Lecture 5: PN Junction...
Transcript of Lecture 5: PN Junction...
ECE 142: Electronic Circuits
Lecture 5:
PN Junction (Diode)
Faculty of EngineeringFaculty of EngineeringFaculty of EngineeringFaculty of Engineering
The PN JunctionSteady State1
P N
- - - - - -
- - - - - -
- - - - - -
- - - - - -
- - - - - -
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
NA ND
Metallurgical Junction
Space Charge Regionionized
acceptorsionized
donors
E-Field
++_ _
h+ drift h+ diffusion e- diffusion e- drift= =
The PN Junction
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
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.
P N
- - - - - -
- - - - - -
- - - - - -
- - - - - -
- - - - - -
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
NA ND
Metallurgical Junction
Space Charge Regionionized
acceptorsionized
donors
E-Field
++_ _
h+ drift h+ diffusion e- diffusion e- drift= =
The Biased PN Junction
P n
+_
Applied Electric
Field
Metal
Contact
“Ohmic
Contact”
(Rs~0)
+_
Vapplied
I
The pn junction is considered biased when an external voltage is applied.
The Biased PN Junction
Forward Bias: • 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.
Reverse Bias: 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.
Vapplied > 0
Vapplied < 0
Properties of DiodesFigure 1.10 – The Diode Transconductance Curve2
• VD = Bias Voltage
• ID = Current through Diode. ID is Negative for Reverse Bias and Positive for Forward Bias
• IS = Saturation Current
• VBR = Breakdown Voltage
• Vφ = Barrier Potential
Voltage
VD
ID (mA)
(nA)
VBR
~Vφ
IS
Diode I-V (Shockley) Equation:
ID = IS(eVD/ηηηηVT – 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.
VT = kTq
k = 1.38 x 10-23 J/K T = temperature in Kelvin q = 1.6 x 10-19 C
• ηηηη 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
Schematic Symbol for a PN
Junction Diode
P N
Representative Structure for a PN
Junction Diode
Zener Diodes: Are specifically designed to operate under reverse breakdown
conditions. These diodes have a very accurate and specific reverse
breakdown voltage.
A K
Schematic Symbol for a Zener
Diode
Types of Diodes and Their Uses
Schottky Diodes: 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.
A K
Schematic Symbol for a Schottky
Diode
Shockley Diodes: 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.
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 K
Schematic Symbol for a Light-
Emitting Diode
The arrows in the LED
representation indicate
emitted light.
Types of Diodes and Their Uses
Photodiodes: • 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.
A K
A Kλ
ECE 142: Electronic Circuits
Lecture 6:
Diode Applications
Faculty of EngineeringFaculty of EngineeringFaculty of EngineeringFaculty of Engineering
Diode Applications
• 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-wave-
rectified sine wave is
+= ∫
π
θθπ
0
0sin2
1dVV MAV
[ ]π
ππ
MM VV=−−= 0coscos
2
Half Wave Rectifier
• So far this rectifier is not very useful.
• Even though the output does not change polarity it has a lot of ripplei.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
capacitor
Output voltage
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:
RC
TV∆V
M≈
Smoothed Half Wave Rectifier
• We can define a ripple factor as
where Vd.c. = (VM - ∆V/2)
The lower the ripple factor the better
d.cV
∆Vfactor Ripple =
Non-Ideal Half Wave Rectifier
VM
Vφ
Vφ
Full-Wave (Bridge) Rectifier
• 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
vi
Full-Wave (Bridge) Rectifier
• 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.
vi
Full-Wave (Bridge) Rectifier
• 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.
vi
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
2RC
TV∆V
M≈
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
i
SL
L
o vRR
Rv
=
+
Diode Clipper Circuits
• If RS << RL
• The level at which the signal is clipped can be
adjusted by adding a d.c. bias voltage in series
with the diode.
v0 ≈≈≈≈ vi
For instance
Diode Clipper Circuits
• Let’s look at a few other examples of clipper
circuits.
Clipper circuits using zeners
Figure 3.24 A voltage regulator supplies constant voltage to a load.
Voltage Regulator
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.