微电子英语ppt.pptVIP

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 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 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 pigments.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 movements,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 lighter 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 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