Recombination phenomena can be classified as direct and indirect processes . Direct recombination , also called band-to-band recombination , usually dominates in direct-bandgap semiconductors , such as gallium arsenide ; while indirect recombination via bandgap recombination centers dominates in indirect bandgap semiconductors , such as silicon.
2.5 Generation and Recombination Processes 载流子产生与复合过程
Whenever the thermal-equilibrium condition is disturbed (i.e.,pn ≠ ni2) , processes exist to restore the syetem to equilibrium(i.e.pn=ni2).In the case of the injection of excess carriers, the mechanism that restores equilibrium is recombination of the injected minority carrier with the majority carriers. Depending on the nature of the recombination process, the released energy that results from the recombination process can be emitted as a photon or dissipated as heat to the lattice. When a photon is emitted , the process is called radiative recombination ;otherwise, it is called nonradiative recombination. 当热平衡条件受到扰乱时（即pn ≠ ni2），会出现一些使系统回复平衡的机制（即 pn=ni2 ）。在超量载流子注入的情形下，回复平衡的机制是将注入的少数载流子 与多数载流子复合。视复合过程的本征而定，复合过程所释放出的能量，一般以 光子形式辐射出或是对晶格产生热而消耗掉。一个光子被辐射出，此过程称为辐 射复合，反之则称为非辐射复合。
当电子从导带向下移到价带， 一个电子 - 空穴对消失。这 种反向过程称为复合，并以 复 合 率 Rth 表 示 ， 如 图 2.11 （ a ）所示。在热平衡状态 下，产生速率 Gth 必定等于 复合率 Rth ，所以载流子浓 度维持常数，且维持 pn=ni2 的状况。
When excess carries are introduced to a direct-bandgap semiconductor, the probability is high that electrons and holes will recombine directly ,because the bottom of the conduction band and the top of the valence band are lined up and no additional crystal momentum is required for the transition across the bandgap . The rate of the direct recombination R is expected to be proportional to the number of holes available in the valence band; that is R=βnp where β is the proportionality constant.
复合现象可分为直接和间接过程。直接复合，又称为带至带复合，通常在直 接禁带的半导体中较为显著，如砷化镓；而通过禁带复合中心的间接复合则 在间接禁带的半导体中较为显著，如硅晶。
Consider a direct-bandgap semicond uctor in thermal equilibrium . The continuous thermal vibration of lattice atoms causes some bonds between neighboring atoms to be broken . When a bond is broken ,an electron-hole pair is generated. In terms of the band diagram, the thermal energy enables a valence electron to make an upward transition to the conduction band leaving a hole in the valence band. This process is called carrier generation and is represented by the generation rate Gth(number of electron-hole pairs generated per cm3 per second) in Fig.2.11(a).
考虑一个在热平衡状态下的直接禁带半导体。 晶格原子连续的热扰动造成邻近原子间的键 断裂。当一个键断裂，一对电子空穴对就产 生了。以能带图的观点而言，热能使得一个 价电子向上移到导带，而留下一个空穴在价 带。这个过程称为载流子产生，并以产生速 率 Gth （每立方厘米每秒产生的电子 - 空穴对 数目）表示，如图2.11（a）所示。
Therefore , the net recombination rate is proportional to the excess minority carrier concentration .The proportionality constant 1/β nno is called the lifetime Tp of the excess minority carriers . The physical meaning of lifetime can best be illustrated by the transient response of a device after the sudden removal of the light source . Consider an n-type sample, as shown in Fid.2.12(a) ,that is illuminated with light and in which the electron-hole pairs are generated uniformly throughout the sample with a generation rate GL. Fig.1.12(b) shows the variation of pn with time. The minority carriers recombine with majority carriers and decay exponentially with a time constant Tp which corresponds to the lifetime. 因此，净复合率正比于超量少数载流子浓 度。比例常数1/β nno称为超量少数载流子 的寿命Tp。寿命的物理意义可通过器件在 瞬间移去光源后的暂态响应作最好的说明。 考虑一个n型样品，如图2.12（a），光照 射其上且整个样品中以一个产生速率 GL 均匀地产生电子-空穴对。图2.12（b）显 示pn随时间的变化。少数载流子与多数载 流子复合，且寿命Tp成指数衰减。
如前面所讨论的，在热平衡下 复合率必定与产生速率保持平 衡，因此，对以n型半导体而言， 我们可以得到 G th=R th=βn no p no 。nno及pno分别表示在热 平衡下，n型半导体中的电子及 空穴浓度。我们在半导体上照 光，使它以GL的速率产生电子空穴对（如图2.11（b）），载 流子浓度将大于平衡时的值。
The above case illustrates the main idea of measuring the carrier lifetime using photoconductivity method . Fig.2.12(c) shows a schematic setup .The excess carriers ,generated uniformly throughout the sample by the light pulse , cause a momentary increase in the conductivity . The increase in conductivity manifests itself by a drop in voltage across the sample when a constant current is passed through it . The decay of the conductivity can be observed on an oscilloserved on an oscilloscope and is a measure of the lifetime of the excess minority carriers. 以上情形说明使用光电导方法来测量 载流子寿命的主要概念。如图2.12（c） 显示一个图示的装置。通过光脉冲照 射，整个样品中均匀产生超量载流子， 因而造成电导率瞬间增加。而电导率 的增加，可由将一定电流通过样品使 样品两端产生一小电压而显示出来。 电导率的衰减可由示波器上观察得知， 它同时又是测量超量少数载流子寿命 的一种方法。