Faculty of Engineering
ECE 142: Electronic Circuits
Lecture 5:
PN Junction (Diode)
The PN Junction
Steady State1
Metallurgical Junction
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N
Space Charge
Region
ionized
acceptors
ionized
donors
E-Field
+
h+ drift
_
+
=
h+ diffusion
e- diffusion
_
=
e- drift
The PN Junction
Metallurgical Junction
NA
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P
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ND
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N
Space Charge Region
ionized
acceptors
ionized
donors
E-Field
+
h+ drift
_
+
=
h+ diffusion
e- diffusion
At steady state, when no
external source is connected to
the pn junction, diffusion and
drift balance each other out for
both the holes and electrons
_
=
e- drift
Depletion Region: This region includes the net positively and negatively charged regions.
The space charge region does not have any free carriers. The width of the space charge
region is denoted by W in pn junction formulae.
Metallurgical Junction: The interface where the p- and n-type materials meet.
The Biased PN Junction
Metal
Contact
“Ohmic
Contact”
(Rs~0)
_
P
+
Applied Electric
Field
n
I
+
_
Vapplied
The pn junction is considered biased when an external voltage is applied.
The Biased PN Junction
Forward Bias:
Vapplied > 0
Reverse Bias:
Vapplied < 0
• Depletion region shrinks slightly in width.
• Energy required for charge carriers to cross the depletion region
decreases exponentially.
• As the applied voltage increases, current starts to flow across the
junction.
• The barrier potential of the diode is the voltage at which appreciable
current starts to flow through the diode.
• The barrier potential varies for different materials.
Depletion region widens.
A small leakage current, Is (saturation current) flows under reverse bias
conditions.
This saturation current is made up of electron-hole pairs being produced
in the depletion region.
Properties of Diodes
Figure 1.10 – The Diode Transconductance Curve2
ID
(mA)
• VD = Bias Voltage
• ID = Current through
Diode. ID is Negative
for Reverse Bias and
Positive for Forward
Bias
IS
VBR
• IS = Saturation Current
~Vφ
VD
• VBR = Breakdown
Voltage
• Vφ = Barrier Potential
Voltage
(nA)
Diode I-V (Shockley) Equation:
ID = IS(eVD/ηηVT – 1)
• As described in the last slide, ID is the current through the diode, IS is the
saturation current and VD is the applied biasing voltage.
k = 1.38 x 10-23 J/K
VT = kT
q
T = temperature in Kelvin
q = 1.6 x 10-19 C
• η is the emission coefficient for the diode. It is determined by the way the
diode is constructed. It somewhat varies with diode current. For a silicon
diode η is around 2 for low currents and goes down to about 1 at higher
currents
Types of Diodes and Their Uses
PN Junction
Diodes:
Are used to allow current to flow in one direction while blocking
current flow in the opposite direction. The pn junction diode is the
typical diode that has been used in the previous circuits.
A
K
P
Schematic Symbol for a PN
Junction Diode
Zener Diodes:
N
Representative Structure for a PN
Junction Diode
Are specifically designed to operate under reverse breakdown
conditions. These diodes have a very accurate and specific reverse
breakdown voltage.
A
Schematic Symbol for a Zener
Diode
K
Types of Diodes and Their Uses
These diodes are designed to have a very fast switching time which
makes them a great diode for digital circuit applications. They are
very common in computers because of their ability to be switched
on and off so quickly.
Schottky Diodes:
A
K
Schematic Symbol for a Schottky
Diode
The Shockley diode is a four-layer diode while other diodes are
normally made with only two layers. These types of diodes are
generally used to control the average power delivered to a load.
Shockley Diodes:
A
K
Schematic Symbol for a four-layer
Shockley Diode
Types of Diodes and Their Uses
Light-Emitting
Diodes (LED):
• Light-emitting diodes are designed with a very large bandgap so
movement of carriers across their depletion region emits
photons of light energy.
• Lower bandgap LEDs (Light-Emitting Diodes) emit infrared
radiation, while LEDs with higher bandgap energy emit visible
light.
• Many stop lights are now starting to use LEDs because they are
extremely bright and last longer than regular bulbs for a
relatively low cost.
A
Schematic Symbol for a LightEmitting Diode
K
The arrows in the LED
representation indicate
emitted light.
Types of Diodes and Their Uses
Photodiodes:
A
A
K
λ
K
• While LEDs emit light, Photodiodes are sensitive to received
light. They are constructed so their pn junction can be exposed
to the outside through a clear window or lens.
• In Photoconductive mode the saturation current increases in
proportion to the intensity of the received light. This type of
diode is used in CD players.
• In Photovoltaic mode, when the pn junction is exposed to a
certain wavelength of light, the diode generates voltage and can
be used as an energy source. This type of diode is used in the
production of solar power.
Faculty of Engineering
ECE 142: Electronic Circuits
Lecture 6:
Diode Applications
Diode Applications
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Half Wave Rectifier
Full Wave Rectifier
Clipping Circuits
Clamping Circuits
Regulator
Regulated Power Supply
Diode Applications
• Half wave rectifier and equivalent circuit with
piece-wise linear model
vi
v i = VM sin (ωt)
Half Wave Rectifier
• We initially consider the diode to be ideal,
such that Vφ =0
Half Wave Rectifier
• The (ideal) diode conducts for vi >0 , thus
v0 ≈ vi
• For vi < 0, the (ideal) diode is an open circuit
(it doesn’t conduct) and
v0 ≈ 0.
Half Wave Rectifier
• In this simplified (ideal diode) case the
input and output waveforms are as shown
The diode must withstand a peak inverse voltage
of VM
Half Wave Rectifier
• The average d.c. value of this half-waverectified sine wave is

1 
=
 ∫ V M sin θ d θ + 0 
2π  0

π
V AV
VM
VM
= − [cosπ − cos0] =
π
2π
Half Wave Rectifier
• So far this rectifier is not very useful.
• Even though the output does not change
polarity it has a lot of ripple
i.e. variations in output voltage about a steady
value.
• To generate an output voltage that more
closely resembles a true d.c. voltage we can
use a reservoir or smoothing capacitor in
parallel with the output (load) resistance.
Smoothed Half Wave Rectifier
Circuit with reservoir
Output voltage
capacitor
The capacitor charges over the period t1 to t2 when the diode is on
and discharges from t2 to t3 when the diode is off.
Smoothed Half Wave Rectifier
• When the supply voltage exceeds the output
voltage the (ideal) diode conducts. During the
charging period (t1 < t< t2)
vo = VM sin (ωt)
Smoothed Half Wave Rectifier
• When the supply voltage falls below the output
voltage the diode switches off and the capacitor
discharges through the load.
• During the discharge period (t2 < t< t3 ) and
vo = VM exp {- t ’ /RC}
where t’= t- t2
• At time t3 the supply voltage once again exceeds
the load voltage and the cycle repeats
Smoothed Half Wave Rectifier
• The resistance in the discharge phase is the
load resistance R.
• RC can be made large compared to the wave
period.
• The change in output voltage (or ripple) can
then be estimated using a linear
approximation to the exponential discharge.
Smoothed Half Wave Rectifier
• vo = VM exp {- t ’ /RC} ≈ VM [ 1- (t ’ /RC)]
• The change in voltage ∆V is therefore
approximately given by VM t ’ /RC
• For a the half wave rectifier this discharge
occurs for a time (t3 - t2 ) close to the period T
= 1/f, with f= frequency.
• Giving the required result:
VMT
∆V ≈
RC
Smoothed Half Wave Rectifier
• We can define a ripple factor as
∆V
Ripple factor =
Vd.c
where Vd.c. = (VM - ∆V/2)
The lower the ripple factor the better
Non-Ideal Half Wave Rectifier
Vφ
VM
Vφ
Full-Wave (Bridge) Rectifier
vi
• We initially consider the diodes to be ideal, such
that VC =0 and Rf =0
• The four-diode bridge can be bought as a package
Full-Wave (Bridge) Rectifier
vi
• During positive half cycles vi is positive.
• Current is conducted through diodes D1, resistor
R and diode D2
• Meanwhile diodes D3 and D4 are reverse biased.
Full-Wave (Bridge) Rectifier
vi
• During negative half cycles vi is negative.
• Current is conducted through diodes D3, resistor
R and diode D4
• Meanwhile diodes D1 and D2 are reverse biased.
Full-Wave (Bridge) Rectifier
• Current always flows the same way through the
load R.
• Show for yourself that the average d.c. value of
this full-wave-rectified sine wave is VAV = 2VM/π
(i.e. twice the half-wave value)
Full-Wave (Bridge) Rectifier
• Two diodes are in the conduction path.
• Thus in the case of non-ideal diodes vo will be
lower than vi by 2VC.
• As for the half-wave rectifier a reservoir
capacitor can be used. In the full wave case
the discharge time is T/2 and
VMT
∆V ≈
2RC
Diode Clipper Circuits
• These circuits clip off portions of signal
voltages above or below certain limits, i.e. the
circuits limit the range of the output signal.
• Such a circuit may be used to protect the
input of a CMOS logic gate against static.
Diode Clipper Circuits
Diode Clipper Circuits
• When the diode is off the output of these
circuits resembles a voltage divider
 RL 
vo = 
v
i

R
L
+
R
S


Diode Clipper Circuits
• If RS << RL
v0 ≈ vi
• The level at which the signal is clipped can be
adjusted by adding a d.c. bias voltage in series
with the diode.
For instance
Diode Clipper Circuits
• Let’s look at a few other examples of clipper
circuits.
Clipper circuits using zeners
Voltage Regulator
Figure 3.24 A voltage regulator supplies constant voltage to a load.
Designing a power supply
Diode Clamper Circuits
• The following circuit acts as a d.c. restorer.
Diode Clamper Circuits
• A bias voltage can be added to pin the output
to a level other than zero.