Lecture #17Carrier Concentration Profiles: Forward BiasCarrier Concentration Profiles: Reverse BiasAlternative Derivation of Formula for I0Junction BreakdownBreakdown Voltage, VBRAvalanche Breakdown MechanismTunneling (Zener) Breakdown MechanismEmpirical Observations of VBRBreakdown Temperature DependenceSummarySlide 12Deviations from the Ideal I-V BehaviorEE130 Lecture 17, Slide 1Spring 2007Lecture #17OUTLINE• pn junctions (cont’d)– Reverse bias current– Reverse-bias breakdownReading: Chapter 6.2EE130 Lecture 17, Slide 2Spring 2007Carrier Concentration Profiles: Forward BiasEE130 Lecture 17, Slide 3Spring 2007Carrier Concentration Profiles: Reverse Bias–Depletion of minority carriers at edges of depletion region–The only current which flows is due to drift of minority carriers across the junction. This current is fed by diffusion of minority carriers toward junction (supplied by thermal generation).EE130 Lecture 17, Slide 4Spring 2007Alternative Derivation of Formula for I0“Depletion approximation”:•I0 represents the rate at which carriers are thermally generated within a diffusion length of the depletion region:PnnpDipnppNnAinpLxxxNnptp-xx-x-LNnntn / /22pDiPnAiNNnqALNnqALI//220EE130 Lecture 17, Slide 5Spring 2007A Zener diode is designed to operate in the breakdown mode.VIVBRP N ARForward CurrentSmall leakageCurrent (a)3.7V R (b) ICZener diodeJunction BreakdownEE130 Lecture 17, Slide 6Spring 2007•If the reverse bias voltage (-VA) is so large that the peak electric field exceeds a critical value CR, then the junction will “break down” (i.e. large reverse current will flow)•Thus, the reverse bias at which breakdown occurs is biCRsBRVqNV 22 sBRbiCRVVqN2Breakdown Voltage, VBREE130 Lecture 17, Slide 7Spring 2007if VBR >> VbiCR increases slightly with N:For 1014 cm-3 < N < 1018 cm-3, 105 V/cm < CR < 106 V/cmqNVCRsBR22Avalanche Breakdown MechanismSmall E-field:High E-field:EE130 Lecture 17, Slide 8Spring 2007Dominant breakdown mechanism when both sides of a junction are very heavily doped. EcEv VA = 0: EvEc Empty StatesFilled States-Tunneling (Zener) Breakdown Mechanism VA < 0:biCRsBRVqNV 22V/cm 106CRTypically, VBR < 5 V for Zener breakdownEE130 Lecture 17, Slide 9Spring 2007Empirical Observations of VBR•VBR decreases with increasing N•VBR decreases with decreasing EGEE130 Lecture 17, Slide 10Spring 2007Breakdown Temperature Dependence•For the avalanche mechanism: –VBR increases with increasing T, because the mean free path decreases•For the tunneling mechanism: –VBR decreases with increasing T, because the flux of valence-band electrons available for tunneling increasesEE130 Lecture 17, Slide 11Spring 2007Summary•The minority-carrier concentrations at the edges of the depletion region change with the applied bias VA, by the factor•The diode saturation current I0 is dominated by the term associated with the more lightly doped side:p+ n diode:p n+ diode:•I0 can be viewed as the drift current due to minority carriers generated within a diffusion length of the depletion regionkTqVAe/ )( 20DPPinPNLDqAnxII )(20ANNipNNLDqAnxIIEE130 Lecture 17, Slide 12Spring 2007Reverse-bias breakdown:•If the peak electric field in the depletion region exceeds a critical value CR, then large reverse current will flow. This happens at a large negative voltage, called the “breakdown voltage”:where N is the dopant concentration on the more lightly doped side•The dominant breakdown mechanism isavalanche, if N < ~1018/cm3tunneling, if N > ~1018/cm3biCRsBRVqNV 22EE130 Lecture 17, Slide 13Spring 2007Deviations from the Ideal I-V BehaviorForward-bias current Reverse-bias
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