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3.3 - 3.3 PN Junction Non-Idealities 3.3.1 Ideal vs Real...

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ECE3080, Chapter 3.3 1 June 13, 2005 ECE 3080: Chapter 3.3 – PN Junction O. Brand, 1 of 18 3.3 PN Junction: Non-Idealities 3.3.1 Ideal vs. Real IV-Characteristic 3.3.2 Junction Breakdown – Avalanching – Zener Breakdown 3.3.3 Generation-Recombination Currents 3.3.4 High-Current Phenomena – Series Resistance – High-Level Injection Pierret, Chapter 6.2-6.3, page 260-289 June 13, 2005 ECE 3080: Chapter 3.3 – PN Junction O. Brand, 2 of 18 3.3.1 Ideal vs. Real IV Characteristic Ideal Diode Equation Characteristics: Current increases exponentially in forward direction Small saturation current I S in reverse direction Saturation current strongly depends on n i , i.e. on the temperature and the semiconductor material Saturation current is proportional to the minority carrier density Contribution of the lower doped side of the pn-junction dominates the saturation current I = I s e qV A /kT ! 1 " # $ % I s = qA D n n p0 L n + D p p n0 L p " # & & $ % = qA D n n i 2 L n N A + D p n i 2 L p N D " # & & $ %
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ECE3080, Chapter 3.3 2 June 13, 2005 ECE 3080: Chapter 3.3 – PN Junction O. Brand, 3 of 18 Experimental IV Characteristic Junction Breakdown Pierret, Fig. 6.9 June 13, 2005 ECE 3080: Chapter 3.3 – PN Junction O. Brand, 4 of 18 “Real” IV Characteristic – Close-Up Deviations from ideal diode characteristic: Reverse-bias breakdown Non-saturating “saturation” current Diode characteristic in forward direction deviates from ideal exponential behavior at low and high currents Pierret, Fig. 6.10 R-G Current R-G Current High Current Phenomena Breakdown
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ECE3080, Chapter 3.3 3 June 13, 2005 ECE 3080: Chapter 3.3 – PN Junction O. Brand, 5 of 18 3.3.2 Junction Breakdown Junction or Reverse-Bias Breakdown: current flowing in a pn-junction under reverse bias suddenly increases drastically if reverse bias is increased over the so-called breakdown voltage V BR ; breakdown process is reversible if the junction is not overheated (current must be limited) The breakdown voltage depends on Junction doping (the non-degenerate doping in case of a n + -p or p + -n junction) Temperature
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