1. Week 11b Lecture Materials Diodes and some of their uses: Review of pn-diode structure Diode I-V characteristics: Actual characteristic – exponential Ideal diode characteristic – switches at V = 0 Large-signal diode model – a switch and a voltage source to represent required turn-on voltage Zener diode, varactor diode, light-emitting diode (LED), solar cell PN-diodes used for isolation in integrated circuits Diodes and MOSFET operation – need for a gate The band picture of semiconductors (if there’s time) Answering one of your questions: A quick picture of superconductivity – EECS Prof. Ted Van Duzer

2. The pn Junction Diode Schematic diagram p-type n-type IDID + V D – Circuit symbol Physical structure: (an example) p-type Si n-type Si SiO 2 metal IDID +VD–+VD– net donor concentration N D net acceptor concentration N A For simplicity, assume that the doping profile changes abruptly at the junction. cross-sectional area A D

3. Water Model of Diode Rectifier Simplistic Simplistic Simplistic view of why a pn-diode conducts differently in forward and reverse bias: When the p side is made positive with respect to the n side (forward bias), the positively charged holes move toward the negatively charged electrons, and they recombine. Then more carriers flow in from the contacts. In reverse bias, the holes and the electrons move away from each other, leaving no mobile carriers in the middle – hence, the diode has an insulator in its middle region and no current flows through.

4. Summary: pn-Junction Diode I-V Under forward bias, current increases exponentially with increasing forward bias Under reverse bias, a potential barrier in the middle of the junction is increased, so that negligible carriers flow across the junction I D (A) V D (V) The net result is an I-V curve that looks like this, with typically nA currents in the reverse direction (V D < 0), and mA or more in the forward direction (V D > 0) 0.7 V for Si |

5. reverse bias forward bias An ideal diode passes current only in one direction. An ideal diode has the following properties: when I D > 0, V D = 0 when V D < 0, I D = 0 Ideal Diode Model of pn Diode I D (A) V D (V) IDID +VD–+VD– +VD–+VD– IDID Circuit symbolI-V characteristic Diode behaves like a switch: closed in forward bias mode open in reverse bias mode Switch model

6. Large-Signal Diode Model reverse bias forward bias I D (A) V D (V) IDID +VD–+VD– +VD–+VD– IDID Circuit symbolI-V characteristic Switch model V turn-on ++ RULE 1: When I D > 0, V D = V turn-on RULE 2: When V D < V turn-on, I D = 0 Diode behaves like a voltage source in series with a switch: closed in forward bias mode open in reverse bias mode For a Si pn diode, V turn-on  0.7 V

7. Application Example: Rectification using the ideal diode model vs(t)vs(t) ++ +vR(t)–+vR(t)– vs(t)vs(t) t R C vR(t)vR(t) t

8. To get a really steady voltage out we can add an integrated circuit regulator to the circuit.

9. Potential plots for forward- and reverse-biased diodes in series with a voltage source and a resistor

10. Clipping circuit using a pn-diode

11. One uses the reverse-bias breakdown voltage as a voltage reference in some circuits

12. Varactor diode Light-emitting diode (LED)

13. Solar cells – two designs

14. Solar cell with load and its I-V characteristic

15. The basic building block in digital ICs is the MOS transistor, whose structure contains reverse-biased diodes. –pn junctions are important for electrical isolation of transistors located next to each other at the surface of a Si wafer. –The junction capacitance of these diodes can limit the performance (operating speed) of digital circuits Why are pn Junctions Important for ICs?

16. p-type Si nn n n n regions of n-type Si No current flows if voltages are applied between n-type regions, because two pn junctions are “back-to-back” n-region p-region => n-type regions isolated in p-type substrate and vice versa Device Isolation using pn Junctions a ba b

17. p-type Si nn Transistor A nn Transistor B We can build large circuits consisting of many transistors without worrying about current flow between devices. The p-n junctions isolate the transistors because there is always at least one reverse-biased p-n junction in every potential current path.