1. Lecture #18 OUTLINE pn junctions (cont’d)
Deviations from the ideal I-V
R-G current
series resistance
high-level injection
Narrow-base diode
Reading: Chapter 6.2, 6.3
EE130 Lecture 18, Slide 1

2. Effect of R-G in Depletion Region
The net generation rate is given by
R-G in the depletion region contributes an additional component of diode current IR-G
EE130 Lecture 18, Slide 2

3. For reverse bias greater than several kT/q,Ip In
EE130 Lecture 18, Slide 3

4. For forward biases, Ip In
EE130 Lecture 18, Slide 4

5. Effect of Series Resistance
EE130 Lecture 18, Slide 5

6. High-Level Injection Effect
As VA increases, the side of the junction which is more lightly doped will eventually reach HLI:
significant gradient in majority-carrier profile
Majority-carrier diffusion current reduces the diode
current from the ideal
(p+n junction)
(n+p junction)
EE130 Lecture 18, Slide 6

7. Summary: Deviations from Ideal I-V
Forward-bias current
Reverse-bias current
EE130 Lecture 18, Slide 7

8. Derivation of Narrow-Base Diode I-V
We have the following boundary conditions:
With the following coordinate system:
Then, the solution is of the form:
x' x' '
NEW:
x' c
EE130 Lecture 18, Slide 8

9. Applying the boundary conditions, we have:
Therefore
Note that so that
EE130 Lecture 18, Slide 9

10. Excess Carrier Profiles: Limiting Cases
Long base (xc’):
EE130 Lecture 18, Slide 10

11. Dpn is a linear function of x  Jp is constant (no recombination)
Narrow base (xc’0):
Dpn is a linear function of x
 Jp is constant (no recombination)
EE130 Lecture 18, Slide 11

12. For a p+n junction, then:
where
EE130 Lecture 18, Slide 12

13. Note:
and
If xc’ << LP:
EE130 Lecture 18, Slide 13

14. Narrow (Short) Base Diode I-V Equation
Let
and
Then,
EE130 Lecture 18, Slide 14

15. Summary: Current Flow in pn Junctions
The diode current is dominated by the term associated with the more lightly doped side:
p+n diode:
pn+ diode:
i.e. current flowing across junction is dominated by carriers injected from the more heavily doped side
EE130 Lecture 18, Slide 15