Reading Materials
Band Structure of Selected Semiconductors
In the following，the band structures of various important and prototype semiconductors are discussed. 
The band below the fundamental energy gap is called the valence band，the band above it the conduction band. The bandgap is the energy separation between the highest valence-band state and the lowest conduction-band state. The maximum of the valence band is for most semiconductors at the Γ point.
1. Silicon
For silicon，an elemental semiconductor，（Fig.3.2）the minimum of the conduction band is located close to the X point at 0.85π/a in the 〈100〉 direction. Thus，it is not at the same point in k-space as the top of the valence band. Such a band structure is called indirect . Since there are six equivalent 〈100〉 directions，there are six equivalent minima of the conduction band.
2. GaAs
GaAs（Fig.3.3）is a compound semiconductor with a direct bandgapsince the top of the valence band and the bottom of the conduction band are at the same position in k-space（at the Γ point）. The next highest（local）minimum in the conduction band is close to the L point.
Fig.3.2 Band structure of silicon（indirect）.
Fig.3.3 Band structure of GaAs（direct）.
Words and Expressions
photoelectron n. 光电子
photon n. 光子
postulate vt. 假设，假定
conjugate v. 使（动词）变化，变化；
adj. 相关的
enormous adj. 巨大的
tremendously adv. 非常地
intuitive adj. 直觉的
quasiparticle n. 准粒子
collide vi . 碰撞
sphere n. 球体
covalent adj. 共有原子价的，共价的
vibration n. 振动
phonon n. 声子
testimonial n. 证明书，推荐书
ubiquitousness n. 到处存在的，普遍存在的
postulate n. 假定，基本条件，基本原理
conduction band n. 导带
Glossary of Important Term
energy quanta 能量量子化
wave-particle duality principle 波粒二相性原理
uncertainty principle 不确定原理
Schrodinger’s wave equation 薛定谔波动方程
bandgap 禁带，带隙
valence band 价带
electron effective mass 电子有效质量
hole effective mass 空穴有效质量
indirect bandgap semiconductor 间接带隙半导体
direct bandgap semiconductor 直接带隙半导体
Exercises
1. Translate the first 3 paragraphs in reading material into Chinese.
2. Answer the following questions in English.
（1）What is effective mass？
（2）What is a direct bandgap semiconductor？What is an indirect bandgap semiconductor？