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 the 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 multiple choice the only solution to each problem.

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

1. phosphorus

2. antimony

3. arsenic

4. gallium

2. What valency does silicon have?

1. I

2. II

3. III

4. IV

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

1. 0

2. 1

3. 2

4. 3

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

1. strong

2. very strong

3. loose

4. very loose

5. According to Reserphord 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

2. neutron

3. hole

4. electron

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

1. donor

2. acceptor

3. extrinsic

4. intrinsic

7. Which name of the chemical element is absent in the text?

1. silicon

2. aluminiium

3. gallium

4. arsenic

11. Refresh the text “Semiconductors of n-type and p-type” in your memory and answer some questions.

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

2. What is the cause of the names of the notions N-type and P-type?

3. What chemical structure does silicon have?

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

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

6. Which types of charge carriers have you known from the text? Can you name them?

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 correctly to use the determinations the majority or minority charge carriers considering pure silicon? Why?