材料科学基础 第12章

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1、第十二章 电学性质一、学习目的在部件或者结构的设计过程中，材料电学性质的考虑对确定要选择的材料以及加工工艺十分重要。不同材料的电学性质差异很大。一些材料需要具有极高的电导率（如连接导线），而另一些材料则需要具有电绝缘性（如保护性的密封包装）。二、本章的主要内容1描述固体材料中4种可能的电子能带结构。2简要描述产生自由电子和空穴的电子激发过程：(a)金属；(b)半导体（本征和杂质情况）；(c) 绝缘体。3给定电荷的载流子浓度和迁移率， 计算金属、半导体(本征和杂质情况)以及绝缘体的电导率。4给出本征半导体和杂质半导体材料的区别。5陈述电导率随温度的增加而变化的方式：(a)金属；(b)半导体；(c

2、)绝缘体。6对于p-n结，根据其子与空穴的运动来解释其整流过程 。7计算平行板电容器的电容。8根据电容率来定义介电常数。9简要解释平板间介电材料的插入与极化是怎样增强电容器的电荷存贮能力的。10定义和描述三种类型的极化。三、重要术语和概念Acceptor state: 受主态When impurity atoms with a valence of 3 is added as a substitutional impurity into silicon crystal, one of the covalent bonds around each of these atoms is defici

3、ent in an electron; such a deficiency may be viewed as a hole that is weakly bound to the impurity atom. Each impurity atom of this type introduces an energy level within the band gap, above yet very close to the top of the valence band which is termed acceptor state.当具有3个价电子的杂质原子取代掺杂到硅晶体中后，这些原子中的一个

4、共价键会缺失一个电子，这种缺陷可以认为是一个空穴微弱的束缚在杂质原子上，每个这种类型的缺陷都会在带隙内部引入一个能级，它十分接近价带的顶端，也被称为受主态。Conduction band: 导带In band structures, one band that is empty with electrons is termed conduction band.在能带结构中，未填充电子的带称为导带。Conductivity, electrical：电导率Electrical conductivity is the reciprocal of the resistivity, or =1/ and

5、 is indicative of the ease with which a material is capable of conducting an electric current.电导率是电阻率的倒数，或者=1/，它意味着材料传导电流的难易。Donor state: 施主态When an impurity atom with a valence of 5 is added as a substitutional impurity into silicon crystal, only 4 of 5 valence electrons of this impurity atom can p

6、articipate in the bonding and the extra nonbonding electron is loosely bound to the region around the impurity atom. For each of such loosely bond electrons, there exists a single energy level, or energy state, which is located within the forbidden band gap just below the bottom of the conduction ba

7、nd and termed donor state.当具有5个价电子的杂质原子取代掺杂到硅晶体中，这个杂质原子只有4个价电子参与成键，而额外那个价电子只是松散的束缚在杂质原子的周围。对于每一个这样松散成键的电子来说，其在禁带的内部相邻导带的位置将形成一个能级或者能态，也被称为施主态。 Doping: 掺杂Controlled concentrations of specific donors or acceptors may be intentionally added into materials to form extrinsic semiconductor using various t

8、echniques. Such an alloying process in semiconducting materials is termed doping.控制特定施主和受主原子的浓度，进而运用各种方法能动地填加，这种在半导体材料中加入合金成分的过程称为掺杂。Electron energy band: 电子能带Each distinct atomic state may split into a series of closely spaced electron states in the solid, to form what is termed an electron energy

9、band.固体中每个分立的原子态会劈裂成一系列的紧密排列的电子态，进而形成了所谓的电子能带Energy band gap: 能隙energy band gap lies between valence band and conduction band.存在于价带和导带之间的部分称之为能隙。Extrinsic semiconductor: 杂质半导体When the electrical characteristics are dictated by impurity atoms, the semiconductor is said to be extrinsic semiconductors.

10、如果半导体的电学性质归因于其中的杂质原子，那么它将被称为杂质半导体。Fermi energy: 费米能The energy corresponding to the highest filled state at 0 K is called the Fermi energy Ef.在绝对零度时，对应最高电子填充态的能量被称为费米能级Ef。 Free electron: 自由电子The electrons that participate in the conduction process are termed free electrons, which have energies greate

11、r than the Fermi energy.参与导电过程的电子，我们称之为自由电子，其能量高于费米能级。Hole: 空穴Hole is an charged electronic entity in semiconductors and insulators which have energies less than the Fermi level and participate in the electric conduction.空穴是半导体和绝缘体中的一种带电体，其能量小于费米能，并且参与导电。Insulator: 绝缘体Insulators are materials with v

12、ery low conductivities, ranging between 10-10and 10-20 (-m) -1.绝缘体是指具有极低电导率的材料，其数值的一般范围在10-10和10-20(-m) -1之间。 Intrinsic semiconductor: 本征半导体Intrinsic semiconductors are those in which the electrical behavior is based on the electronic structure inherent to the pure material.本征半导体是电学性质基于纯材料的电子结构的半导体。

13、Ionic conduction: 离子导电For ionic materials a net motion of charged ions is possible that produces a current; such is termed ionic conduction.对于离子型材料来说，带电荷离子的净运动可以产生电流，这被称为离子导电。Matthiessens rule: Matthiessen规则The total resistivity of a metal is the sum of the contributions from thermal vibrations, imp

14、urities, and plastic deformation and this is sometimes known as Matthiessens rule.金属的总电阻率可以表示为热振动、杂质以及塑性形变所导致的电阻率的总和，这一规则也被称为Matthiessen 规则。Metal: 金属Metals are good conductors, typically having conductivities on the order of 107 (-m)-1.金属是指良导体，通常具有107 (-m)-1量级的电导率。Mobility: 迁移率The magnitude of elect

15、ron mobility is indicative of the frequency of the events that free electrons, being acted on by an electric field, are scattered by imperfection in the crystal lattice.自由电子在电场的作用下，会由于晶体中的杂质而发生散射，迁移率就是指这种散射发生的频率。Ohms law: 欧姆定律Ohms law relates the current Ior time rate of charge passage to the applie

16、d voltage V as follows: V=IR. Where R is the resistance of the material through which the current is passing.欧姆定律陈述了电流I（或者单位时间流经的电量）与所加电压的关系：V=IR，这里R是电流通过的材料的电阻。Resistivity, electrical: 电阻率The resistivityis independent of specimen geometry but related to R through the expression=RA/l. where I is the

17、 distance between the two points at which the voltage is measured, and A is the cross-sectional area perpendicular to the direction of the current .电阻率与样品的几何形状无关，与R的关系是=RA/l. 这里l是指测量电压所加的两点间的距离，A指垂直与电流方向的截面面积。Semiconductor: 半导体Materisls with intermediate conductivities generally from 10-6to 104(-m)-

18、1, are termed semiconductors.半导体是指具有中等程度导电性的材料，其电导率一般在10-6至104(-m)-1之间。Valence band: 价带In the band structure, one band that is completely filled with electrons is termed valence band.在能带结构中，完全填满电子的带被称为价带。四、主要例题、习题的分析例12.1 对于本征硅，室温下电导率为；电子和空穴迁移率分别为0.14m2/V-s.和0.048m2/V-s。计算室温下电子和空穴的浓度。解：因为材料是本征的，电子和空

19、穴浓度相同,因此.从方程12.15可知例12.2 将磷加入到高纯度硅中,室温下得到载流子的浓度为。(a) 判断此材料是n型还是p型？(b) 假定电子和空穴的迁移率和本征材料相同，计算室温下此材料的电导率。解：(a) 磷是VA族无素，因此, 磷在硅中作为施主物质，从而浓度为的载流子全部为电子。这个电子浓度要比本征材料的浓度（）大，因此, 这种材料是n型杂质半导体。(b) 此题中电导率可以由公式12.16得出：例12.3 如果室温25(298K)情况下, 本征锗的电导率是2.2(-m)-1，估算其在150(423K)时的电导率为多少。解：此题可以用公式12.18来解决。首先, 用室温数据来测定常数C的值, 然后, 可知150的值可以计算.从表12.2中可知, 锗的Eg值是0.67eV, 因此,在150(423K)时，五、背景资料以人名命名的组织大体如此，其他的组织命名则各有不同。铁素体Ferrite，命名自拉丁文的铁（Ferrum）；珠光体Pearlite，得名自其珍珠般（pearl-like）的光泽；渗碳体Cementite则因发现者称其为水泥（法语Ciment）以描述它在凝固过程中粘结先析出的晶胞的作用而得名。参考书目 1. 钱临照：晶体缺陷研究的历史回顾，物理,第9卷，第4期,1980。