EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu1Lecture #11OUTLINEThe pn Junction Diode-- Uses: Rectification, parts of transistors, light-emitting diodesand lasers, solar cells, electrically variable capacitor (varactordiode),voltage reference (zener diode) Depletion region & junction capacitanceI-V characteristicCircuit applications and analysisReference ReadingHambley, Chapter 10.1 to 10.4Howe and Sodini, Chapter 3.3-3.6EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu2The 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 ADEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu3When 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 ionsElectrons diffuse from the n side to the p side, leaving behind positively charged immobile donor ions A 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+++++–––––p nacceptor ionsdonor ionsEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu4quasi-neutral p regionCharge Density Distribution+++++–––––p nacceptor ionsdonor ionsdepletion region quasi-neutral n regioncharge density (C/cm3)distanceCharge is stored in the depletion region.EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu5DopingTypical doping densities: 1016~1019cm-3Atomic density for Si: 5 x 1022atoms/cm3 1018cm-3is 1 in 50,000two persons in entire Berkeley wearing a green hatP-n junction effect is like EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu6Electric Field and Built-In Potential φφφφ0=20lniDAnNNqkTφ+++++–––––p nelectric 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( == TqkTEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu7Effect of Applied VoltageThe 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.p n+++++–––––VDEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu8Forward BiasAs 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.p n+++++–––––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.EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu9Reverse BiasAs |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.p n+++++–––––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.EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu10Note that e0.6/0.026= 1010 and e0.72/0.026= 1012 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)EE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu11Depletion Region Width WjThe 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 −≅02φεEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu12Junction CapacitanceThe charge stored in the depletion region changes with applied voltage. This is modeled as junction capacitancejSiDjWACε=p n+++++–––––VD charge density (C/cm3)distanceEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu13Summary: pn-Junction Diode ElectrostaticsA 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 regionsA 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 junctionWidth of depletion regiondecreases 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 capacitance( )DSijVqNW −≅02φεjSiDjWACε==20lniDAnNNqkTφEE40 Summer 2006: Lecture 11 Instructor: Octavian Florescu14Summary: pn-Junction Diode I-VUnder forward bias, the potential barrier is reduced, so that carriers flow (by diffusion) across the junctionCurrent increases exponentially with increasing forward biasThe carriers become minority carriers once they cross the junction; as they diffuse in the quasi-neutral regions, they recombine with
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