4、Semiconductor-2.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,Specialized English,School of Physics&Optoelectronic Engineering,W.Zhang,电子科学与技术专业英语,1,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：,2,Where m,0,is the free-electron mass,q is the electronic charge,0,is 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-13.6eV for the ground level(n=1),-3.4eV for the first excited(n=2),etc.,电子科学与技术专业英语,式中,m,0,代表自由电子质量,q,是电荷量,0,是真空中电导率,h,是普朗克常数，,n,是正整数称为主量子数。

不连续能量在基态为,-13.6eV(n=1),第一激发态为,-3.4eV(n=2),etc.,3,We now consider two identical,（相同）,atoms.When they are far apart,（远离）,the allowed energy levels for a given principal quantum number(e.g.,n=1)consist of one doubly degenerate,（双重简并）,level,that is,each atom has exactly then same energy(e.g.,-13.6eV for n=1).As the two atoms approach one another,the doubly degenerate energy level will spilt into two levels by the interaction,（,相互作用）,between the atoms.When we bring N atoms together to form a crystal,the N-fold degenerate energy level will split into N separate but closely spaced levels due to atomic interaction.,电子科学与技术专业英语,我们考虑两个相同原子,.,当它们 远离时,对所给主量子数,(e.g.,n=1),的允态能级具有双重简并能级,也就是说，每个原子具有相同能级,(e.g.,-13.6eV for n=1).,当两个原子相互靠近，这个双重简并能级将被原子间相互作用分成两个能级。

当从晶体中引入,N,个原子，,N,重简并能级将会分成,N,个能级，但原子相互作用能级相互接近4,This results in an essentially,（基本）,continuous band of energy.,The detailed energy band structures of crystalline solids have been calculated using quantum mechanics,(,量子理论）,.Figure 1-3 is a schematic diagram of the formation of a diamond lattice crystal from isolated,（孤立）,silicon atoms.Each isolated atom has its discrete,（不连续）,energy levels(two levels are shown on the far right of the diagram).As the,interatomic,（原子间）,spacing decreases,each degenerate energy level splits to form a band.,电子科学与技术专业英语,这导致一个基本连续的能带。

结晶固体的详细能带结构能够用量子理论计算而得图,1,3,是孤立硅原子的金刚石结构晶体形成的原理图每个孤立原子有不连续能带（在右图给出的两个能级）如原子间隔的减少，每个简并能级将分裂产生带5,电子科学与技术专业英语,6,Further decrease in spacing causes the bands originating,（引起）,from different discrete levels to lose their identities,（同性）,and merge,（合并）,together,forming a single band.When the distance between atoms approaches the equilibrium,interatomic,spacing of the diamond lattice(with a lattice constant of 0.543 nm for silicon),this band splits again into two bands.,电子科学与技术专业英语,在空间更多减少将导致能带从不连续能级到失去其特性并合并起来，产生一个简单的带。

当原子间距离接近金刚石结构的平衡原子间距（对硅而言晶格常数,0.543 nm,），这个带分为两个带区7,T,hese bands are separated by a region which designates,（指明）,energies that the electrons in the solid cannot possess.This region is called the forbidden gap,or bandgap,（带隙）,Eg,.The upper band is called the conduction band,（导带,）,while the lower band is called valence band,（价带,）,as shown on the far left of Figure 1-3.,电子科学与技术专业英语,这些带被固态电子不能够拥有的能量区域分开这个区域称为禁带或带隙,Eg,如图,1,3,左侧所示上面称为导带下面称为价带8,Figure1-4 shows the energy band diagrams of three classes of solids insulators,semiconductors,and conductors.In an insulator such as silicon dioxide(SiO,2,),the valence electrons form strong bonds between neighboring atoms.These bonds are difficult to break,and consequently,（因此）,there are no free electrons to participate,（产生）,in current conduction.,电子科学与技术专业英语,图,1,4,给出了三种固体（绝缘体、半导体、导体）的能带图。

在绝缘体（如,SiO,2,),价电子在相邻原子间产生强的价键这些键很难断开，因此没有自由电子在电流传导过程中产生9,As shown in the energy band diagram Figure1-4(a),there is a large bandgap,（带隙）,.Note that all energy levels in the valence band are occupied,（填充）,by electrons and all energy levels in the conduction band are empty.Thermal energy or an applied electric field,（外加电场）,cannot raise the uppermost electron in the valence band to the conduction band.Therefore,silicon dioxide is an insulator,which cannot conduct current.,.,电子科学与技术专业英语,如图,1,4,（,a,）能带图所示，有一个大带隙。

注意到所有的价带都被电子充满而导带中能级是空的热能量和外加电场不能够提高在价带中最上层电子到导带因此，二氧化硅是绝缘体不能导电,10,电子科学与技术专业英语,11,As we discussed in section 1.1.3,bonds between neighboring atoms in a semiconductor are only moderately,（适中）,strong.Therefore,thermal vibrations,（振动）,will break some bonds.When a bond is broken,a free electron along with a free hole result.Figure 1-4(b)shows that the bandgap of a semiconductor is not as large as that of an insulator(e.g.,Si with a bandgap of 1.12eV).,电子科学与技术专业英语,如,1.1.3,节所述，在半导体中相邻原子价键仅仅一定程度强因此，热振动将断开其中的价键。

当价键断开后，产生了自由电子连同自由空穴图,1,4,（,c,）给出的半导体的带隙不如绝缘体宽如硅带隙,1.12eV,）,12,Because of this,some electrons will be able to move from the valence band to the conduction band,leaving holes in the Valence band.when An electric field is applied,both the electrons in the conduction band and the holes in the valence band will gain kinetic energy,（动能）,and conduct electricity.,电子科学与技术专业英语,正因为此，一些电子能够从价带移动到导带，在在价带中留下空穴当外加电场后，所有在导带中电子和在价带中的空穴将得到动能并能够导电13,In conductors such as metals,Figure 1-4(c),the conduction band either is partially,（部分）,fill。