Understanding Narrow-Base Diodes and pn Junction Breakdown Mechanisms

1. Lecture 11 OUTLINE • pn Junction Diodes (cont’d) • Narrow-base diode • Junction breakdown Reading: Pierret 6.3.2, 6.2.2; Hu 4.5

2. Introduction • The ideal diode equation was derived assuming that the lengths of the quasi-neutral p-type & n-type regions (WP’ , WN’) are much greater than the minority-carrier diffusion lengths (Ln , Lp) in these regions. • Excess carrier concentrations decay exponentially to 0. • Minority carrier diffusion currents decay exponentially to 0. • In modern IC devices, however, it is common for one side of a pn junction to be shorter than the minority-carrier diffusion length, so that a significant fraction of the “injected” minority carriers reach the end of the quasi-neutral region, at the metal contact. Recall that Dp = Dn = 0 at an ohmic contact  In this lecture we re-derive the diode I-V equation with the boundary condition that Dp = 0 at a distance xc’ (rather than ) from the edge of the depletion region. EE130/230A Fall 2013 Lecture 11, Slide 2

3. Excess Carrier Distribution (n side) • From the minority carrier diffusion equation: • For convenience, let’s use the coordinate system: • So the solution is of the form: • We have the following boundary conditions: x’’ 0 0 x’ xc' EE130/230A Fall 2013 Lecture 11, Slide 3

4. Applying the boundary conditions, we have: Therefore Since this can be rewritten as We need to take the derivative of Dpn’ to obtain the hole diffusion current within the quasi-neutral n region: EE130/230A Fall 2013 Lecture 11, Slide 4

5. Thus, for a one-sided p+n junction (in which the current is dominated by injection of holes into the n-side) with a short n-side: Evaluate Jp at x=xn (x’=0) to find the injected hole current: EE130/230A Fall 2013 Lecture 11, Slide 5

6. and Therefore if xc’ << LP: For a one-sided p+n junction, then: EE130/230A Fall 2013 Lecture 11, Slide 6

7. Excess Hole Concentration Profile If xc’ << LP: Dpn is a linear function: • Jp is constant (No holes are lost due to recombination as they diffuse to the metal contact.) Dpn(x) slope is constant x' 0 x'c 0 EE130/230A Fall 2013 Lecture 11, Slide 7

8. General Narrow-Base Diode I-V • Define WP‘ and WN’ to be the widths of the quasi-neutral regions. • If both sides of a pn junction are narrow (i.e. much shorter than the minority carrier diffusion lengths in the respective regions): e.g. if hole injection into the n side is greater than electron injection into the p side: J JP JN x xn -xp EE130/230A Fall 2013 Lecture 11, Slide 8

9. Summary: Narrow-Base Diode • If the length of the quasi-neutral region is much shorter than the minority-carrier diffusion length, then there will be negligible recombination within the quasi-neutral region and hence all of the injected minority carriers will “survive” to reach the metal contact. • The excess carrier concentration is a linear function of distance. For example, within a narrow n-type quasi-neutral region: • The minority-carrier diffusion current is constant within the narrow quasi-neutral region. Shorter quasi-neutral region  steeper concentration gradient  higher diffusion current Dpn(x) location of metal contact (Dpn=0) x 0 xn WN’ EE130/230A Fall 2013 Lecture 11, Slide 9

10. pn Junction Breakdown C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 4-10 Breakdown voltage, VBR VA AZener diodeis designed to operate in the breakdown mode: EE130/230A Fall 2013 Lecture 11, Slide 10

11. Review: Peak E-Field in a pn Junction E(x) -xp xn x E(0) For a one-sided junction, where N is the dopant concentration on the lightly doped side EE130/230A Fall 2013 Lecture 11, Slide 11

12. 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 EE130/230A Fall 2013 Lecture 11, Slide 12

13. Avalanche Breakdown Mechanism R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.12 High E-field: if VBR >> Vbi Low E-field: • ECR increases slightly with N: • For 1014 cm-3 < N < 1018 cm-3, • 105 V/cm < ECR < 106 V/cm EE130/230A Fall 2013 Lecture 11, Slide 13

14. Tunneling (Zener) Breakdown Mechanism Dominant breakdown mechanism when both sides of a junction are very heavily doped. VA = 0 VA < 0 Ec Ev Typically, VBR < 5 V for Zener breakdown C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 4-12 EE130/230A Fall 2013 Lecture 11, Slide 14

15. Empirical Observations of VBR R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.11 • VBR decreases with increasing N • VBR decreases with decreasing EG EE130/230A Fall 2013 Lecture 11, Slide 15

16. VBR 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 EE130/230A Fall 2013 Lecture 11, Slide 16

17. Summary: Junction Breakdown • If the peak electric field in the depletion region exceeds a critical value ECR, then large reverse current will flow. This occurs at a negative bias voltage called the breakdown voltage, VBR: 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 EE130/230A Fall 2013 Lecture 11, Slide 17