Chapter 1

1. Chapter 1 Fundamental Solid-State Principles

2. Semiconductor • Semiconductor - An element that is neither an insulator nor a conductor.

3. The Atom • The nucleus contains protons and neutons. • Electrons orbit the nucleus in orbital paths, called shells. • The outermost shell is called the valence shell.

4. Semiconductors • Semiconductors contain four valence-band electrons. • Three common semiconductor elements are silicon (Si), germanium (Ge), and carbon (C).

5. Electrons and Orbital Shells • Energy gap – The difference between the energy levels of any two orbital shells. • Band – Another name for an orbital shell. • Electron-volt (eV) – The energy absorbed by an electron when it is subjected to a 1 V difference of potential. • Conduction band – The band outside the valence shell.

6. Covalent Bonding • A means of holding atoms together by sharing valence electrons. • The center atom (at right) is electrically stable because its covalent bond is complete. • Intrinsic (pure) silicon is a poor conductor.

7. Electron-Hole Pair • Electron-hole pair – A free electron and its matching valence band hole. • Recombination – The return of a free electron to the valence shell. • Lifetime – The time from the generation of an electron- hole pair until recombination occurs.

8. Doping • Trivalent element – One that has three valence electrons. • p-type material – A semiconductor that has added trivalent impurities. • Pentavalent element – One that has five valence electrons. • n-type material – A semiconductor that has added pentavalent impurities. The process of adding impurity elements to intrinsic semiconductors.

9. n-Type Material • A semiconductor that has added pentavalent impurities. • The pentavalent atom (As) has a fifth valence electron that is not a part of the covalent bond. • Relatively little energy is required to force the excess electron into the conduction band.

10. n-Type Material Energy Diagram • The conduction band electrons outnumber the valence band holes. • Conduction-band electrons are the majority carriers. • Valence-band holes are the minority carriers. • The material contains the same overall number of protons and electrons, so it remains electrically neutral.

11. p-Type Material • A semiconductor that has added trivalent impurities. • The bond requires one more valence electron than the trivalent atom (Al) is capable of providing. • The electron shortage results in the bond having a valence-band hole.

12. p-Type Material Energy Diagram • The valence-band holes outnumber the conduction-band electrons. • Valence-band holes are the majority carriers. • Conduction-band electrons are the minority carriers. • The material contains the same overall number of protons and electrons, so it remains electrically neutral.

13. The pn Junction • When the junction is formed, free-electrons in the n-type material diffuse (wander) across the junction to the p-type material. • An electron crossing the junction into the p-type material gets trapped in a valence-band hole.

14. The Depletion Layer • The diffusion of an electron from the n-type material to the p-type material results in: • One net positive charge in the n-type material. • One net negative charge in the p-type material. • On a larger scale, the area surrounding the junction is depleted of charge carriers. This is the depletion layer. • The difference of potential between the two sides of the junction is called the barrier potential.

15. Bias A potential applied to a pn junction to obtain a desired mode of operation. • Forward bias – A potential used to reduce the resistance of a pn junction. • Reverse bias – A potential used to increase the resistance of a pn junction.

16. Forward Bias A pn junction is forward biased when the applied potential causes the n-type material to be more negative than the p-type material.

17. Bulk Resistance The combined resistance of the n-type and p-type materials in a forward-biased pn junction. RB = Rp+ Rn

18. Forward Voltage • Forward voltage (VF) is the voltage across a forward biased pn junction. • VF is greater than the junction barrier potential. VF 0.7 V (for silicon) VF 0.3 V (for germanium) • Any differences between the measured and approximated values of VF result primarily from the current through the diode bulk resistance.

19. How to Forward Bias a pn Junction • Apply a potential to the n-type material that drives it more negative than the p-type material. • Apply a potential to the p-type material that drives it more positive than the n-type material.

20. Reverse Bias • A pn junction is reverse biased when the applied potential causes the n-type material to be more positive than the p-type material.

21. How to Reverse Bias a pn Junction • Apply a potential to the n-type material that drives it more positive than the p-type material. • Apply a potential to the p-type material that drives it more negative than the n-type material.

22. Bias Polarities and Effects Bias TypeJunction PolaritiesJunction Resistance Forward n-type material is Extremely low more negative than p-type material. Reverse p-type material is Extremely high more negative than n-type material.