Reading Materials

Why do the deep levels act as effective centres of recombination？

Let us imagine an electron and hole wandering in the crystal. In order to meet，recombine and disappear，it is necessary that they should be close to each other in the vicinity of one and the same atom of the crystal lattice. Such a situation is，in general，possible but seldom occurs.

Let us now assume that there is an impurity centre in the crystal whose activation energy ΔE is great. Should an electron appear in the vicinity of this centre，it is sure to be trapped by the impurity centre.

The centre will keep the electron trapped until a hole appears in the vicinity. As soon as that happens the electron and hole recombine. The“killer” has committed this task of annihilating the electron-hole pair，and is ready to start all over again.

Sometimes it is important for electrons and holes to perish in the device as soon as possible. It is often quite essential for the fast switching of semiconductor devices. Then impurities creating effective recombination centres should be incorporated into the material. Sometimes，on the contrary，electrons and holes must live long. In this case，the semiconductor is to be thoroughly purified.

Words and Expressions

constitute vt. 制定（法律），建立（政府），组成，任命

annihilate vt. 消灭

Glossary of Important Term

ambipolar transport 双极输运

excess carriers 过剩载流子

nonequilibrium excess charges 非平衡过剩载流子

generation 产生

recombination 复合

carrier injection 载流子注入

band-band recombination 带间复合（直接复合）

spontaneous recombination 自发复合

absorption 吸收

stimulated emission 受激发射

Auger recombination 俄歇复合

impact ionization 碰撞离化

band-impurity recombination 能带—杂质能级复合（SRH复合）

surface recombination 表面复合

majority carriers 多子，多数载流子

minority carriers 少子，少数载流子

minority carrier lifetime 少子寿命

Exercises

1. Translate the reading material into Chinese.

2. Answer the following questions in English.

（1）Describe the concept of excess generation and recombination.

（2）Why are the electron generation rate and recombination rate equal in the thermal equilibrium？

（3）Describe the concept of an excess carrier lifetime.