Reading Materials

Degenerate and Nondegenerate Semiconductors

In our discussion of adding dopant atoms to a semiconductor，we have implicitly assumed that the concentration of dopant atoms added is small when compared to the density of host or semiconductor atoms. The small number of impurity atoms are spread far enough apart so that there is no interaction between donor electrons，for example，in an n-type material. We have assumed that the impurities introduce discrete，noninteracting donor energy states in the n-type semiconductor and discrete，noninteracting acceptor states in the p-type semiconductor. These types of semiconductors are referred to as nondegenetate semiconductors .

If the impurity concentration increases，the distance between the impurity atoms decreases and a point will be reached when donor electrons，for example，will begin to interact with each other. When this occurs，the single discrete donor energy will split into a hand of energies. As the donor concentration further increases，the band of donor states widens and may overlap the bottom of the conduction band. This overlap occurs when the donor concentration becomes comparable with the effective density of states. When the concentration of electrons in the conduction band exceeds the density of states Nc，the Fermi energy lies within the conduction band. This type of semiconductor is called a degeneraten-type semiconductor .

In a similar way，as the acceptor doping concentration increases in a p-type semiconductor，the discrete acceptor energy states will split into a band of energy and may overlap the top of the valence band. The Fermi energy will lie in the valence band when the concentration of holes exceeds the density of states Nv，This type semiconductor is called a degenerate p-type semiconductor.

Schematic models of the energy-band diagrams for a degenerate n-type and degenerate p-type semiconductor are shown in Fig.4.6. The energy states below are mostly filled with electrons and the energy states above EF are mostly empty. In the degenerate n-type semiconductor，the states between EF and Ec are mostly fill with electrons；thus，the electron concentration in the conduction band is very large. Similarly，in the degenerate p-type semiconductor，the energy states between Ev and EF，are mostly empty；thus，the hole concentration in the valence band is very large.

Words and Expressions

intrinsic adj. 本征的

extrinsic adj. 非本征的

discrete adj. 不连续的，分立的

Glossary of Important Term

cyclotron resonance experiments 回旋共振实验

density of states 态密度

Fermi distribution 费米分布

effective density of states function in the conduction band 导带有效态密度

ionization energy 离化能

effective density of states function in the valenceband 价带有效态密度

donor 施主

acceptor 受主

nondegenetate semiconductor 非简并半导体

degenerate semiconductor 简并半导体

Exercises

1. Translate the first 2 paragraphs in the reading material into Chinese.

2. Answer the following questions in English.

（1）Under what condition would the intrinsic Fermi level be at the midgap energy？

（2）What is a donor impurity？What is an acceptor impurity？

（3）Describe the meaning of degenerate and nondegenerate semiconductors.