OUTLINE

1
Lecture 5

• OUTLINE
• PN Junction Diodes
• I/V
• Capacitance
• Reverse Breakdown
• Large and Small signal models
• Reading Chapter 2.2-2.3,3.2-3.4

2
Recap Law of the Junction
Law of the junction
0
a
-b
3
Recap Minority Carrier Concentrations at the
Edges of the Depletion Region
x'
0
-b
x0
xa
Notation Ln ? electron diffusion length (cm)
4
Hole Diffusion
x
0
X
5
Distribution of Diffusion Current
x
0
a
-b

• 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
0
a
-b
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 10-14
  to 10-17 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
--(2)
-b
x
on the N side
-qNA
--(3)
V(x)
Let us set the reference point at xa Then
V(a)0 V(-b)V0 Built in potential
V0
a
-b
x
0
12
Depletion Width at Equilibrium
V(x)
--(1)
V0
--(2)
a
-b
x
--(3)
0
V(a)0 V(-b)V0 Built in potential
13
Depletion Width at Equilibrium
V(x)
V0
a
-b
x
0
14
Depletion Width at biased conditions
V(x)
V0
a
-b
x
0
15
PN Junction Depletion Capacitance

• A reverse-biased PN junction can be viewed as a
  capacitor, for incremental changes in applied
  voltage.

esi ? 10-12 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
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