1 SEMICONDUCTORS Semiconductor atomic structure

2 SEMICONDUCTORS We are going to look at the basic structure of an atom and compare and contrast the properties of an element that makes it a good conductor, insulator, or semi- conductor Then we will look at how current is produced in a semiconductor We are going to look at the basic structure of an atom and compare and contrast the properties of an element that makes it a good conductor, insulator, or semi- conductor Then we will look at how current is produced in a semiconductor

3 SEMICONDUCTORS Electrons, protons, and neutrons of atoms are thought to be arranged in a manner similar to a miniature solar system. Electrons have a negative charge, protons have a positive charge and neutrons are neutral, all atoms are comprised of at least two of these three sub atomic particles Electrons, protons, and neutrons of atoms are thought to be arranged in a manner similar to a miniature solar system. Electrons have a negative charge, protons have a positive charge and neutrons are neutral, all atoms are comprised of at least two of these three sub atomic particles Hydrogen atom

4 SEMICONDUCTORS Notice the helium atom in the figure, protons and neutrons form the heavy nucleus with a positive charge around which the electrons revolve. The path each electron takes around the nucleus is called an orbit Notice the helium atom in the figure, protons and neutrons form the heavy nucleus with a positive charge around which the electrons revolve. The path each electron takes around the nucleus is called an orbit

5 SEMICONDUCTORS The orbiting electrons do not follow random paths, instead they are confined to definite energy levels. Visualize these levels as shells with each successive shell being spaced a greater distance from the nucleus. The orbiting electrons do not follow random paths, instead they are confined to definite energy levels. Visualize these levels as shells with each successive shell being spaced a greater distance from the nucleus.

6 SEMICONDUCTORS The outer shell of an atom is called VALENCE SHELL, and the electrons contained in this shell are called VALENCE ELECTRONS. The valence shell of an atom determines its ability to gain or lose an electron, which in turn determines the electrical properties of the atom. The outer shell of an atom is called VALENCE SHELL, and the electrons contained in this shell are called VALENCE ELECTRONS. The valence shell of an atom determines its ability to gain or lose an electron, which in turn determines the electrical properties of the atom.

7 SEMICONDUCTORS Conductor: A material in which the outer electrons (valence) of the atoms are loosely bound and free to move through the material. All conductors contain electric charges which will move when an electric potential difference (volts) is applied across separate points on the material Simply stated, most metals are good electrical conductors, most nonmetals are not. Conductor: A material in which the outer electrons (valence) of the atoms are loosely bound and free to move through the material. All conductors contain electric charges which will move when an electric potential difference (volts) is applied across separate points on the material Simply stated, most metals are good electrical conductors, most nonmetals are not.

8 SEMICONDUCTORS Insulator: a material in which the outer electrons of the atoms or tightly bound and there is essentially zero electron flow through them. Glass, ceramic, and mica are good insulators. Insulator: a material in which the outer electrons of the atoms or tightly bound and there is essentially zero electron flow through them. Glass, ceramic, and mica are good insulators.

9 SEMICONDUCTORS A simple model of the copper atom looks like this:

10 SEMICONDUCTORS Copper atom Copper atom

11 SEMICONDUCTORS 2)

12 SEMICONDUCTORS 3)

13 SEMICONDUCTORS Remember, the valence electrons are weakly attracted to the nuclei. Remember, the valence electrons are weakly attracted to the nuclei. copper’s single valence electron makes it a good conductor

14 SEMICONDUCTORS The rule of eight states that a material like this would be stable since it’s valence orbit is full, it holds the maximum number of electrons in it’s valence shell It acts as an electrical insulator The rule of eight states that a material like this would be stable since it’s valence orbit is full, it holds the maximum number of electrons in it’s valence shell It acts as an electrical insulator

15 SEMICONDUCTORS A semiconductor is a material that is between conductors and insulators, they have electrons in their valence orbit.

16 SEMICONDUCTORS When an electron in the valence shell receives enough additional energy, it can leave the valence band, and become a free electron. A free electron exists in what is known as the conduction band. The difference in the energy levels of these bands is called an energy gap. When an electron in the valence shell receives enough additional energy, it can leave the valence band, and become a free electron. A free electron exists in what is known as the conduction band. The difference in the energy levels of these bands is called an energy gap.

17 SEMICONDUCTORS Here we have a diagram showing energy band gaps.

18 SEMICONDUCTORS The silicon atom has four valence electrons and feels an attractive force of +4. The copper atom has one valence electron and feels an attractive force of +1. The silicon atom has four valence electrons and feels an attractive force of +4. The copper atom has one valence electron and feels an attractive force of +1.

19 SEMICONDUCTORS The copper atom has four electron shells compared to the three shells in the silicon atom.

20 SEMICONDUCTORS Notice that both atoms have four electrons in their outer shell (valence). Silicon has three shells, germanium has four. Which valence electrons are easier to remove? Notice that both atoms have four electrons in their outer shell (valence). Silicon has three shells, germanium has four. Which valence electrons are easier to remove?

21 SEMICONDUCTORS Each silicon atom will position itself adjacent to four silicon atoms. By sharing electrons with its neighbors, the silicon atoms create a lattice that provides eight shared electrons with each atom. Each silicon atom will position itself adjacent to four silicon atoms. By sharing electrons with its neighbors, the silicon atoms create a lattice that provides eight shared electrons with each atom.

22 SEMICONDUCTORS The sharing of electrons produces a covalent bond that holds the atoms together.

23 SEMICONDUCTORS When sufficient thermal energy is applied, valence electrons can jump the gap from the valence band to the conduction band, becoming conduction electrons. When the electron jumps the band, it creates a hole. When sufficient thermal energy is applied, valence electrons can jump the gap from the valence band to the conduction band, becoming conduction electrons. When the electron jumps the band, it creates a hole.

24 SEMICONDUCTORS Together this is known as an electron-hole pair. Recombination occurs when the free electron loses energy and falls back into a hole. Together this is known as an electron-hole pair. Recombination occurs when the free electron loses energy and falls back into a hole.

25 SEMICONDUCTORS When voltage is applied to the semiconductor, the free electrons are attracted toward the positive terminal, this is the electron current. Other electrons can now move into the hole created, causing the hole to move toward the negative terminal, this is the hole current. When voltage is applied to the semiconductor, the free electrons are attracted toward the positive terminal, this is the electron current. Other electrons can now move into the hole created, causing the hole to move toward the negative terminal, this is the hole current.