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. Recall that Dp = Dn = 0 at an ohmic contact
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’’ 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. Evaluate Jp at x=xn (x’=0) to find the injected hole current:
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:
EE130/230A Fall 2013
Lecture 11, Slide 5

6. Therefore if xc’ << LP:
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(x)
Dpn is a linear function:
Jp is constant
(No holes are lost due to recombination as they diffuse to the metal contact.)
slope is
constant x'
x'c
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): x JN xn
-xp J JP
e.g. if hole injection into the n side is greater than electron injection into the p side:
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 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
A Zener diode is 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:
VBR decreases 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