应用电子二班 156-157B26 温机问2008201226.doc

Figure4.14|Creation of excess electron and hole densities by photons. Figure 4.15|Recombination of excess carries reestablishing thermal equilibrium. A steady-state generation os excess electrons and holes will not will not cause a continual buildup of the carrier concentrations.As in the case of thermal equilibrium, an electron in the conduction band may “fall down ” into the valence band ,leading to the process of excess electron-hole recombination. Figure 4.15 shows this process. The recombination rate for excess electrons is denoted dy and for excess holes dy Both parameters have units of . The excess electrons and holes recombine in pairs， so the recombination rates must be equal. We can then write (4.53)In the direct band-to-band recombination that we are considering，the recombination occurs Spontaneously；thus，the probability of an electron and hole recombining is constant with time. The rate at which electrons recombine must be proportional to the electron concentration and must also be proportional to the bole concentration. If there are on electrons or holes,there can be no recombination. The nett rate of change in the electron concentration can be written as (4.54)Where (4.55a)and (4.55b)The first term，in Equation(4.54) is the thermal-equilibrium generation rate.Since excess electrons and holes are created and recombine in pairs, we have that . (Excwss electron and hole concentrations are equal so we can simply use the phrase excess carriers to mean either.) The thermal-equilibrium parameters, and being independent of time, Equation(4.54)becomes (4.56)Equation(4.56) can easily easily be solved if we impose the condition of low-level injection. Low-level injection puts limits on the magnitude of the excess carrier concentration compared with the thermal-equilibrium carrier concentrations. In an extrinsic n-type material, we generally have and ,in an extrinsic p-type material,We generally have . Low-level injection means that the excess carrier concentration becomes comparable to or greater than the thermal-equilibrium majority-carrier concentrations.If we consider a p-type material() under low-level injection (),then Equation(4.56) becomes (4.57)The solution to the equation is an exponential decay from the initial excess concentration, or (4.58)Where and is a constant for the low-level injection. Equation(4.58) describes the decay of excess minority carrier electrons so that isOften referred to as the excess minority-carrier lifetime.4 Excess minoritycarrier lifetime The recombination rate-which is defined as a positive quantity-of excess minority-carrier electrons can be writen, using Equation(4.57),as (4.59)In Section 4.12 we defined as mean time between collisions. We define here as the mean time before a recombination event occurs. The two parameters are not related. 译文： Figure4.14 |创作的过剩电子和光子的孔密度。 图4.15 |过剩的重组进行重新建立热平衡。 一个稳定的状态产生多余的电子和空穴的操作系统不会将不会导致承运人的浓度不断积累。由于在热平衡情况下，在导带电子可能会“倒”到价带，导致过剩的电子空穴复合过程。图4.15显示了这个过程。对于过量电子重组率表示颐和这两个参数下道溢有个单位超额孔。多余的电子和空穴在对重组，使重组率必须是平等的。然后，我们可以写 (4.53) 在直接带至波段重组，我们正在考虑，在重组发生自发性，因此，一个电子和空穴重组的概率随时间不变。其中电子的速率必须重组是成正比的电子浓度和还必须是成正比的树干浓度。如果对电子或孔，不可能有重组。 这种变化实价率在电子浓度可写为 (4.54)凡 (4.55a)和 (4.55b)第一项，个方程，（4.54）是热平衡产生率。由于多余的电子和空穴的创建和在对重组，我们有。（溢电子和空穴浓度是相等的，所以我们可以简单地使用多余的词语意味着运营商任。）热平衡参数，和正在与时间无关，方程（4.54）变为 (4.56)方程（4.56）可以很容易很容易得到解决，如果我们征收的低水平注入条件。低水平注入穿的大小限制过剩的载流子浓度比热平衡载流子浓度。在一个外在的n型材料，我们一般有 ，并以一种外在的P型材料，我们一般有。低水平注入也就是说，成为多余的载流子浓度相当于或大于热平衡多数载流子浓度。 如果我们考虑一个P型材料()下低水平注入()，然后方程（4.56）变为 (4.57)对方程的解是一个指数