1. Spring 2007EE130 Lecture 17, Slide 1 Lecture #17 OUTLINE pn junctions (cont’d) – Reverse bias current – Reverse-bias breakdown Reading: Chapter 6.2

2. Spring 2007EE130 Lecture 17, Slide 2 Carrier Concentration Profiles: Forward Bias

3. Spring 2007EE130 Lecture 17, Slide 3 Carrier 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).

4. Spring 2007EE130 Lecture 17, Slide 4 Alternative Derivation of Formula for I 0 “Depletion approximation”: I 0 represents the rate at which carriers are thermally generated within a diffusion length of the depletion region:

5. Spring 2007EE130 Lecture 17, Slide 5 A Zener diode is designed to operate in the breakdown mode. V I V BR PN A R Forward Current Small leakage Current (a) 3.7V R (b) IC Zener diode Junction Breakdown

6. Spring 2007EE130 Lecture 17, Slide 6 If the reverse bias voltage (-V A ) 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 Breakdown Voltage, V BR

7. Spring 2007EE130 Lecture 17, Slide 7 if V BR >> V bi  CR increases slightly with N: For 10 14 cm -3 < N < 10 18 cm -3, 10 5 V/cm <  CR < 10 6 V/cm Avalanche Breakdown Mechanism Small E-field: High E-field:

8. Spring 2007EE130 Lecture 17, Slide 8 Dominant breakdown mechanism when both sides of a junction are very heavily doped. E c E v V A = 0: E v E c Empty States Filled States - Tunneling (Zener) Breakdown Mechanism V A < 0: Typically, V BR < 5 V for Zener breakdown

9. Spring 2007EE130 Lecture 17, Slide 9 Empirical Observations of V BR V BR decreases with increasing N V BR decreases with decreasing E G

10. Spring 2007EE130 Lecture 17, Slide 10 Breakdown Temperature Dependence For the avalanche mechanism: –V BR increases with increasing T, because the mean free path decreases For the tunneling mechanism: –V BR decreases with increasing T, because the flux of valence-band electrons available for tunneling increases

11. Spring 2007EE130 Lecture 17, Slide 11 Summary The minority-carrier concentrations at the edges of the depletion region change with the applied bias V A, by the factor The diode saturation current I 0 is dominated by the term associated with the more lightly doped side: p+ n diode: p n+ diode: I 0 can be viewed as the drift current due to minority carriers generated within a diffusion length of the depletion region

12. Spring 2007EE130 Lecture 17, Slide 12 Reverse-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 is avalanche, if N < ~10 18 /cm 3 tunneling, if N > ~10 18 /cm 3

13. Spring 2007EE130 Lecture 17, Slide 13 Deviations from the Ideal I-V Behavior Forward-bias currentReverse-bias current