Writing

Speaking

3. With your group, study the energy band structures in Fig. 1.3 and try to explain the difference between the energy bands in metals, semiconductors and insulators. Use such patterns to help you:

· There is / are…

· The band gap is…

· The valence band / conduction band is unfilled / filled / partially filled / completely filled with electrons.

· The Fermi level is

· A semiconductor has a band gap between…

Note

The Fermi Level is the highest energy level which an electron can occupy at the absolute zero temperature. Levels of lower energy than the Fermi level tend to be entirely filled with electrons, whereas energy levels higher than the Fermi tend to be empty.

Fig. 1.3. Energy band structures in metals, semiconductors and insulators.

4. With your group, fill in the column about the valence band with the sentences antonymous to those in the column dealing with the description of the conduction band.

When two atoms are brought nearer to each other, there are alterations in energy levels and they spread in the form of bands. Energy bands are of the following types.

Valence band (VB)	Conduction band (CB)	Forbidden energy gap (Eg)
· · · ·	· This band is partially filled with electrons. · It is also called empty band of minimum energy. · In this band the electrons can gain energy from external electric field. · Current flows due to such electrons.	· There are no free electrons. · The width of a forbidden energy gap depends on the nature of a substance. · As temperature increases, the forbidden energy gap decreases very slightly. · It is also called band gap or energy gap.

5. With your group, write a storyline on the topic: “How semiconductors work”. Use all the information from the previous tasks.

Procedure:

Each group has a sheet of paper. One group member writes the first sentence of a story at the top of the page and passes it along to the next person in the group. That person reads the first sentence and adds one more to it to continue the story. Each group member should work on different stages. Check and correct each other’s work before you finalize the complete story.

Дата добавления: 2014-11-29