2015-08-21
741
Read the text “Electrons and Holes” only once. How much can you remember? Answer these questions without additional reading.
1. What takes place in an atom when one electron frees itself from a covalent bond?
2. Is the hole able to attract electrons?
3. Can you define the process of recombination?
4. Can you say anything about the rate of recombination?
5. What is intrinsic semiconductor?
If you failed try to answer these questions again after doing the exercises given below the text.
If the temperature of a silicon crystal is above absolute zero, some of the valence electrons will receive sufficient energy to free themselves from covalent bonds.
When an electron leaves the covalent bond it leaves behind it an ‘absence of an electron’. This absence of an electron consists of a missing negative charge ‘e’, and it is equivalent to a positive charge called ‘a hole’.
Each hole is able to attract electrons and can be filled by a passing electron. This process is called recombination. Recombination causes a continual loss of holes and free electrons. The rate of recombination of holes and electrons is always equal to the rate of production of new holes and electrons. The total number of free electrons and holes is constant. The material having equal concentrations of mobile electrons and holes is called an intrinsic semiconductor.
14. Read the text again and complete the sentences.
1. The rate of recombination of holes and electrons is always equal to ….
2. … causes a continual loss of holes and free electrons.
3. The material having equal concentrations of mobile electrons and holes is called ….
4. If the temperature of a silicon crystal is above absolute zero, ….
5. … is equivalent to a positive charge called ‘a hole’.
6. The total number of free electrons and holes is ….
7. … is able to attract electrons and can be filled by a passing electron.
8. The absence of an electron consists of ….
9. When an electron leaves the covalent bond it leaves ….
Match the terms in Table A with their definitions in Table B.
| Table A | Table B |
| 1. rate 2. magnitude 3. hole 4. loss 5. mobility | a. the speed at which something happens over a period of time. b. the great size or importance of something c. an empty space in something solid. d. the fact of no longer having something, or of having less of it than you used to have, or the process by which this happens e. the ability to move easily |
SPEAKING
16. Summarize the text “ Electrons and holes” in 150 words.
