Lecture #17

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

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

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). EE130 Lecture 17, Slide 3

4. Alternative 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: EE130 Lecture 17, Slide 4

5. Junction Breakdown I Forward Current VBR V R Small leakage Current A P N (a) R 3.7V IC Zener diode (b) AZener diodeis designed to operate in the breakdown mode. EE130 Lecture 17, Slide 5

6. Breakdown Voltage, VBR • If the reverse bias voltage (-VA) is so large that the peak electric field exceeds a critical value eCR, then the junction will “break down” (i.e. large reverse current will flow) • Thus, the reverse bias at which breakdown occurs is EE130 Lecture 17, Slide 6

7. Avalanche Breakdown Mechanism High E-field: if VBR >> Vbi • eCRincreases slightly with N: • For 1014 cm-3 < N < 1018 cm-3, • 105 V/cm < eCR < 106 V/cm Small E-field: EE130 Lecture 17, Slide 7

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

9. Empirical Observations of VBR • VBR decreases with increasing N • VBR decreases with decreasing EG EE130 Lecture 17, Slide 9

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

11. Summary • 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 region EE130 Lecture 17, Slide 11

12. Reverse-bias breakdown: • If the peak electric field in the depletion region exceeds a critical value eCR, 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 < ~1018/cm3 tunneling, if N > ~1018/cm3 EE130 Lecture 17, Slide 12

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