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Transcript
ECE 4991 Electrical and Electronic Circuits Chapter 9 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 2 What’s Important in Chapter 9 1. 2. 3. 4. 5. 6. Definitions Semiconductor basics Diode behavior Ideal diode model Offset diode model Diodes in circuits 3 1. Definitions • • • • Semiconductor Diode Majority Carrier Minority Carrier • • • • Forward bias Reverse bias Ideal model Offset model 4 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 5 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 6 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) 7 Why do they work the way they do? - P side Dn ni2 qVa Jn x q exp Ln N a kT + + + + + + + + + + N side x 1 exp Ln x0 Which leads to the diode equation qVa I I 0 exp kT 1 8 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 SiO SiO 2 2 n-type Si p-type Si 9 3. Diode behavior • Plot diode current versus diode bias x 10 -15 qVa I I 0 exp kT 1 Ideal Diode I-V Characteristic Near the Origin 12 10 Current (A) 8 6 4 2 -3kT/q +3kT/q 0 -1 -0.8 -0.6 -0.4 Applied Voltage (V) -0.2 0 0.2 10 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. 11 Diode Breakdown 12 4. Ideal Diode Model • Forward bias – Diode turns on hard at zero volts • Reverse bias x 10 -15 Ideal Diode I-V Characteristic Near the Origin 12 – Zero current independent of bias 10 Current (A) 8 6 Ideal Diode Model 4 2 -3kT/q +3kT/q 0 -1 -0.8 -0.6 -0.4 -0.2 Applied Voltage (V) 0 0.2 13 5. Offset Diode Model Offset Diode Model • Forward bias – Diode turns on hard at 0.6 volts • Reverse bias – Zero current independent of bias 14 For Either Model… Reverse breakdown is abrupt, at either the Zener or the avalanche voltage Or 15 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? 16 An ideal diode in a circuit + + V R R R D V VD < 0 VD 0 D + D V 17 An offset model diode behaves like an ideal diode in series with a battery Offset model diode = + Ideal diode 0.6 volt battery 18 An offset model diode in a circuit R + + V R R VD 0.6 V D V VD < 0.6 V + 0.6 V D + D V 19 A Zener diode in a circuit R 0 < VD < VZ R + R VD VZ + V D + +VZ V V D V R D + D V 20 Procedure for Circuit Analysis 1. Think about the circuit, then assume a conduction state (on, off, or zenering) 2. Substitute in the assumed diode model 3. Solve the circuit 4. Figure out the resultant diode current and voltage 5. If consistent with assumption, congrats! If not, try again with another assumption 21 Practice with Circuits 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