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1、微电子专业英语,柳沐璇 0301080106,2.3 Carrie Diffusion,2.4 Carrier Injection,In the preceding section we considered the drift current,that is the transport of carrier when an electric field is applied.Another important current component can exist if there is a spatial variation of carrier concentration in the
2、semiconductor material,that is,the carrier tend to move from a region of high concentration to a region of low concentration.This current component is called diffusion current. 前面部分我们提到了漂移电流，就是外加电场下载流子的运输。如果半导体中载流子浓度发生空间变化，也就是说载流子倾向于从浓度高的地方向浓度低的地方移动，那么另外一种电流成分也存在。这种电流成分被称为扩散电流。,Carrier Diffusion,Let
3、 us consider the effect of dropping a tiny amount of highly concentrated red ink in a glass of water.Suppose that it is carefully done so that the water is not disturbed.We wil observe that the ink spot is slowly enlarged.Physically,the ink droplet contains a large number of molecules of color pigme
4、nts.The individual molecules move around because of thermal energy. 让我们来考虑将一滴高浓度的红色墨水滴入一杯水中。假定这个过程非常小心以至于不会扰乱杯中的水。我们将会观察到墨水滴慢慢变大。从物理上来说，这个墨水滴包含了大量的色素分子。每个单独的分子由于热能到处移动。,Their motion is random and they experience collisions just like electrons scattering described in Section 2.1.In truly random movem
5、ents,half the scattered color pigments will move toward the center of the ink droplet and the other half will move away from the center.Now,the number of molecules is fixed.The migration of the molecles reduces the density at the center of the color spot so that the spot becomes enlarged with a ligh
6、ter color. 它们的移动是任意的并且在这个过程中会经历撞击就像2.1中描述的电子被驱散是一样的。在真正的任意的移动中。半数的被驱散的的色素分子会向着墨水滴中心移动而另外一半会远离中心。现在分子被混合在了一起。分子的迁移降低了色素中心的密度，这样这滴墨水会以更浅的颜色扩大。,Let the curve at t1 represent the color-pigment concentration of the ink droplet shortly after it reaches the water.It has a peak density of unity and a small
7、half-width.As time goes by,more molecules migrate away so that the density at the center goes down and the half-width increases.Eventually,the whole glass of water will be permeated with red ink to become uniform light red liquid. 曲线t1处代表墨水滴刚接触水面时色素的浓度。它有最高的结合密度和一个较小的半径。随着时间增加，越来越多的分子迁移走导致中心处密度减小而半径
8、增加。最终，这整杯水将被红墨水渗入而变成一杯均匀的浅红色的液体。,Notice that the movement of color pigments is related to the concentration gradient so that the molecules have a net tendency to move from high concentration to low concentration.This is the basic physics of particle diffusion.When the concentration becomes uniform t
9、hroughout the liquid,the molecules continue to move around. 注意到色素分子的移动跟浓度梯度有关，也就是说分子最终倾向由高浓度从低浓度移动。这也是微粒扩散的基本原理。单整杯液体的浓度变成一致时，分子继续到处运动。,However,this random motion requires that movements in all directions are the same so that the concentration uniformity remains unchanged.Macroscopically,the density
10、 of the color pigments is the same throughout so that the diffusion stops.Therefore,the progress of diffusion results from nonuniform distribution of the color. 然而，这种任意运动要求更个方向的移动是一样的，这样的话浓度一致性保持不变。宏观来说，色素的密度整体是不变的这样扩散也就停止了。因此，扩散过程起因于色素分子的不均匀分布。,In a semiconductor,the diffusion of electrons or holes
11、 results from their movement from high concentration to low concentration.Because electrons and holes are charge carriers,their motion gives rise to a current flow known as the diffusion current.The diffusion flux obeys Ficks law where F=flux of carriers=numbers passing through 1cm2/s; D=diffusion c
12、onstant; N=carrier density. With a cross-sectional area A,the current is given by qAF so that the diffusion currents of electrons and holes are where and are the diffusion constant for electrons and holes,respectively. 在半导体中，电子和空穴的扩散起因于他们由高浓度向低浓度移动。由于电子和空穴是电荷载流子，所以他们的移动引起电流流动就是扩散电流。扩散的变化遵循菲克第一定律，其中F
13、是载流子通量；D是扩散系数；N是载流子密度。 在部分混合的区域A，电流由qAF导出所以电子和空穴的扩散电流如下，其中 和 分别是电子空穴的扩散系数。,The diffusion constant will be shown later to be related to the mobilty by Einstein relationship For the lightly doped silicon at room temperature =38cm2/s and =13cm2/s. The total electron and hole currents are obtained by ad
14、ding the drift and diffusion component These equations will be used throughout the text to characterize semiconductor devices. 扩散系数与迁移率的关系如下。 对于室温下轻参杂的硅 =38cm2/s， =13cm2/s。 总的电子空穴电流通过漂移电流和扩散电流叠加得到。 这些等式将在后面的用来描述半导体器件的特性。,Carrier Injection,In thermal equilibrium the relationship pn=ni2 is valid.If ex
15、cess carriers are introduced to a semiconductor so that pnni2,we have a none-equilibrium situation.The process of introducing excess carriers is called carrier injection.We can inject carriers by using various methods including optical excitation and forward biasing a p-n junction. 在热平衡条件下关系式pn=ni2是
16、成立的。如果过量载流子被引入半导体中导致pnni2，我们就得到了一个非平衡状态。引入过剩载流子的过程称为载流子的注入。我们能通过各种方法注入载流子包括光激发和正向偏置下的pn结。,In case of optical excitation,we shine a light on a semiconductor.If the photon energy of the light is greater than the bandgap energy of the semiconductor,where h is the Plank constant and v is the optical frequency,the photon is absorbed by semiconductor and a electron-hole pair (EHP) is generated.The optical excitation increases the electron an
