Download Module 3 session 1 d

EEE1001 - ELECTRIC
CIRCUITS AND SYSTEMS
Module 3 Session 1
Semiconductor Devices
• Introduction to semiconductors PN Junction Diode: Construction
and operation with characteristics - Rectifiers: Half-wave and
Full-wave. Zener Diode: Construction and operation with
characteristics - Zener Regulator. BJT - Configuration –
Characteristics – CEAmplifier, SCR & MOSFET.
Introduction to semiconductors
What is a Semiconductor?
Microprocessors
LED
Transistors
Capacitors
Range of Conduciveness
The semiconductors fall somewhere
between conductors and insulators.
midway
Range of Conduciveness
Semiconductors have special electronic properties
which allow them to be insulating or conducting
depending on their composition.
Resistance (Ohms)
1833
Michael Faraday
Temperature (ºC)
Discovers that electrical resistively decreases as
temperature increases in silver sulfide.
This is the first investigation of a semiconductor.
Lab: Metals vs. Semiconductors
Data Chart
Temperature
0ºC
25ºC
50ºC
75ºC
100ºC
Copper
31Ω
33Ω
37Ω
41Ω
44Ω
Germanium
5.2Ω
4.2Ω
1.2Ω
0.63Ω
.029Ω
1874
Ferdinand Braun
The first semiconductor device was born.
Scientific Principle of Conduction
Valence Band
• The highest occupied energy band is called the valence band.
• Most electrons remain bound to the atoms in this band.
Conduction Band
• The conduction band is the band of orbitals that are high in energy
and are generally empty.
• It is the band that accepts the electrons from the valence band.
Energy Gap
• The “leap” required for electrons from the Valence Band to
enter the Conduction Band.
Conduction Band
Band Gap
Valence Band
Conductors
In a conductor, electrons can move freely among these orbitals within
an energy band as long as the orbitals are not completely occupied.
Conductors
• In conductors, the valence band is empty.
Conductors
Also in conductors, the energy gap is nonexistent or
relatively small.
Insulators
In insulators, the valence band is full.
Insulators
Also in insulators, the energy gap is relatively large.
Semiconductors
In semiconductors, the valence band is full but the energy
gap is intermediate.
Semiconductors
Only a small leap is required for an electron to enter the
Conduction Band.
Band Diagrams
Semiconductor Materials
Silicon is a very common element, the main element
in sand & quartz.
Silicon’s Arrangement
Types of Semiconductor
• Intrinsic semiconductor
• Extrinsic semiconductor
Intrinsic Silicon
• At any temperature above absolute zero temperature, there is a finite
probability that an electron in the lattice will be knocked loose from its
position.
Intrinsic Silicon
• The electron in the lattice knocked loose from its position
leaves behind an electron deficiency called a "hole".
Current Flow
• If a voltage is applied, then both the electron and the hole
can contribute to a small current flow.
Doping
• Doping (adding an impurity) can
produce 2 types of semi-conductors
depending upon the element
added.
• P- Type
• N- Type
P-Type Doping
In P-type doping, boron or gallium is the dopant.
P-Type Doping
• Boron and gallium each have only three outer
electrons.
• When mixed into the silicon lattice, they form
"holes" in the lattice where a silicon electron has
nothing to bond to.
P-Type Doping
The absence of an electron creates the effect of a
positive charge, hence the name P-type.
Holes can conduct current. A hole happily accepts
an electron from a neighbor, moving the hole over a
space. P-type silicon is a good conductor.
N-Type
• In N-type doping, phosphorus or arsenic is added
to the silicon in small quantities.
N-Type
• Phosphorus and arsenic each have five outer
electrons, so they're out of place when they get
into the silicon lattice.
• The fifth electron has nothing to bond to, so it's
free to move around.
N-Type
• It takes only a very small quantity of the impurity to create
enough free electrons to allow an electric current to flow
through the silicon. N-type silicon is a good conductor.
• Electrons have a negative charge, hence the name N-type.
P-N Junction
• A p-n junction is created by joining together two pieces of
semiconductor, one doped n-type, the other p-type.
P-N Junction
• In the n-type region there are extra electrons and in the ptype region, there are holes from the acceptor impurities .
P-N Junction
In the p-type region there are holes from the
acceptor impurities and in the n-type region there
are extra electrons.
P-N Junction
When a p-n junction is formed, some of the
electrons from the n-region which have reached the
conduction band are free to diffuse across the
junction and combine with holes.
P-N Junction
• Filling a hole makes a negative ion and leaves
behind a positive ion on the n-side.
• A space charge builds up, creating a depletion
region.
P-N Junction
This causes a depletion zone to form around the
junction (the join) between the two materials.
This zone controls the behavior of the diode.
Forward Biasing
Forward biasing the p-n junction drives holes to the
junction from the p-type material and electrons to
the junction from the n-type material.
Forward Biasing
At the junction the electrons and holes combine so
that a continuous current can be maintained.
Diode
A diode is the simplest possible semiconductor
device.
One Way Electric “Turnstile”
A diode allows current to flow in one direction but
not the other.
Jumping
If you apply enough reverse voltage, the junction
breaks down and lets current through.
Reverse Biasing
The application of a reverse voltage to the p-n
junction will cause a transient current to flow as
both electrons and holes are pulled away from the
junction.
Reverse Biasing
When the potential formed by the widened depletion
layer equals the applied voltage, the current will
cease except for the small thermal current.
When forward-biased, there is a small amount of
voltage necessary to get the diode going. In silicon,
this voltage is about 0.7 volts.
This voltage is needed to start the hole-electron
combination process at the junction.
Diode Characteristic
When reverse-biased, an ideal diode would block all
current. A real diode lets perhaps 10 microamps
through -- not a lot, but still not perfect.
Zener diodes
• A Zener diode is a silicon
semiconductor device that
permits current to flow in
either a forward or reverse
direction.
• The diode consists of a
special, heavily doped p-n
junction,
designed
to
conduct in the reverse
direction when a certain
specified
voltage
is
reached.
Working Principle
• If reverse voltage is increased continuously, junction breaks
down and suddenly a large reverse current flows.
• This reverse current is controlled or limited by connecting a
suitable series resistance so that excessive heat produced due
to heavy current flow may not burn the diode.
• Two ways breakdown occur
• Zener breakdown
• Avalanche break down
Zener breakdown
• If the depletion layer of a diode is narrow and a reverse voltage is applied, the
voltage per unit of width of the depletion layer becomes high.
• This establishes a strong electric field intensity which causes electrons to break
away from their parent atoms.
• Thus, a depletion layer which was insulating in nature, becomes a conducting
path.
• This kind of breakdown due to the creation of a strong electric field intensity, i.e.,
V/μm is called zener breakdown
Avalanche breakdown
• If the width of the depletion layer is wide for a zener breakdown, a
sufficient reverse voltage may provide the free electrons (minority carriers
causing saturation current) to gain sufficient energy to knockout electrons
from the atoms of the semiconductor in the depletion region.
• This is called ionization by collision.
• The breakdown occurring this way is called avalanche breakdown.
• The breakdown voltage of a diode can be accurately control at
the time of the doping level. The breakdown voltage of a
commonly available Zener diodes can vary from 1.2 V to 200 V.
• Diodes which are lightly doped and the breakdown voltage is
less than 5.6 V, is due to Zener effect.
• Regulating the voltage of a circuit is its ability to keep the output
voltage fixed regardless of the variation in input voltage or load
current.
• Here, RS is the current limiting resistor, VS is the voltage source
and RL represents the load resistance. RS absorbs the voltage
fluctuations so as to give a constant voltage at the output. Until
load voltage is less than the breakdown voltage, the zener
diode does not show conduction.
• As the voltage at the load increases than breakdown voltage,
the device now starts conduction in the breakdown region.
Thus, at the breakdown region, a constant voltage is maintained
Rectifiers
• Rectifier is a device which convert AC voltage in to pulsating DC
• A rectifier utilizes unidirectional conducting device
Ex : P-N junction diodes
Types
• Depending up on the period of conduction
 Half wave rectifier
 Full wave rectifier
• Depending up on the connection procedure
Bridge rectifier
Half wave rectifier
• The ripple factor is quite high(1.21)
• Rectifier efficiency is very low(40%)
• Transformer Utilization Factor (TUF) is low(0.21)
• The half wave rectifier circuit is normally not used as a power
rectifier circuit
Half wave Rectifiers
As diodes conduct current in one direction and block in other.
When connected with ac voltage, diode only allows half cycle passing
through it and hence convert ac into dc.
As the half of the wave get rectified, the process called half wave
rectification.
 A diode is connected to an ac source and a load resistor
forming a half wave rectifier.
 Positive half cycle causes current through diode, that causes
voltage drop across resistor.
• Advantages of Half Wave Rectifier
• Affordable
• Simple connections
• Easy to use as the connections are simple
• Number of components used are less
• Disadvantages of Half Wave Rectifier
• Ripple production is more
• Harmonics are generated
• Utilization of the transformer is very low
• The efficiency of rectification is low
• Applications of Half Wave Rectifier
• Power rectification: Half wave rectifier is used along with a
transformer for power rectification as powering equipment.
• Signal demodulation: Half wave rectifiers are used for
demodulating the AM signals.
• Signal peak detector: Half wave rectifier is used for detecting the
peak of the incoming waveform.
Diode as Rectifiers
Reversing diode.
Average value of Half wave output voltage:
VAVG = VP / pi
VAVG is approx 31.8% of Vp
 PIV: Peak Inverse Voltage = Vp
Full wave rectifier
• Ripple factor is (0.48)
• Rectifier efficiency is high(81.2%)
• TUF is high(0.693)
Full wave rectifiers
A full wave rectifier allows unidirectional current through the load during the entire 360
degree of input cycle.
Full Wave Rectifier
The output voltage have twice the input frequency.
VAVG = 2VP / pi
VAVG is 63.7% of Vp
70
The Center-Tapped Full wave rectifiers
• A center-tapped transformer is used with two diodes that conduct on
alternating half-cycles.
F
+
+
–
I
Vin
0
D1
During the positive half-cycle,
the upper diode is forwardbiased and the lower diode is
reverse-biased.
Vout
–
0
+
+
RL
–
–
–
D2
+
F
–
D1
+
–
During the negative half-cycle,
the lower diode is forwardbiased and the upper diode is
reverse-biased.
Vin
Vout
+
0
0
–
I
+
+
D2
+
RL
–
–
71