Lecture 3 ANNOUNCEMENTS HW2 is posted due Tu 9 11 TAs will hold their office hours in 197 Cory Prof Liu s office hours are changed to TuTh 12 1PM in 212 567 Cory EE105 accounts can access EECS Windows Remote Desktop servers OUTLINE PN Junction Diodes cont d Electrostatics cont d I V characteristics Reverse breakdown Small signal model Reading Chapter 2 2 2 3 3 4 EE105 Fall 2007 Lecture 3 Slide 1 Prof Liu UC Berkeley The Depletion Approximation In the depletion region on the N side dE qN D dx si si qN D x b E si x qND a b qNA x In the depletion region on the P side dE qN A dx si si E qN A a x si aN A bN D EE105 Fall 2007 Lecture 3 Slide 2 Prof Liu UC Berkeley More on the Built In Potential V0 Q Why can t we harness V0 and use the PN junction as a battery V0 A A built in potential also exists at a junction between a metal and a semiconductor e g at a contact If we connect the P and N regions together there is no net voltage drop across the device No net current flows across the junction when the externally applied voltage is 0 V V x V0 Vbn V0 Vbp 0 Vbn EE105 Fall 2007 V0 Vbp Lecture 3 Slide 3 b 0 a x Prof Liu UC Berkeley Effect of Applied Voltage The quasi neutral N type and P type regions have low resistivity whereas the depletion region has high resistivity Thus when an external voltage VD is applied across the diode almost all of this voltage is dropped across the depletion region Think of a voltage divider circuit If VD 0 reverse bias the potential barrier to carrier diffusion is increased by the applied voltage If VD 0 forward bias the potential barrier to carrier diffusion is reduced by the applied voltage VD ID EE105 Fall 2007 Lecture 3 Slide 4 Prof Liu UC Berkeley PN Junction under Forward Bias A forward bias decreases the potential drop across the junction As a result the magnitude of the electric field decreases and the width of the depletion region narrows x qND a b qNA x ID V x V0 b EE105 Fall 2007 0 a x Lecture 3 Slide 5 Prof Liu UC Berkeley Minority Carrier Injection under Forward Bias The potential barrier to carrier diffusion is decreased by a forward bias thus carriers diffuse across the junction The carriers which diffuse across the junction become minority carriers in the quasi neutral regions they recombine with majority carriers dying out with distance np x np0 0 EE105 Fall 2007 x edge of depletion region x Equilbrium concentration n p0 of electrons on the P side Lecture 3 Slide 6 ni2 NA Prof Liu UC Berkeley Minority Carrier Concentrations at the Edges of the Depletion Region The minority carrier concentrations at the edgesqVof kT V e the depletion region are changed by the factor e D D VT There is an excess concentration pn np of minority carriers in the quasi neutral regions under forward bias Within the quasi neutral regions the excess minoritycarrier concentrations decay exponentially with distance from the depletion region to zero n p x n p 0 n p x n p x 2 i VD VT n e NA e 1 x Ln J n diff Notation Ln electron diffusion length cm dn p qDn ni2 VD VT qDn e 1 e x Ln dx N A Ln x EE105 Fall 2007 Lecture 3 Slide 7 Prof Liu UC Berkeley Diode Current under Forward Bias The current flowing across the junction is comprised of hole diffusion and electron diffusion components J tot J p drift x 0 J n drift x 0 J p diff x 0 J n diff x 0 Assuming that the diffusion current components are constant within the depletion region i e no recombination occurs in the depletion region J n diff x 0 qDn ni2 VD VT e 1 N A Ln VD VT J tot J S e EE105 Fall 2007 J p diff x 0 qD p ni2 N D Lp e VD VT 1 Dn Dp 1 where J S qn N L N L D p A n 2 i Lecture 3 Slide 8 Prof Liu UC Berkeley Current Components under Forward Bias For a fixed bias voltage Jtot is constant throughout the diode but Jn x and Jp x vary with position Jtot b EE105 Fall 2007 Lecture 3 Slide 9 0 a x Prof Liu 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 A n 2 i EE105 Fall 2007 Lecture 3 Slide 10 Prof Liu UC Berkeley Parallel PN Junctions Since the current flowing across a PN junction is proportional to its cross sectional area two identical PN junctions connected in parallel act effectively as a single PN junction with twice the cross sectional area hence twice the current EE105 Fall 2007 Lecture 3 Slide 11 Prof Liu UC Berkeley Diode Saturation Current IS Dn Dp I S Aqni L N L N n A p D 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 side is negligible If the P side is much more heavily doped If the N side is much more heavily doped EE105 Fall 2007 Lecture 3 Slide 12 Dp I S Aqni L N p D 2 Dn I S Aqni Ln N A 2 Prof Liu UC Berkeley Reverse Breakdown As the reverse bias voltage increases the electric field in the depletion region increases Eventually it can become large enough to cause the junction to break down so that a large reverse current flows breakdown voltage EE105 Fall 2007 Lecture 3 Slide 13 Prof Liu UC Berkeley Reverse Breakdown Mechanisms a Zener breakdown occurs when the electric field is sufficiently high to pull an electron out of a covalent bond to generate an electron hole pair b Avalanche breakdown occurs when electrons and holes gain sufficient kinetic energy due to acceleration by the E field in between scattering events to cause electronhole pair generation upon colliding with the lattice EE105 Fall 2007 Lecture 3 Slide 14 Prof Liu UC Berkeley Constant Voltage Diode Model If VD VD on The diode operates as an open circuit If VD VD on The diode operates as a constant voltage source with value VD on EE105 Fall 2007 Lecture 3 Slide 15 Prof Liu UC Berkeley Example Diode DC Bias Calculations IX VX I X R1 VD I X R1 VT ln IS I X 2 2mA for VX 3V …
View Full Document
Unlocking...