Understanding Semiconductor Basics and Diode Behavior for Circuit Analysis

1. ECE 4991 Electrical and Electronic CircuitsChapter 9

2. Where We Are • Chapter 2 - The basic concepts and practice at analyzing simple electric circuits with sources and resistors • Chapter 3 – More harder networks to analyze and the notion of equivalent circuits • Chapter 4 – Capacitors and inductors added to the mix • Chapter 5 – Analyzing transient situations in complex passive networks • Chapter 8 – New subject – the wonders of operational amplifiers as system elements • Chapter 9 – Introduction to semiconductors – the basics and diodes – more network analysis • Chapter 10 – Bipolar junction transistors and how they work – now you can build your own op amp

3. What’s Important in Chapter 9 • Definitions • Semiconductor basics • Diode behavior • Ideal diode model • Offset diode model • Diodes in circuits

4. 1. Definitions • Semiconductor • Diode • Majority Carrier • Minority Carrier • Forward bias • Reverse bias • Ideal model • Offset model

5. 2. Semiconductor Basics • Metals are a “sea of electrons” – conduct electricity extremely well • Insulators have no available charge carriers to make an electric current • Semiconductors are in between • Silicon

6. Semiconductor Basics • Silicon can be “doped” to provide charge carriers – either negative or positive • N-type or P-type • One-part-per-million doping is normal • ~ 1 x 1016 /cm3

7. What’s a diode? • A volume of n-type silicon touching a volume of p-type silicon forms a “diode” • Electric current flows easily in one direction but not in the reverse direction • This is the electronic symbol for a diode (P N)

8. Why do they work the way they do? P side N side Which leads to the diode equation

9. 2 SiO n-type Si p-type Si 2 SiO How to make a diode • Start with p-type silicon • Grow silicon dioxide (glass) • Pattern and etch a hole in the oxide • Dope the exposed silicon n-type and diffuse it into the substrate • Apply metal contacts to top and bottom

10. 3. Diode behavior • Plot diode current versus diode bias

11. There are two types of diode reverse breakdown • Avalanching • Caused by strong electric fields in the diode • Impact ionization – chain reaction • Zener Breakdown • Quantum mechanical effect • Can be set and controlled very tightly • Used in circuits to set voltages – usually in the 5 to 25 volt range.

12. Diode Breakdown

13. 4. Ideal Diode Model • Forward bias • Diode turns on hard at zero volts • Reverse bias • Zero current independent of bias Ideal Diode Model

14. 5. Offset Diode Model Offset Diode Model • Forward bias • Diode turns on hard at 0.6 volts • Reverse bias • Zero current independent of bias

15. For Either Model… Reverse breakdown is abrupt, at either the Zener or the avalanche voltage Or

16. 6. Diodes in Circuits • Diodes are often found in LCR circuits • Use a simple model for the diode in the circuit • Ideal • Offset • Circuit analysis by hand involves some initial guesswork • Is it conducting or not?

17. R D V An ideal diode in a circuit R R VD< 0 VD 0 + + D D + V V

18. + Ideal diode Offset model diode 0.6 volt battery An offset model diode behaves like an ideal diode in series with a battery =

19. An offset model diode in a circuit R R R VD < 0.6 V VD  0.6 V + + 0.6 V + D D + D V V V

20. R 0 < VD < VZ R R VD  VZ + R D + +VZ V + V D V V D + D V A Zener diode in a circuit

21. Procedure for Circuit Analysis • Think about the circuit, then assume a conduction state (on, off, or zenering) • Substitute in the assumed diode model • Solve the circuit • Figure out the resultant diode current and voltage • If consistent with assumption, congrats! If not, try again with another assumption

22. Practice with Circuits

23. Practice with Circuits

24. Practice with Circuits

25. Practice with Circuits

26. Practice with Circuits

27. Practice with Circuits

28. Practice with Circuits