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Berkeley ELENG 42 - Lecture Notes

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Week 8a, Slide 1EECS42, Spring 2005 Prof. WhiteWeek 8aOUTLINE• The pn Junction Diode• -- Uses: Rectification, parts of transistors, light-emitting diodesand lasers, solar cells, electrically variable capacitor (varactor diode),voltage reference (zener diode) – Depletion region & junction capacitance– I-V characteristic– Circuit applications and analysisReference Reading• Rabaey et al.– Chapter 3.2.1 to 3.2.2• Hambley– Chapter 10.1 to 10.4Week 8a, Slide 2EECS42, Spring 2005 Prof. WhiteThe pn Junction DiodeSchematic diagramp-type n-typeID+ VD–Circuit symbolPhysical structure:(an example)p-type Sin-type SiSiO2SiO2metalmetalID+VD–net donorconcentration NDnet acceptorconcentration NAFor simplicity, assume thatthe doping profile changes abruptly at the junction.cross-sectional area ADWeek 8a, Slide 3EECS42, Spring 2005 Prof. White• When the junction is first formed, mobile carriers diffuseacross the junction (due to the concentration gradients)– Holes diffuse from the p side to the n side, leaving behind negatively charged immobile acceptor ions– Electrons diffuse from the n side to the p side, leaving behind positively charged immobile donor ionsÆA region depleted of mobile carriers is formed at the junction.• The space charge due to immobile ions in the depletion region establishes an electric field that opposes carrier diffusion.Depletion Region+++++–––––pnacceptor ionsdonor ionsWeek 8a, Slide 4EECS42, Spring 2005 Prof. Whitequasi-neutral p regionCharge Density Distribution+++++–––––pnacceptor ionsdonor ionsdepletion region quasi-neutral n regioncharge density (C/cm3)distanceCharge is stored in the depletion region.Week 8a, Slide 5EECS42, Spring 2005 Prof. WhiteElectric Field and Built-In Potential φ0⎟⎟⎠⎞⎜⎜⎝⎛=20lniDAnNNqkTφ+++++–––––pnelectric field (V/cm)distancepotential (V)distancebuilt-in potential φ0No net current flowsacross the junctionwhen the externallyapplied voltage is 0 V.300K for mV 60)10ln( == TqkTWeek 8a, Slide 6EECS42, Spring 2005 Prof. WhiteWeek 8a, Slide 7EECS42, Spring 2005 Prof. WhiteEffect of Applied Voltage• The quasi-neutral p and n regions have low resistivity, whereas the depletion region has high resistivity. Thus, when an external voltage VDis applied across the diode, almost all of this voltage is dropped across the depletion region. (Think of a voltage divider circuit.)•If VD> 0 (forward bias), the potential barrier to carrier diffusion is reduced by the applied voltage.•If VD< 0 (reverse bias), the potential barrier to carrier diffusion is increased by the applied voltage.pn+++++–––––VDWeek 8a, Slide 8EECS42, Spring 2005 Prof. WhiteForward Bias•As VDincreases, the potential barrier to carrier diffusion across the junction decreases*, and current increases exponentially.ID(Amperes)VD(Volts)* Hence, the width of the depletion region decreases.pn+++++–––––VD > 0The carriers that diffuse across the junction become minority carriers in the quasi-neutral regions; they thenrecombine with majority carriers,“dying out” with distance.Week 8a, Slide 9EECS42, Spring 2005 Prof. WhiteReverse Bias•As |VD| increases, the potential barrier to carrier diffusion across the junction increases*; thus, no carriers diffuse across the junction.ID(Amperes)VD(Volts)* Hence, the width of the depletion region increases.pn+++++–––––VD < 0A very small amount of reverse current (ID< 0) does flow, due to minority carriersdiffusing from the quasi-neutral regions into the depletion region and drifting across the junction.Week 8a, Slide 10EECS42, Spring 2005 Prof. WhiteNote that e0.6/0.026= 1010 and e0.72/0.026= 1012Æ IDis in the mA range for VDin the range 0.6 to 0.7 V, typically.I-V Characteristic300K for Volts 026.0 == TqkTExponential diode equation: )1(/−=kTqVSDDeIIISis the diode saturation current• function of ni2, AD, NA, ND, length of quasi-neutral regions• typical range of values: 10-14to 10-17A/µm2ID(A)VD(V)Week 8a, Slide 11EECS42, Spring 2005 Prof. WhiteWeek 8a, Slide 12EECS42, Spring 2005 Prof. WhiteWeek 8a, Slide 13EECS42, Spring 2005 Prof. WhiteFigure 0.1 Schematic symbol and water model of a pn-diodeWater Model of Diode RectifierWeek 8a, Slide 14EECS42, Spring 2005 Prof. WhiteDepletion Region Width Wj• The width of the depletion region is a function of the bias voltage, and is dependent on NAand ND:• If one side is much more heavily doped than the other (which is commonly the case), then this can be simplified:where N is the doping concentration on the more lightly doped side()DDADASijVNNNNqW −⎟⎟⎠⎞⎜⎜⎝⎛+=02φεF/cm 1012−=Siε()DSijVqNW −≅02φεWeek 8a, Slide 15EECS42, Spring 2005 Prof. WhiteJunction Capacitance• The charge stored in the depletion region changes with applied voltage. This is modeled as junction capacitancejSiDjWACε=pn+++++–––––VD charge density (C/cm3)distanceWeek 8a, Slide 16EECS42, Spring 2005 Prof. WhiteSummary: pn-Junction Diode Electrostatics• A depletion region (in which n and p are each much smaller than the net dopant concentration) is formed at the junction between p- and n-type regions– A built-in potential barrier (voltage drop) exists across the depletion region, opposing carrier diffusion (due to a concentration gradient) across the junction:• At equilibrium (VD=0), no net current flows across the junction– Width of depletion region• decreases with increasing forward bias (p-type region biased at higher potential than n-type region)• increases with increasing reverse bias (n-type region biased at higher potential than p-type region)– Charge stored in depletion region Æ capacitance()DSijVqNW −≅02φεjSiDjWACε=⎟⎟⎠⎞⎜⎜⎝⎛=20lniDAnNNqkTφWeek 8a, Slide 17EECS42, Spring 2005 Prof. WhiteSummary: pn-Junction Diode I-V• Under forward bias, the potential barrier is reduced, so that carriers flow (by diffusion) across the junction– Current increases exponentially with increasing forward bias– The carriers become minority carriers once they cross the junction; as they diffuse in the quasi-neutral regions, they recombine with majority carriers (supplied by the metal contacts)“injection” of minority carriers• Under reverse bias, the potential barrier is increased, so that negligible carriers flow across the junction– If a minority carrier enters the depletion region (by thermal generation or diffusion


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Berkeley ELENG 42 - Lecture Notes

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