Lecture 6 OUTLINE PN Junction Diodes Reverse Breakdown Large and Small signal models Reading Chapter 2 2 2 3 3 2 3 4 Bipolar Junction Transistors Chapter 4 EE105 Fall 2011 Lecture 6 Slide 1 Prof Salahuddin UC Berkeley Depletion Width at Equilibrium E V x V0 E b 0 a x qN A a x si qN D x b si aN A bN D 1 2 3 V a 0 V b V0 Built in potential EE105 Fall 2011 Lecture 6 Slide 2 Prof Salahuddin 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 2011 Lecture 6 Slide 3 Prof Salahuddin 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 2011 Lecture 6 Slide 4 Prof Salahuddin 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 2011 Lecture 6 Slide 5 Prof Salahuddin UC Berkeley Small and Large Signal Models Basic Concept EE105 Fall 2011 Lecture 6 Slide 6 Prof Salahuddin UC Berkeley Example Diode DC Bias Calculations Vx 1 V IX VX I X R1 VD I X R1 VT ln IS I X 0 2mA EE105 Fall 2011 Lecture 6 Slide 7 Prof Salahuddin UC Berkeley Constant Voltage Diode Model for Large Signal Analysis 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 2011 Lecture 6 Slide 8 Prof Salahuddin UC Berkeley Example Diode DC Bias Calculations Vx 1 V Say VD V0 ND 1017 NA 1016 EE105 Fall 2011 Lecture 6 Slide 9 Prof Salahuddin UC Berkeley Large signal model key points This example shows the simplicity provided by a constant voltage model over an exponential model Using an exponential model iteration is needed to solve for current Using a constant voltage model only linear equations need to be solved EE105 Fall 2011 Lecture 6 Slide 10 Prof Salahuddin UC Berkeley Small Signal Analysis Small signal analysis is performed at a DC bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation If two points on the I V curve are very close the curve inbetween these points is well approximated by a straight line I D dI D VD dVD VD VD1 I s VD1 VT I D1 e VT VT EE105 Fall 2011 Lecture 6 Slide 11 Prof Salahuddin UC Berkeley Diode Model for Small Signal Analysis Since there is a linear relationship between the smallsignal current and small signal voltage of a diode the diode can be viewed as a linear resistor when only small changes in voltage are of interest Small Signal Resistance or Dynamic Resistance EE105 Fall 2011 VT rd ID Lecture 6 Slide 12 Prof Salahuddin UC Berkeley Diode Model Summary EE105 Fall 2011 Lecture 6 Slide 13 Prof Salahuddin UC Berkeley Bi Polar Junction Transistors EE105 Fall 2011 Lecture 6 Slide 14 Prof Salahuddin UC Berkeley Reverse Biased PN Junction as a Current Source PN junction diode current is independent of the reverse bias voltage It depends only on the rate at which minority carriers are introduced into the depletion region Key Insight We can increase the reverse current by injecting minority carriers near to the depletion region EE105 Fall 2011 Lecture 6 Slide 15 Prof Salahuddin UC Berkeley BJT Structure and Circuit Symbol A bipolar junction transistor consists of 2 PN junctions that form a sandwich of three doped semiconductor regions The outer two regions are doped the same type the middle region is doped the opposite type EE105 Fall 2011 Lecture 6 Slide 16 Prof Salahuddin UC Berkeley NPN BJT Operation Qualitative In the forward active mode of operation The collector junction is reverse biased The emitter junction is forward biased current gain EE105 Fall 2011 Lecture 6 Slide 17 IC IB Prof Salahuddin UC Berkeley Base Current The base current consists of two components 1 Injection of holes into the emitter and 2 Recombination of holes with electrons injected from the emitter I C I B EE105 Fall 2011 Lecture 6 Slide 18 Prof Salahuddin UC Berkeley BJT Design Important features of a well designed BJT large Injected minority carriers do not recombine in the quasi neutral base region Emitter current is comprised almost entirely of carriers injected into the base rather than carriers injected into the emitter EE105 Fall 2011 Lecture 6 Slide 19 Prof Salahuddin UC Berkeley Carrier Transport in the Base Region The length of the quasi neutral base region WB x2 x1 is much smaller than the minority carrier diffusion length so that very few of the carriers injected from the emitter into the base recombine before they reach the collector junction depletion region Minority carrier diffusion current is constant in the quasi neutral base The minority carrier concentration at the edges of the collector junction depletion region are 0 EE105 Fall 2011 Lecture 6 Slide 20 Prof Salahuddin UC Berkeley Minority Carrier Diffusion in the Quasi Neutral P type Base Region Long Base nB J n diff ni2 e q VBE VT NB exp x 1 nB LB dn qDB B dx Short Base J n diff ni2 e q VBE VT NB 1 x 1 WB dnB qDB dx ni2 q VBE VT 1 x qDB e 1 exp LB NB LB ni2 q VBE VT 1 qDB e 1 NB WB LB diffusion length of minority carriers in the quasi neutral base WB width of quasi neutral base EE105 Fall 2011 Lecture 6 Slide 21 Prof Salahuddin UC Berkeley Collector Current IC AE qDB ni2 N BWB I C I S exp VBE exp 1 V T VBE VT AE qDB ni2 where I S N BWB The equation above shows that the BJT is indeed a voltage dependent current source thus it can be used as an amplifier EE105 Fall 2011 Lecture 6 Slide 22 Prof Salahuddin UC Berkeley Common Emitter Current Gain Assuming that no minority carrier recombination occurs within the quasi neutral base region The collector current is equal to the current due to minority carrier injection from the emitter into the base IC AE qDB ni2 VBE e N BWB VT 1 The base current is equal to the current due to minority carrier injection from the base into the short emitter IB AE qDE ni2 VBE e N EWE VT 1 IC The current
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