Lecture 5Hole DiffusionDistribution of Diffusion CurrentDiode Current under Forward BiasI-V Characteristic of a PN JunctionPractical PN JunctionsSlide 7Diode Saturation Current ISDepletion WidthPN Junction under Reverse BiasPN Junction Small-Signal CapacitanceVoltage-Dependent CapacitanceReverse-Biased Diode ApplicationReverse Breakdown MechanismsReverse BreakdownParallel PN JunctionsConstant-Voltage Diode Model for Large-Signal AnalysisExample: Diode DC Bias CalculationsSmall-Signal AnalysisDiode Model for Small-Signal AnalysisSmall Sinusoidal AnalysisEE105 Fall 2010 Lecture 5, Slide 1 Prof. Salahuddin, UC BerkeleyLecture 5OUTLINE•PN Junction Diodes–I/V–Capacitance–Reverse Breakdown–Large and Small signal modelsReading: Chapter 2.2-2.3,3.2-3.4EE105 Fall 2011 Lecture 4, Slide 2 Prof. Salahuddin, UC BerkeleyHole Diffusionx0 pTDLxVVpDipdiffpeeLNnqDJ//2,1xEE105 Fall 2011 Lecture 4, Slide 3 Prof. Salahuddin, UC BerkeleyDistribution of Diffusion Currentx0 a-b nTDLxVVnAindiffneeLNnqDJ//2,1 pTDLxVVpDipdiffpeeLNnqDJ//2,1•Assume: No Recombination in the depletion region•Known: Total Current is the same everywhereEE105 Fall 2011 Lecture 4, Slide 4 Prof. Salahuddin, UC BerkeleyDiode Current under Forward Bias•The current flowing across the junction is comprised of hole diffusion and electron diffusion components:0,0,0,0,xdiffnxdiffpxdriftnxdrif tptotJJJJJx0 a-b nTDLxVVnAindiffneeLNnqDJ//2,1 pTDLxVVpDipdiffpeeLNnqDJ//2,1J_totalEE105 Fall 2011 Lecture 4, Slide 5 Prof. Salahuddin, UC BerkeleyI-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. pDpnAniSSVVSDLNDLNDAqnAJIeIITD2/1“Ideal diode” equation:EE105 Fall 2011 Lecture 4, Slide 6 Prof. Salahuddin, UC BerkeleyPractical 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)1( kTVqSDDeIIDppAnniSNLDNLDAq nI2VD (V)ID (A)EE105 Fall 2011 Lecture 4, Slide 7 Prof. Salahuddin, UC BerkeleyHow to make sure that current flow in a forward-biased p-n junction diode is mainly due to electrons?EE105 Fall 2011 Lecture 4, Slide 8 Prof. Salahuddin, UC BerkeleyDiode Saturation Current IS•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,DppAnniSNLDNLDAqnI2DppiSNLDAqnI2AnniSNLDAqnI2EE105 Fall 2011 Lecture 4, Slide 9 Prof. Salahuddin, UC BerkeleyDepletion Width on the P side: xaqNEsiA(see slide 3) 2)(2xaqNVxaqNdXdVsiAsiA(x)x-qNAqNDa-bV(x)xa-bV00EE105 Fall 2011 Lecture 4, Slide 10 Prof. Salahuddin, UC BerkeleyPN Junction under Reverse Bias •A reverse bias increases the potential drop across the junction. As a result, the magnitude of the electric field in the depletion region increases and the width of the depletion region widens. 112 0 RDAsidepVVNNqW EE105 Fall 2011 Lecture 4, Slide 11 Prof. Salahuddin, UC BerkeleyPN Junction Small-Signal Capacitance•A reverse-biased PN junction can be viewed as a capacitor, for incremental changes in applied voltage. depsijWCEE105 Fall 2011 Lecture 4, Slide 12 Prof. Salahuddin, UC BerkeleyVoltage-Dependent Capacitance•The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR.si 10-12 F/cm is the permittivity of silicon.0000121VNNNNqCVVCCDADAsijRjjVDEE105 Fall 2011 Lecture 4, Slide 13 Prof. Salahuddin, UC BerkeleyReverse-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. LCfres121EE105 Fall 2011 Lecture 4, Slide 14 Prof. Salahuddin, UC BerkeleyReverse Breakdown Mechanismsa) 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 electron-hole pair generation upon colliding with the lattice.EE105 Fall 2011 Lecture 4, Slide 15 Prof. Salahuddin, UC BerkeleyReverse 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 voltageEE105 Fall 2011 Lecture 4, Slide 16 Prof. Salahuddin, UC BerkeleyParallel 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 4, Slide 17 Prof. Salahuddin, UC BerkeleyConstant-Voltage Diode Modelfor 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 4, Slide 18 Prof. Salahuddin, UC BerkeleyExample: Diode DC Bias Calculations•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.SXTXDXXIIVRIVRIV ln11V1for mA 2.0V3for mA 2.2XXXXVIVIEE105 Fall 2011 Lecture 4, Slide 19 Prof. Salahuddin, UC BerkeleySmall-Signal Analysis•Small-signal analysis is performed at a DC bias point by perturbing the voltage by a
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