8. Choose as many words from the table of ex. 1 as you can and form sensible sentences.

For example: Each gallium atom will attempt to form a covalent bond with each of its four neighboring silicon atoms.

Specialist reading

11. Read the text “Semiconductors of n-type and p-type”. Mark the following sentences as true (t) or false (f).

1. If we are interested in creation N-type or P-type conduction of a semiconductor material, we will use chemical elements of group III and V of Periodic Table.

2. The minority charge carriers of a P-type semiconductor are holes.

3. Each atom of arsenic introduces one hole into the molecular structure of silicon.

4. Under special conditions silicon can be a semiconductor of either P-type or N-type.

5. The process of the formation of covalent bonds between atoms of the specimen and an impurity requires a great amount of doping material.

6. Gallium is a donor impurity.

7. In an N-type semiconductor the process of recombination occurs very often.

Semiconductors of n-Type and p-Type

If we add a small amount of arsenic to silicon, several valence electrons of each arsenic atom will form covalent bonds with the neighbouring atoms of silicon. But only four electrons of each atom can take part in this process, because the fifth electron has no pair. As it is attached to the arsenic nucleus very loosely, it easily leaves the atom of arsenic and becomes a free electron. Thus each atom of arsenic will add one negative charge carrier to the silicon crystal and the resulting semiconductor will be called an N-type semiconductor. In this type of semiconductor electrons are majority charge carriers. Compared to the electrons the holes are in the minority in this semiconductor and they are called minority charge carriers. Sometimes an electron and a hole meet and recombination takes place.

If instead of arsenic, a group III element such as gallium is introduced into a silicon crystal, then each gallium atom will attempt to form a covalent bond with each of its four neighbouring silicon atoms. But gallium has only 3 valence electrons and only 3 bonds can be formed. Thus each atom of gallium introduces one hole into the crystal lattice. In this case holes are majority charge carriers. This type of impurity is called an acceptor impurity and the resulting semiconductor is known as a P-type semiconductor.

11. Read the text again and select from the multiple choice the only solution to each problem.

1. If you want to get a semiconductor of P-type, what impurity material will you introduce into a silicon crystal?

1. phosphorus c) arsenic

2. antimony d) gallium

2. What valency does silicon have?

1. It is univalent c) trivalent

2. bivalent d) tetravalent

3. How many electrons may a pentavalent material introduce into the crystal lattice of silicon?

1. 0 c) 2

2. 1 d) 3

4. Characterize the bond between the arsenic nucleus and the electron, which is going to free itself.

1. strong c) loose

2. very strong d) very loose

5. According to Reserphord an atom of each substance consists of three elements, but the fourth element, which has a great role in semiconductor conductivity is mentioned in the text. Mark the odd word from the list below.

1. proton c) hole

2. neutron d) electron

6. If we need a P-type semiconductor, we will use an impurity called …

1. donor c) extrinsic

2. acceptor d) intrinsic

7. Which chemical element is not mentioned in the text?

1. silicon c) gallium

2. aluminium d) arsenic

11. Read the text “Semiconductors of N-type and P-type” again and answer the following questions.

1. How many valency electrons should an acceptor impurity have?

2. Why are semiconductors called N-type and P-type?

3. What chemical properties does silicon have?

4. Why do we usually use the elements of group III and V as impurities for silicon crystal to cause a certain type of conductivity?

5. Is the process of recombination possible in an N-type semiconductor? Why?

6. Which types of charge carriers have you met in the text?

7. How do chemists and physicists call the bonds between impurity electrons and neighbouring atoms of the material?

8. What does the valency of an element show us in general?

9. Can you name some other chemical elements which may be used as impurities?

10. Is it correct to use the terms majority/minority charge carriers considering pure silicon? Why?