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18 Pages

### lecture5_annotated

Course: EE 105 EE105, Fall 2008
School: Berkeley
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Word Count: 862

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5 OUTLINE Lecture PN Junction Diodes I/V Capacitance Reverse Breakdown Large and Small signal models Reading: Chapter 2.2-2.3,3.2-3.4 EE105 Fall 2011 Lecture 5, Slide 1 Prof. Salahuddin, UC Berkeley Recap: Law of the Junction Law of the junction: n(a ) p (a ) = ni2 eVD / VT -b 0 a EE105 Fall 2011 Lecture 5, Slide 2 Prof. Salahuddin, UC Berkeley Recap: Minority Carrier Concentrations at the Edges of...

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5 OUTLINE Lecture PN Junction Diodes I/V Capacitance Reverse Breakdown Large and Small signal models Reading: Chapter 2.2-2.3,3.2-3.4 EE105 Fall 2011 Lecture 5, Slide 1 Prof. Salahuddin, UC Berkeley Recap: Law of the Junction Law of the junction: n(a ) p (a ) = ni2 eVD / VT -b 0 a EE105 Fall 2011 Lecture 5, Slide 2 Prof. Salahuddin, UC Berkeley Recap: Minority Carrier Concentrations at the Edges of the Depletion Region -b 0 x' ;x=0 x=a n p ( x) = n p 0 + n p ( x) ( ) ni2 eVD / VT 1 x / Ln n p ( x) = e NA J n ,diff EE105 Fall 2011 ( Notation: Ln electron diffusion length (cm) ) dn p qDn ni2 VD / VT = qDn = e 1 e x / Ln dx N A Ln Lecture 5, Slide 3 Prof. Salahuddin, UC Berkeley Hole Diffusion X x 0 J p ,diff = EE105 Fall 2011 qD p ni2 N D Lp (e VD / VT Lecture 5, Slide 4 ) 1 e x '' / L p Prof. Salahuddin, UC Berkeley Distribution of Diffusion Current J p ,diff = qD p ni2 N D Lp (e VD / VT ) 1 e x / Lp J n ,diff ( ) qDn ni2 VD / VT = e 1 e x / Ln N A Ln x -b 0 a Assume: No Recombination in the depletion region Known: Total Current is the same everywhere EE105 Fall 2011 Lecture 5, Slide 5 Prof. Salahuddin, UC Berkeley Diode Current under Forward Bias The current flowing across the junction is comprised of hole diffusion and electron diffusion J J = components: +J +J +J tot p , drift x = 0 n , drift x = 0 p , diff x = 0 n , diff x = 0 J_total J p ,diff = qD p ni2 N D Lp (e VD / VT ) 1 e x / Lp J n ,diff ( ) qDn ni2 VD / VT = e 1 e x / Ln N A Ln x -b EE105 Fall 2011 0 a Lecture 5, Slide 6 Prof. Salahuddin, UC Berkeley I-V Characteristic of a PN Junction Current increases exponentially with applied forward bias voltage, and saturates at a relatively small negative current level for reverse bias voltages. Ideal diode equation: ( ) I D = I S eVD / VT 1 Dn Dp I S = AJ S = Aqn + N L N L D p An 2 i EE105 Fall 2011 Lecture 5, Slide 7 Prof. Salahuddin, UC Berkeley Practical PN Junctions Typically, pn junctions in IC devices are formed by counter-doping. The equations provided in class (and in the textbook) can be readily applied to such diodes if NA net acceptor doping on p-side (NA-ND)p-side ND net donor doping on n-side (ND-NA)n-side ID (A) I D = I S (e qVD kT 1) Dn Dp I S = Aqni + L N Lp N D n A 2 VD (V) EE105 Fall 2011 Lecture 5, Slide 8 Prof. Salahuddin, UC Berkeley How to make sure that current flow in a forwardbiased p-n junction diode is mainly due to electrons? EE105 Fall 2011 Lecture Slide 5, 9 Prof. Salahuddin, UC Berkeley Diode Saturation Current IS Dn Dp I S = Aqni + L N Lp N D n A 2 IS can vary by orders of magnitude, depending on the diode area, semiconductor material, and net dopant concentrations. typical range of values for Si PN diodes: 10-14 to 10-17 A/m2 In an asymmetrically doped PN junction, the term associated with the more heavily doped side2 negligible: is D p I S Aqni L N p D If the P side is much more heavily doped, Dn I S Aqni L N n A 2 If the EE105 Fall 2011 N side is much more heavily doped, Lecture 5, Slide 10 Prof. Salahuddin, UC Berkeley Depletion Width at Equilibrium (see slide 3 in lecture 4) (x) qND a -b -qNA on the P side: E = x qN A ( a x ) --(1) si qN D ( x + b) E= on the N side: si aN A = bN D V(x) EE105 Fall 2011 0 --(3) Let us set the reference point at x=a Then V(a)=0 V(-b)=V0; Built in potential V0 -b --(2) a x Lecture 5, Slide 11 Prof. Salahuddin, UC Berkeley Depletion Width at Equilibrium E= V(x) V0 -b E= 0 a x qN A ( a x ) --(1) si qN D ( x + b) si aN A = bN D --(2) --(3) V(a)=0 V(-b)=V0; Built in potential EE105 Fall 2011 Lecture 5, Slide 12 Prof. Salahuddin, UC Berkeley Depletion Width at Equilibrium V(x) V0 -b 0 EE105 Fall 2011 a x Lecture 5, Slide 13 Prof. Salahuddin, UC Berkeley Depletion Width at biased conditions V(x) V0 -b 0 EE105 Fall 2011 a x Lecture 5, Slide 14 Prof. Salahuddin, UC Berkeley PN Junction Depletion Capacitance A reverse-biased PN junction can be viewed as a capacitor, for incremental changes in applied voltage. si 10-12 F/cm is the permittivity of silicon si Cj = Wdep EE105 Fall 2011 Lecture 5, Slide 15 Prof. Salahuddin, UC Berkeley Voltage-Dependent Capacitance The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR. VD EE105 Fall 2011 Lecture 5, Slide 16 Prof. Salahuddin, UC Berkeley Reverse-Biased Diode Application A very important application of a reverse-biased PN junction is in a voltage controlled oscillator (VCO), which uses an LC tank. By changing VR, we can change C, which changes the oscillation frequency. f res EE105 Fall 2011 Lecture 5, Slide 17 1 = 2 1 LC Prof. Salahuddin, UC Berkeley Forward Bias Diffusion capacitance x At small forward bias, putting a small ac signal changes the concentration of diffused minority carriers and therefore gives a capacitance. This is called a diffusion capacitance EE105 Fall 2011 Lecture 5, Slide 18 Prof. Salahuddin, UC Berkeley
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