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1. Semiconductors Prepared by Dr . Abdelhamid Khattab Lecture 4 in Electronics ( BS1102)

2. Content The Atom Material Used in Electronics Current in Semiconductors N- Type & P- Type Semiconductors The PN Junction

3. The Atom An atom is the smallest particle of an element that retains the characteristics of that element . Each of the known 118 elements has atoms that are different from the atoms of all other element s . This gives each element a unique atomic structure.

4. The outermost occupied shell is called the valence shell and electrons that occupy this shell are called valence electrons . Shell 1 Shell 2 Nucleus The Bohr model of the atom is that electrons can circle the nucleus only in specific orbits , which correspond to discrete energy levels called shells . Energy The atomic number is the number of protons in the nucleus. Bohr Atom

5. Bohr Atom

6. Conductors Materials can be classified by their ability to conduct electricity. This ability is related to the valence electrons. Copper is an example of an excellent conductor . It has only one electron in its valence band , which can easily escape to the conduction band , leaving behind a positive ion (the core). Like all metals, copper has many free electrons which are loosely held by the attraction of the positive metal ions. +29 Core  Material Used in Electronics

7. Insulators have tightly bound electrons with few electrons available for conduction. Nonmetals , such as glass, air, paper, and rubber are excellent insulators and widely used in electronics. Even these materials can break down and conduct electricity if the voltage is high enough. Insulators Material Used in Electronics

8. Semiconductors Silicon is an example of a single element semiconductor . It has four electrons in its valence band. Unlike metals, silicon forms strong covalent bonds (shared electrons) with its neighbors. Intrinsic silicon is a poor conductor because most of the electrons are bound in the crystal and take part in forming the bonds between atoms. + 4 + 4 + 4 + 4 +4 Semiconductors are between conductors and insulators in their ability to conduct electricity. +14 Core  Material Used in Electronics

9. Material Used in Electronics

10. Material Used in Electronics

11. Band Gap When an electron acquires enough additional energy, it can leave the valence shell , become a free electron, and exist in what is known as the conduction band The difference in energy between the valence band and the conduction band is called an energy gap or band gap. Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom

12. Band Gap

13. Semiconductors In intrinsic silicon, a few electrons can jump the energy gap between the valence and conduction band. Having moved into the conduction band, a “hole” (vacancy) is left in the crystal structure. Conduction band Valence band Energy gap Hole Free electron Electron-hole pair Heat Energy Current in Semiconductors

14. Semiconductors Within the crystalline structure, there are two types of charge movement (current): 1) The conduction band electrons are free to move under the influence of an electric field. (electron Current ) Current in Semiconductors

15. Current in Semiconductors 2) The bound (valence) electrons move between atoms, effectively moving holes from one atom to another as illustrated. Holes act like positive charges, with their own mobility.

16. N- Type & P- Type Semiconductors Semi- conductive materials do not conduct current well . This is because of the limited number of free electrons in the conduction band and holes in the valence band . Semiconductors conductivity can be drastically increased by the controlled addition of impurities to the intrinsic pure.

17. Doping Process Doping process , increases the number of current carriers (electrons or holes). The two categories of impurities are n- type and p- type.

18. N- Type Semiconductors To increase the number of conduction- band electrons in intrinsic silicon, pentavalent ( نكافئ خماسى ) impurity atoms are added. These are atoms with five valence electrons such asarsenic (As), Phosphorus (P), bismuth (Bi), and antimony (Sb)

19. N- Type Semiconductors Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an n- type semiconductor ( the n stands for the negative charge on an electron )

20. P- Type Semiconductors To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga)

21. P- Type Semiconductors The p stands for the positive charge on an holes )

22. The pn junction Take a block of silicon and dope part of it with a trivalent impurity and the other part with a pentavalent impurity, a boundary called the pn junction is formed. The pn junction is the basis for diodes, certain transistors, solar cells, and other devices

23. The pn junction

24. Quiz 1. A semiconductor is a crystalline material with many free electrons held by the attraction of positive ions strong covalent bonds between neighboring atoms only one electron in its outer shell a filled valence shell

25. 1. A semiconductor is a crystalline material with many free electrons held by the attraction of positive ions strong covalent bonds between neighboring atoms only one electron in its outer shell a filled valence shell Quiz

26. Quiz 2. A metallic conductor has many free electrons held by the attraction of positive ions covalent bonds between neighboring atoms four electrons in its outer shell a filled valence shell Quiz

27. Quiz In a semiconductor, the concept of an energy gap is used to show the difference between the energies of the nucleus and outer shell electrons nucleus and the free electrons conduction band electrons and valence electrons core electrons and valence electrons Quiz

28. Quiz An impurity such as Antimony (Sb) has five electrons in its outer shell. When silicon has Sb impurities, an n material is formed the crystal will be negatively charged both of the above none of the above Quiz

29. Quiz Compared to a p - material, the energy levels in an n- material are the same greater lower Quiz

30. Quiz When a pn junction is formed, electrons move across the junction and fill holes in the p - region. The filled hole is a neutral atom minority carrier positive ion negative ion Quiz

31. Quiz Answers: b 6. d a c a c Quiz

32. eNd Next Lecture , Chapter 2 Diodes and Application