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 superconductivity EECS Prof Ted Van Duzer A quick picture of The pn Junction Diode Schematic diagram p type net acceptor concentration NA Circuit symbol ID n type net donor concentration ND cross sectional area AD Physical structure an example For simplicity assume that the doping profile changes abruptly at the junction ID VD metal SiO2 VD SiO2 p type Si n type Si metal Water Model of Diode Rectifier Figure 0 1 Schematic symbol and water model of a pn diode 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 S electrons move away from each other leaving no mobile carriers in the middle hence the diode has an insulator in its i middle region and no current flows through m p l i 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 ID A The net result is an I V curve that looks like this with typically nA currents in the reverse direction VD 0 and mA or more in the forward direction VD 0 0 7 V for Si VD V Ideal Diode Model of pn Diode Circuit symbol ID I V characteristic ID A VD Switch model ID forward bias reverse bias VD V An ideal diode passes current only in one direction An ideal diode has the following properties when ID 0 VD 0 when VD 0 ID 0 Diode behaves like a switch closed in forward bias mode open in reverse bias mode VD Large Signal Diode Model Circuit symbol ID I V characteristic ID A VD Switch model ID forward bias reverse bias Vturn on VD V Vturn on VD For a Si pn diode Vturn on 0 7 V RULE 1 When ID 0 VD Vturn on RULE 2 When VD Vturn on ID 0 Diode behaves like a voltage source in series with a switch closed in forward bias mode open in reverse bias mode Application Example Rectification using the ideal diode model vs t C R vs t vR t t vR t t To get a really steady voltage out we can add an integrated circuit regulator to the circuit 2500 F 20 1 Figure 0 7 Regulated power supply 120V MC7805 regulator 1 F Potential plots for forward and reverse biased diodes in series with a voltage source and a resistor a b Potential 0 7 V Figure 0 1 Battery and resistor circuits with a forward and b reverse biased diodes Clipping circuit using a pn diode Vi n Vo ut volts Vi n Vo ut 4 3 2 1 0 1 2 3 4 Vi n Time s Vi n Vo ut Figure 0 1 Diode circuit with alternating supply volt age ID ID VD Zener breakdown voltage Idealized diode characteristic VD Zener Diode Symbol Diode turn on voltage Zener diode I V characteristic Figure 0 1 The Zener diode One uses the reverse bias breakdown voltage as a voltage reference in some circuits Varactor diode Light emitting diode LED Solar cells two designs Screen Printed contact finger Single layer antireflection coating n P finger Double layer antireflection coating n doping P P Rear Contact Oxide Solar cell with load and its I V characteristic V V o lta g e P h o t o v o l ta i c ju n c t io n I 1 0 5 Short circuit current 0 0 5 Maximum Power 1V Open circuit voltage Dark characteristic lightg e n e r a te d current lightge nera ted current I Why are pn Junctions Important for ICs 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 Device Isolation using pn Junctions regions of n type Si a b n n n n n p type Si No current flows if voltages are applied between n type regions because two pn junctions are back to back n region a b n region p region n type regions isolated in p type substrate and vice versa Transistor A Transistor B n n n n p type Si 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
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