1. Lecture 5 OUTLINE PN Junction Diodes I/V Capacitance Reverse Breakdown
Large and Small signal models
Reading: Chapter ,

2. Recap: Law of the Junction-b a

3. Recap: Minority Carrier Concentrations at the Edges of the Depletion Regionx' -b
;x‘=0
x=a
Notation:
Ln  electron diffusion length (cm)

4. Hole Diffusion
x’’
X’’

5. Distribution of Diffusion Currentx -b a
Assume: No Recombination in the depletion region
Known: Total Current is the same everywhere

6. Diode Current under Forward Bias
The current flowing across the junction is comprised of hole diffusion and electron diffusion components:
J_total x -b a

7. I-V Characteristic of a PN Junction
Current increases exponentially with applied forward bias voltage, and “saturates” at a relatively small negative current level for reverse bias voltages.
“Ideal diode” equation:

8. Practical PN Junctions
Typically, pn junctions in IC devices are formed by counter-doping. The equations provided in class (and in the textbook) can be readily applied to such diodes if
NA  net acceptor doping on p-side (NA-ND)p-side
ND  net donor doping on n-side (ND-NA)n-side
ID (A)
VD (V)

9. How to make sure that current flow in a forward-biased p-n junction diode is mainly due to electrons?

10. Diode Saturation Current IS
IS can vary by orders of magnitude, depending on the diode area, semiconductor material, and net dopant concentrations.
typical range of values for Si PN diodes: to A/mm2
In an asymmetrically doped PN junction, the term associated with the more heavily doped side is negligible:
If the P side is much more heavily doped,
If the N side is much more heavily doped,

11. Depletion Width at Equilibrium
(see slide 3 in lecture 4)
r(x)
qND
on the P side:
--(1) a -b x
--(2)
on the N side:
-qNA
--(3)
V(x)
Let us set the reference point at x=a
Then V(a)=0
V(-b)=V0; Built in potential V0 -b a x

12. Depletion Width at Equilibrium
V(x)
--(1) V0
--(2) -b a x
--(3)
V(a)=0
V(-b)=V0; Built in potential

13. Depletion Width at Equilibrium
V(x) V0 -b a x

14. Depletion Width at biased conditions
V(x) V0 -b a x

15. PN Junction Depletion Capacitance
A reverse-biased PN junction can be viewed as a capacitor, for incremental changes in applied voltage.
esi  F/cm is the permittivity of silicon

16. Voltage-Dependent Capacitance
The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR. VD

17. Reverse-Biased Diode Application
A very important application of a reverse-biased PN junction is in a voltage controlled oscillator (VCO), which uses an LC tank. By changing VR, we can change C, which changes the oscillation frequency.

18. Forward Bias Diffusion capacitance
x’’
At small forward bias, putting a small ac signal changes the concentration of diffused minority carriers and therefore gives a capacitance. This is called a diffusion capacitance