7、Semiconductor-5.ppt

1、1,Specialized English,School of Physics hence the mobility decreases with increasing temperature. Theoretical analysis shows that the mobility due to lattice uL will decrease in proportion to T-3/2.,电子科学与技术专业英语 ,在方程1-15中迁移率直接和相邻两次碰撞间的平均自由时间有关,平均自由时间是由各种散射机制决定的。最重要的两种散射机制是晶格散射和杂质散射。由于在绝对零度以上的任何温度晶格原子

2、都有热振动，晶格散射就是由晶格原子的热振动引起。这些振动微扰了晶格的周期势能,使得载流子和晶格之间的能量传递成为可能。由于随着温度提高,晶格原子的振动加剧,在高温时晶格散射占主要地位；于是温度升高其迁移率降低。理论分析表明晶格散射引起的迁移率L随温度按T3/2的关系下降。,4,Impurity scattering results when a charge carrier travels past an ionized dopant（掺杂） impurity (donor or acceptor). The charge carrier path will be deflected（偏转）

3、due to coulomb force（库仑力） interaction. The probability of impurity scattering depends on the total concentration of ionized impurity, that is, the sum of the concentration of negatively and positively charged ions. However, unlike lattice scattering, impurity scattering becomes less significant（显著）

4、at higher temperatures.,电子科学与技术专业英语 ,当一个带电的载流子靠近一个电离的杂质(施主或受主)时就产生了杂质散射。由于受到库仑力的作用，带电的载流子的运动曲线会偏斜。杂质散射的几率取决于电离杂质的总浓度,即负离子和正离子的浓度总和。但是,和晶格散射不同的是,在高温时杂质散射就不重要了。,5,At higher temperatures, the carrier move faster; They remained near the impurity atom for a short time and are therefore less effectively s

5、cattered（分散）. The mobility due to impurity scattering I can theoretically be shown to vary as T3/2/NT, where NT is the total impurity concentration. The probability of a collision taking place in unit time ,1/c,is the sum of the probabilities of collisions due to the various scattering mechanisms.,电

6、子科学与技术专业英语 ,高温时,载流子运动得更快,它们在杂质原子附近的时间很短,所以不会被有效散射。杂质散射引起的迁移率理论上I随温度按T3/2/NT改变,NT是总的杂质浓度。 单位时间的碰撞几率1/c是由各种散射机制引起的碰撞几率之和。,6,The measured electron mobility as a function（函数） of temperature for silicon with five different donor concentration is shown. The insert shows the theoretical temperature depende

7、nce（曲线） of mobility due to both lattice and impurity scatterings. For lightly doped（掺杂） samples (e.g., the sample with doping of 1014 cm-3), the lattice scattering dominates（占优势）, and the mobility decreases as the temperature increases.,电子科学与技术专业英语 ,测得的5种不同施主浓度的硅样品的电子迁移率随温度的变化如图所示。插图显示了理论计算得到的晶格散射和杂

8、质散射引起的电子迁移率和温度的依赖关系。对于掺杂较少的样品(例如掺杂浓度为1014cm-3)，晶格散射占主要地位，迁移率随温度上升而下降。,7,For heavily doped samples, the effect of impurity scattering is most pronounced at lower temperatures. The mobility increases as the temperature increases, as can be seen for the sample with doping of 1019 cm-3. For a given temp

9、erature, the mobility decreases with increasing impurity concentration, because of enhanced（增加） impurity scattering.,电子科学与技术专业英语 ,对掺杂很多的样品，在低温时杂质散射更显著。迁移率随温度上升而上升。例如样品掺杂浓度为1019cm-3时，在一定的温度，迁移率随着杂质浓度提高而下降，因为杂质散射增强了。,8,The measured mobilities in silicon and gallium arsenide as function of impurity con

10、centration at room temperature are shown. Mobility reaches a maximum value at low impurity concentration; this corresponds to（与 相同） the lattice-scattering limitation. Both electron and hole mobilities decrease with increasing impurity concentration and eventually（最后） approach a minimum value at high

11、 concentrations. Note also that the mobility of electrons is greater than that of holes. Greater electron mobility is due mainly to the smaller effective mass of electrons.,电子科学与技术专业英语 ,室温时测得的硅和砷化镓的迁移率随杂质浓度的变化如图所示。在低杂质浓度时迁移率为最大值。随着杂质浓度提高电子和空穴的迁移率都下降了,最后在高浓度时达到一个最小值。请注意电子的迁移率比空穴大,这主要是因为电子的有效质量比空穴的有效质

12、量小。,9,Carrier diffusion（扩散） In the preceding（前） section we considered the drift current, that is , the transport of carrier when an electric filed is applied. Another important current component can exist if there is a spatial（空间） variation of carriers concentration in the semiconductor material, th

13、at is ,the carriers tend to move from a region of high concentration to a region of low concentration . This current component is called diffusion current（扩散电流）.,电子科学与技术专业英语 ,上一节我们讨论了迁移电流,即载流子在外加电场下的运动.如果在半导体材料中载流子的浓度在空间上有变化,就会有另外一种重要的电流成分存在,即载流子会从高浓度区运动到低浓度区,这种电流成分叫做扩散电流。,10,To understand the diffu

14、sion process, let us assume an electron density that varies in the x-direction. The semiconductor is at uniform（均匀） temperature, so that the average thermal energy of electrons does not vary with x, only the density n(x) varies. We shall consider the number of electrons crossing the plane（面） at x=0

15、per unit time and per unit area.,电子科学与技术专业英语 ,为了理解扩散过程，我们假定电子密度在x方向上变化。半导体温度均匀，所以电子的平均热运动能量不随x改变，只有密度N(x)改变。我们来讨论单位时间内通过x=0平面单位面积的电子数。,11,Because of finite（有限） temperature, the electrons have random thermal motions with a thermal velocity vth and a mean free path l.(note that l=vthc, where c is the

16、mean free time.) the electrons at x=-l, one mean free path away on the left side, have equal chances of moving left or right; and in a mean free time c, one half of time will move across the plane x=0.,电子科学与技术专业英语 ,因为温度一定，电子做随机的热运动，运动速度为vth,平均自由程为l (注意l=vthc, c是平均自由时间)，电子在x=-l ，离左边一个平均自由程，向左或向右运动的几率

17、相等；在一个平均自由时间内有一半的时间会通过x=0平面。,12,Because each electron carries a charge -q, the carrier flow gives rise to（产生） a current Where Dnvth l is called diffusivity.（扩散系数） The diffusion current is proportional to the spatial derivative（导数） of electron density. Diffusion current results from the random therma

18、l motion of carriers in a concentration gradient. For an electron density that increases with x, the gradient is positive, and the electrons will diffuse toward the negative x-direction. The current is positive and flows in the direction opposite to that of the electrons.,电子科学与技术专业英语 ,因为每个电子有一个负电荷-q

19、，载流子流动就产生了电流： 扩散电流正比于电子密度的空间导数，扩散电流是由于载流子在浓度梯度下的随机热运动。如果电子浓度随x提高，梯度为正，电子将朝x负方向扩散。电流为正，电流的方向与电子的运动方向相反。,13,Carrier Injection（注入） In thermal equilibrium the relationship pn=n2i is valid. If excess carriers are introduced to a semiconductor so that pn n2i , we have a none-equilibrium situation. The pro

20、cess of introducing excess carriers is called carrier injection（载流子注入）. We can inject carriers by using various methods including optical excitation and forward biasing a p-n junction (discussed in section 1.3).,电子科学与技术专业英语 ,在热平衡条件下关系式是成立的。如果在半导体材料中注入过量载流子以致，这就是非平衡情况。引入过量载流子的过程叫做载流子注入。我们可以用光激发的方法或者在

21、PN结上加前向偏压的方法（下一节讨论）注入载流子,14,In the case of optical excitation, we shine a light on a semiconductor. If the photon energy hv of the light is greater than the bandgap energy Eg of the semiconductor, where h is the Plank constant and v is the optical frequency, the photon is absorbed by semiconductor a

22、nd an electron-hole pair is generated. the optical excitation increases the electron and hole concentration above their equilibrium values. These additional carriers are called excess carriers.,电子科学与技术专业英语 ,在光激发的情况,我们是把光照射在一个半导体上。如果光的光子能量大于半导体的带隙,是普朗克常数, 是光的频率,光子会被半导体吸收,产生电子空穴对,光激发使电子空穴浓度高于平衡值。这些附加的

23、载流子叫做过量载流子。,15,The magnitude（值） of the excess carrier concentration relative to the majority（大部分） carrier concentration determines（决定） the injection level. We shall use an example to clarify（阐明） the meaning of injection level. The majority carrier concentration is approximately equal to the donor co

24、ncentration ,that is, nno=105cm-3. The minority carrier concentration is given by pno= n2i/nno ,=1.45105cm-3 . in this notation（符号） ,the first subscript refers to the type of semiconductor and the subscript o refers to the thermal equilibrium condition. Thus,nno and pno denote（代表） the electron and h

25、ole concentrations, respectively, in an n-type semiconductor in equilibrium.,电子科学与技术专业英语 ,过量载流子浓度相对于主要载流子浓度的大小决定了注入水平。我们用一个例子来说明注入水平的含义。多数载流子浓度近似等于施主浓度,即，少数载流子浓度为,在这个记法中,第一个下标表示半导体的类型,下标o指热平衡条件,于是, 和 分别表示热平衡态下N型半导体中的电子和空穴浓度。,16,When we introduce (e.g., by optical excitation) excess carriers of both

26、types into the semiconductor, the excess electron concentration n must equal to the excess hole concentration p, because electrons and holes are produced in pairs. In the example shown in figure 1-8(b),we have added 1012/cm2 minority carriers (holes in an n-type semiconductor). Therefore, the hole c

27、oncentration is increased by seven orders of magnitude (from 105/cm3 to 1012/cm3). At the same time we have added 1012 cm-3 majority carriers (electrons) to the semiconductor.,电子科学与技术专业英语 ,当我们在半导体中引入(例如用光激发的方法)两类过量载流子时,过量电子浓度一定等于过量空穴浓度,因为电子和空穴成对产生。在图所示的例子中,我们注入了的少数载流子(N型半导体中的空穴)。所以空穴浓度提高了7个数量级(从到)。同

28、时给半导体注入了的多数载流子(电子).,17,However, this concentration of electrons is negligibly（可忽略） small compared to the original（原有） electron concentration. The percentage change in the majority carrier concentration is only 0.1%. This condition, in which the excess carrier concentration is small in comparison to

29、the doping（掺杂） concentration, that is n=pND, is referred to as low-level injection.,电子科学与技术专业英语 ,但是,过量电子的浓度和它本来的电子浓度比较起来可以忽略.多数载流子浓度的百分比变化只有0.1%。注入的载流子浓度比掺杂浓度小的条件,即，叫做低水平注入。,18,Figure 1-8 shows an example of high level injection in which the number of injected excess carriers is comparable to or lar

30、ge than the number of carriers due to doping concentration .In this case, the injected-carrier concentrations may overwhelm（占优势） the equilibrium majority carrier concentration, and p, becomes comparable to n, as indicated（所示） in figure. High-level injection is sometime encountered（遇到） in device oper

31、ations. However, because of the complexities（复杂） involved in its treatment, we shall be concerned mainly with low-level injection.,电子科学与技术专业英语 ,图1-8显示了一个高水平注入的例子,注入的载流子的数目大于或等于掺杂引起的载流子数目。在这种情况下,注入载流子浓度可能超过平衡时的多数载流了浓度,并且P可以变得和N差不多大,如图所示。设备运行中有时会遇到高水平注入的情况。但是,由于处理这类问题的复杂性,我们下面主要关心低水平注入的情况。,19,PN Junction Most semico