Chapter 6. Doping and dopants (допирование и допанты)

Exercise 1. Read and translate the text, using the words below.

The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic. By adding impurity to pure semiconductors, the electrical conductivity may be varied not only by the number of impurity atoms but also, by the type of impurity atom and the changes may be thousand folds and million folds.

The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. A donor atom that activates (that is, becomes incorporated into the crystal lattice) donates weakly bound valence electrons to the material, creating excess negative charge carriers. These weakly bound electrons can move about in the crystal lattice relatively freely and can facilitate conduction in the presence of an electric field. (The donor atoms introduce some states under, but very close to the conduction band edge. Electrons at these states can be easily excited to the conduction band, becoming free electrons, at room temperature.) Conversely, an activated acceptor produces a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material's majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier.

In general, an increase in doping concentration affords an increase in conductivity due to the higher concentration of carriers available for conduction. Degenerately (very highly) doped semiconductors have conductivity levels comparable to metals and are often used in modern integrated circuits as a replacement for metal. Often superscript plus and minus symbols are used to denote relative doping concentration in semiconductors. For example, n⁺ denotes an n-type semiconductor with a high, often degenerate, doping concentration. Similarly, p^- would indicate a very lightly doped p-type material. It is useful to note that even degenerate levels of doping imply low concentrations of impurities with respect to the base semiconductor.

Words to be remembered.

impure–нечистый; с примесью

fold –складка; сгиб; фальц

acceptor–1. акцептор, акцепторная примесь 2. акцепторный уровень

donor–1. донорная примесь, донор 2. донорный уровень

charge carrier–носитель заряда

excess–избыток; излишек

edge–край; лезвие

designation–указание; назначение

superscript–1. знак сноски 2. надстрочный знак 3. показатель степени

excitation–возбуждение

Exercise 2. Answer the following questions.

1. How may the conductivity of semiconductors be easily modified?

2. What is doping?

3. Are doped semiconductors referred to as extrinsic or intrinsic?

4. What may happen if we add impurity to pure semiconductors?

5. What do the materials chosen as suitable dopants depend on?

6. How are dopants classified?

7. What produces a hole?

8. What do you know about n-type and p-type?

9. Why does an increase in doping concentration afford an increase in conductivity?

10. What is used as a replacement for metal?

11. What is used to denote relative doping concentration in semiconductors?

12. Even degenerate levels of doping imply low concentrations of impurities, don`t they?

Exercise 3. Look through the following word expressions and make up sentences of your own with them.

Amount of impurity, level of conductivity, suitable dopants, weakly bound, charge carriers, relatively freely, room temperature, majority carrier, minority carrier, thermal excitation, integrated circuits.

Exercise 4. Choose the word with the same meaning as the definition.

1. The process of adding controlled impurities to a semiconductor is known as …

a) dopant

b) doping

c) doper

2. Doped semiconductors are often referred to as …

a) extrinsic

b) intrinsic

c) offtrinsic

3. An activated … produces a hole.

a) donor

b) receiver