How semiconductors work

Start here

1. Work in small groups. Study the electron orbital zone in Fig.1.2 and discuss these questions.

Fig. 1.2. Electron orbital zone.

1) Does the information depicted in the diagram have any connection with our previous lessons?

2) How do you think it works?

Reading

2. With your group, read the text, divide it into parts and entitle them. Come up with the headline. Answer the questions below.

To understand how semiconductors work, you must first understand a little about how electrons are organized in an atom. The electrons in an isolated atom are organized in energy levels. The outermost level is called the valence level. The electrons in this level are the ones, known as valence electrons that form bonds with neighboring atoms. Such bonds are called covalent bonds. If all the neighboring atoms are of the same type, it's possible for all the valence electrons to bind with valence electrons from other atoms. When that happens, the atoms arrange themselves into structures called crystals. Semiconductors are made out of such crystals, usually silicon crystals.

Atoms within a crystal have a marked effect upon each other. The forces that bind these atoms together greatly modify the behavior of the other electrons. One consequence of this close proximity of atoms is to cause the individual energy levels of an atom to break up and form bands of energy. If for isolated atoms we have energy levels, for big spread molecules, macromolecules, and also crystals we have energy bands.

The energy band formed by a series of energy levels containing valence electrons is known as valence band. Electrons in this band are more tightly bound to the individual atom than the electrons in the conduction band. However, the electrons in the valence band can still be moved to the conduction band with the application of energy, usually thermal energy.

The upper band in the solid is called the conduction band because electrons in this band are easily removed by the application of external electric fields. These electrons are free enough to move and thereby carry an electric current.

The energy difference between a valence band and a conduction band is called the forbidden band. Electrons are never found in this band, but may travel back and forth through it. Technically, the band gap is the energy it takes to move electrons from the valence band to the conduction band. The width of the forbidden band determines whether a substance is an insulator, semiconductor, or conductor.

Questions:

1) How are electrons in an isolated atom organized?

2) How is the outermost energy level in an isolated atom called?

3) What electrons form bonds with neighboring atoms? How are such bonds called?

4) When do atoms arrange themselves into crystals?

5) Are semiconductors made out of isolated atoms?

6) How are atoms arranged in crystals and molecules?

7) What is the difference between a valence band and a conduction band?

8) What is a forbidden band?

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.

Writing

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.