PN JUNCTION DIODES
The V-I characteristics of a junction diode
from the physics point of view.
1
Physical structure
Junction
Acceptor ion
Donor ion
_
_
_
+
+
+
_
_
_
+
+
+
_
_
_
+
+
+
p - type
n - type
If donor impurities are introduced into one side and
acceptors into the other side of a single crystal of a
semiconductor, a pn-junction is formed.
2
Diffusion Current
Junction
Acceptor ion
_
_
_
+
+
Donor ion
+
Hole
Electron
_
_
_
+
+
+
_
_
_
+
+
+
p - type


n - type
A density gradient across the junction established.
Holes from the p-type will diffuse to the right and
electrons from the n-type to the left.

Diffusion current
3
Re-combination
Depletion region
5
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
p - type


M
n - type
Holes in p-type combine with the diffused electrons.
Electrons in the n-type combine with diffused holes.
4
Depletion Region
Depletion region
5
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
p - type


M
n - type
At the junction, there are no mobile charges.
This region is called the depletion region, the spacecharge region, or the transition region.
5
Drift Current
Depletion region
5
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
p - type


M
n - type
At the junction, there are positive charges to the right
and negative charges to the left.
An electric field is built up across the junction.
6
Contact Potential
Depletion region
5
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
_
_
_
_
+
+
+
+
p - type


M
n - type
Holes from the n-type will tend to flow across the junction.
Electrons from the p-type tends to flow into the n-type.

Drift current
7
Contact Potential


The potential established by this electric field is called
the contact potential VD.
Under steady state conditions
 Diffusion current = drift current
 Net current = 0
8
Reverse Biased
p
n
+
The holes in the p-type and the electrons in the n-type move
away from the junction.
9
Reverse Biased (cont.)



As there are very few holes (electrons) in the n-type (ptype), zero current results.
In practice, a small current does flow because a small
amount of hole-electron pair is generated throughout the
crystal as a result of thermal energy.
This current is called the reverse saturation current, Is.
10
Forward Biased
p
n
+

The potential barrier established by the electric field will
be lowered by the applied voltage.
11
Forward Biased (cont.)


The equilibrium initially established at the junction will be
disturbed.
 Current starts to flow
If the applied voltage is increased to the contact potential VD,
the current will become arbitrary large.
12
Forward Biased (cont.)
I
Cutin voltage
Offset voltage
Threshold voltage
V
The V-I characteristics of a pn-junction with forwardbiased is approximated as a straight line.
13
The V-I Characteristics
I
Breakdown
V
When a reversed-bias voltage is increased to a large value, a
large reverse current will flow.

Breakdown
14
The V-I Characteristics of an Ideal
Junction Diode
I
V
For most applications in the course, we can use this diode
model to maximize simplicity.
15
Modeling of a Junction Diode
I
V
0.7 V
In practice, a real diode can be modeled by an ideal diode in
series with a small battery.
16
Schematic Symbol
+
vD
_
iD
17
Summary




The diode is a non-linear 2-terminal device.
It allows current to flow in one direction (i.e. the
forward direction).
In the forward direction, it acts almost like a short
circuit.
It does not allow any current to flow in the opposite
direction (i.e. reverse direction).

In practice, only the reverse saturation current flows in this
direction.
18
Summary (cont.)



In the reverse direction, it acts almost like an open
circuit.
In practice, a real diode can be modeled by an ideal
diode in series with a small battery.
In most analyses, an ideal diode is considered either as a
short-circuit or an open-circuit, i.e. two states.
19
Diode Applications
1. Rectification
Rectification is the process of turning an
alternating signal (ac) into one that is restricted to
only one direction (dc).
Rectification is classified as

(i) half-wave

(ii) full-wave
20
(i) Half-wave Rectifier
ri
vi

= Vim cos w O t
+_
+vD
_
iD
RL
VL
With one diode, there are two possible states to
consider.


ON
OFF.
21
Half-wave Rectifier (cont.)
(i) The diode is ON.
ri
iD
iD =
vi
+
_
RL
vi
ri + RL
VL
The diode characteristics indicates that only positive
current can flow in this circuit. This requires vi > 0.
22
Half-wave Rectifier (cont.)
(ii) The diode is OFF.
ri
vi
+
_
iD
RL
VL
iD = 0
The happens when the voltage across the diode is
negative. This requires vi < 0.
23
Half-wave Rectifier (cont.)
vi
Vim
wt
2
iD
wt
2
24
Half-wave Rectifier (cont.)

The average output voltage VL(avg) is
!
1 2
VL ( avg ) =
# VLmcos "t d("t )
2! $!
2
VLm
=
!
25
(ii) Full-wave Rectifier
iL
+
RL
vi
1
3
2
4
vL
_
+_
A full-wave rectifier transfers input energy to the output
during both halves of the cycle.
26
Full-wave Rectifier (cont.)


It provides increased average current per cycle over
that obtained by using the half-wave rectifier.
Since there are 4 diodes, there are 16 possible states
to consider. However, only 2 are self-consistent.
27
Full-wave Rectifier (cont.)
vi
Vim
iL
+
wt
2
RL
vi
1
3
2
4
vL
_
+_
i D1
&
i D4
wt
When the input voltage is positive, diodes #1 and #4 are ON
and diodes #2 and #3 are OFF.
28
Full-wave Rectifier (cont.)
vi
Vim
iL
+
wt
2
RL
vi
1
3
2
4
vL
_
+_
i D2
&
i D3
wt
When the input voltage is negative, diodes #2 and #3 are ON
and diodes #1 and 4 are OFF.
29
Full-wave Rectifier (cont.)
iL
wt
vL
VLm
wt
30
(iii) Filtering
vi
+
_
C
RL
VL
The waveform resulted from the half-wave or full-wave
rectifier can be converted to a nearly constant level by
using a capacitor as a simple filter.
31
Filtering (cont.)
VL
t
This circuit can be used to detect the peak value of the
input signal.
 Half-wave peak detector
32
(iv) Full-wave peak rectifier


As in the half-wave case, the output voltage will be
almost equal to the peak value of the input signal.
The ripple frequency, however, will be twice that of
the input.
33
(v) The Peak detector as an AM demodulator
Consider a simple radio broadcast system.
Music
Speech
Transduction
Amplification
Transmission
Remote
listeners
34
The Peak detector as an AM demodulator
(cont.)
Two problems arise:
(i) The transmitting antenna required to convert
the signals to electromagnetic radiation would
have to be very long.
(ii) If all radio stations were to transmit the same
frequency band (audio), the listener would find
it difficult to distinguish those signals.
35
The Peak detector as an AM demodulator
(cont.)
A solution:
To shift the frequency band to be transmitted from
the audio range to a location at a much higher
frequency.
 This reduce the required antenna length.

36
The Peak detector as an AM demodulator
(cont.)
To assign different radio stations with different
radio-frequencies for their transmissions.
 The listener can then select a desired station
by tuning receiver to the broadcasting
frequency.

37
The Peak detector as an AM demodulator
(cont.)
The information contained in the AM signal can
then be extracted or detected using a diode peak
rectifier.
38
The Peak detector as an AM demodulator
(cont.)
2
wC
vC
wt
2
wS
vs
39
Diode Applications (cont.)
2. Clipping Circuits
To select part a signal that lies above or
below a reference level.

Two general categories:



Series
parallel
40
Clipping Circuits (cont.)
In solving this type of circuits, we need to
(i) determine the transition voltage that will cause
a change in state for the diode and the
corresponding applied voltage.

For the ideal diode, the transition will occur at the
point on the characteristics where vD = 0 and iD = 0.
(ii) sketch the output voltage above and below the
transition voltage.
41
(i) Simple series clippers
Example 3. Determine the output waveform for
the following clipping circuit.
vi
V
Vim
o
+
T
__
2
T
t
vi
_
+
R
vo
_
42
Simple series clippers (cont.)
1. The diode changes state
(vd=0 and id=0) when vi = V .
vi
Vim
V
T
__
2
vo
t
T
V
vd=0 i d=0
+
vi
=
V
( diodes change state )
_
Vim V
vi
R
_
t
3. When vi < V, the diode is OFF
and vo=0.
+
vo
_
2. When vi > V, the diode is ON.
By KVL, we have
vo = vi ! V
43
(ii) Simple parallel clippers
Example 4. Determine the output waveform for the
following clipping circuit.
vi
R
+
+
16 V
t
_ 16 V
vi
vo
V = 4V
_
_
44
Simple parallel clippers (cont.)
vi
1. The diode changes state (vd=0 and id=0)
when vi = 4 V . vo = 4 V.
16 V
V = 4V
_ 16 V
t
T
__
2
T
R
+
i d =0
vi
vd =0
+
vo = V = 4 V
V = 4V
16 V
_
4V
t
T
__
2
T
_
2. When vi > V, the diode is OFF. vo = vi.
3. When vi < V, the diode is ON. vo=4 V
45
Diode Applications (cont.)
3. Clamping Circuits
 Clamping circuits “clamp” a signal to a different
dc level.
 Consist of:
 a capacitor
 a diode
 a resistor
 DC supply (for an additional DC shift)
46
Clamping Circuits (cont.)
In solving this type of circuits, we need to
(i) determine the applied voltage that will forwardbias the diode.
(ii) determine the voltage level during the diode
“ON” state by assuming that the capacitor will
charge up instantaneously.
(iii) determine the voltage level during the diode
“OFF” state by assuming that the capacitor
will hold on its established voltage level.
47
Clamping Circuits (cont.)
Example 5. Determine the output waveform for the
following clamping circuit.
vi
C
V
+
T
__
2
_
V
T
t
vi
+
R
vo
_
_
48
1. The diode will be forwardbiased when vi = V. vo = V+VC.
Clamping Circuits (cont.)
vi
C
+
+
_
+
V
V
R
_
V
vo
_
_
V
T
__
2
t
T
C
_
+
V
V
R
+
vo
+
_
vo
_
t
_ 2
1. The diode is OFF when vi = -V. vo =
0.
V
49
OTHER DIODE TYPES (cont.)
1. Shockley diodes
_
+
n+


A Shockley diode is formed by bonding a metal,
such as platinum, to a n-type silicon.
This type of diodes has no depletion layer and can
switch faster than ordinary diodes.
50
Shockley diodes (cont.)


The most important application of Shockley
diodes is in digital computers.
Since a Shockley diode has a cut-in voltage of 0.25
V, they are frequently used in low-voltage
rectifiers.
51
OTHER DIODE TYPES (cont.)
2. Light-emitting diodes (LEDs)


Special materials (e.g. gallium, arsenic and
phosphorous) are used to convert a portion of
this energy into light.
Emit light when forward-biased.
52
OTHER DIODE TYPES (cont.)
3. Zener diodes


Optimize to operate in the breakdown region.
Use mainly in voltage regulators

circuits that hold load voltage almost constant
despite large changes in input voltage and/or
load resistance.
53
Zener diodes (cont.)
The V-I characteristics
I
Breakdown
_ VZ
V
I
Z
min
I
Z
max
54
Zener diodes (cont.)
Schematic symbol
Zener resistance
Ideal
RE
55
Zener regulators
iL
Ri
+
vs
+
VZ
_
iZ
RL
_


vS > Vz
Vo remains constant even when the input voltage
varies over a relatively wide range.
56
Summary



Half-wave and full-wave rectifiers convert an ac signal
to an dc signal.
A filtering capacitor can be added to rectifier circuits to
reduce ripples.
The filtered rectifier circuits can be used in AM
demodulation process.
57
Summary (cont.)


Diodes can be used in wave-shaping circuits that either
clip portions of a signal or shift the dc voltage level. These
circuits are called clipping and clamping circuits
respectively.
Shockley diodes, because of their fast switching nature
and low cut-in voltage, are used extensively in digital
computers and low-voltage rectifications.
58
Summary (cont.)


The LED converts an electrical current into light and is
used extensively in applications such as the sevensegment display.
Zener diodes operate in the reverse breakdown region
and behave like a voltage source in that region. These
devices are used mainly in regulator circuits.
59