. The wavelength λof a standing wave is related to the length of the semiconductor by L where h is the plank constant and px , is the crystal momentum in the x-direction, Substituting Eq.1-2 into Eq.1-1 gives The volume dpxdpydpz in the momentum space for a unit cube(L=1) is thus equal to h3.Each incremental change in n corresponds to a unique set of integers(nx,ny,nz) ,which in turn corresponds to an allowed energy state Thus, the volume in momentum space for an energy state is h3.The volume between two concentric spheres from p to p+dp is 4πp2dp). The number of energy states contained in this volume is then 2(4πp2dp)/h3 , where the factor 2 accounts for the electron spins. We can substitute E for p and obtain: Where N(E) is called the density of states, that is, the density of allowed energy states per unit volume.（N(E) A young lady came to take the examination for a driver’s license. The first question was too difficult for her to answer. Completely stumped(受困惑） for an answer, she wrote, “God knows!” The results came through immediately. She found that the examiner had written, “ God passes, you fail!” 有个小妹来参加驾驶执照考试。第一个问题对她来说太难回答了。完全被答案难道的她，只好写上：“上帝知道！” 考试结果立刻出来了。她发现主考官写着：“上帝通过了，你失败了！” * * 电子科学与技术专业英语 》》 ?Energy Bands For an isolated (孤立）atom, the electrons of the atom can have only discrete（不连续） energy levels. For example, the energy levels for an isolated hydrogen atom are given by the bohr model 电子科学与技术专业英语 》》 ?对于孤立原子，原子的电子有不连续的能级。如，孤立氢原子的能级可由玻尔模型得出： Where m0 is the free-electron mass, q is the electronic charge,ε0is the free-space permittivity （电导率）, h is the plank constant, and n is a positive integer（整数） called the principal quantum（量子） number. The discrete energies are -13.6e