Chapter 29 Solid State Electronics

1. Chapter 29 Solid State Electronics

2. Objectives • 29.1Compare and contrast n-type and p-type semiconductors • 29.1 Describe electron motion in conductors and semiconductors • 29.2 Describe how diodes limit current to motion in only one direction • 29.2 Explain how a transistor can amplify or increase voltage changes

3. Conduction in Solids • Review • Conductors (easily allow electrons or electrical charge to move) • Insulators (resist electron and electrical charge movement) • Electrons are only allowed to occupy certain energy levels • Under most conditions, electrons are found in the lowest available energy level (ground state)

4. Conduction in Solid • What happens to the energy levels as atoms come close to one another? • The energy levels are raised or lowered as they interfere with one another

7. (a) the interatomic spacing between two atoms is reduced • (b) the electron wave function of five atoms interact • (c) in the formation of a continuous band of allowed energies the number of interacting atoms is very large.

14. Terms • Forbidden Gaps: The bands of energy which no electrons possess • Valence Band: The outermost band that contains electrons • Conduction Band: The lowest energy band that is not filled to capacity

15. Conduction Band • If a potential difference (voltage) is placed across a material, then the electrons are able to accelerate and gain energy • If the potential energy is large enough, the gap is bridged and the electrons go from one atom to the next (current)

16. How many free electrons? • How many valence electrons determines that amount of free electrons • Sodium (1st Column, Alkali Metal) has one free electron. It has 11 electrons total, but only 1 electron is in the valence band • Most metals provide only 1 or 2 free electrons

17. Insulator/Semiconductor/Conductor • How big is the gap? • Insulator will be larger (typically 5 to 10 eV) • Semiconductor (1 eV or less) • Conductor (what gap?!)

18. The values are for individual atoms • As the atoms come together, the gaps between the bands reduces • Reason: Electrons in different atoms push the other electrons closer

19. Insulators • At room temperature, no electrons have enough kinetic energy (as a result of their thermal energy) to jump the forbidden gap. • Even small electric fields are usually not sufficient to cause them to conduct electricity • Very HIGH ionization energies

20. Semi-Conductors • At room temperature, a few electrons are able to jump the forbidden gap • A small electric field is able to make more electrons jump the gap, so the current increases • As temperature increases, conductivity goes up so you have more current

21. Metals • Room temperature is more than enough energy to move electrons around. • What forbidden gap?! The electron goes and pleases as he wants • Different than semi-conductors, increasing temperature means less current due to increase in collisions

22. Holes • When an electron jumps, it leaves a positive hole where the electron used to be

23. Holes • An electron from another atom can jump over, but ultimately leaves a hole where he was • Musical Chairs • The negative goes one way, positive other way

24. Types of Semi-Conductors • Intrinsic: Are pure and conduct electricity in very tiny quantities at room temperature (very high resistance) • Extrinsic: Impurity atoms added which increase the conductivity by either adding more electrons (n-doping) or more holes (p-doping)

25. P-Doping and N-doping • P = Add a hole (less electrons than your semi-conductor) • N = Add an electron (more electrons)

26. P- Doping and N-Doping

27. Periodic Table Connection

28. How conductive do you want it? • Vary the amount of impurities • Even just a few different atoms can change the conductivity by factors as large as a 1000

29. Thermistors • Resistance depends highly on temperature • Used as a very sensitive thermometer • Used to compensate for temperature variations of components in an electrical circuit • Used to measure power of light sources

30. Diodes • Consists of a joined p-doped and n-doped semiconductor • Boundary between is called a pn-junction diode

31. Diodes • The free electrons on the n-side are attracted to the p-side (holes) • Thus, the electrons move easily from the n-side to the p-side, but not the other way

32. Diodes • Region in the middle with no charge carriers is called the depletion layer • So, two ends are great conductors with middle being an insulator

33. No Current

34. Purpose for diodes • Current in one direction (turns AC into DC)

35. Transistor • pnp • npn • Middle is base • ends are collectors and emitters

36. Purpose for Transistor • Used as off/on switches • Modify voltages (amplify) • Store information (as an off/on switch)