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Transcript of Semiconductors
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Semiconductors
Semiconductors are crystalline materials that are
characterized by specific energy bands for electrons.
The last energy band is the
conduction band, where electrons
are mobile.
Between the bands are gaps;
these gaps represent energiesthat electrons cannot posses.
Nucleus
First band
Second band
Valence band
Conduction band
Energy gap
Energy gap
Energy gap
Energy
The next to the last band is the
valence band, which is the energy
level associated with electrons
involved in bonding.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Electron and hole current
At room temperature, some electrons have enough
energy to jump into the conduction band.
Valence band
Conduction band
Energy gap
Energy
After jumping the gap, these electrons are free to drift throughout
the material and form electron current when a voltage is applied.
Heatenergy
Electron-
hole pairFor every electron
in the conduction
band, a hole is leftbehind in the
valence band.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Electron and hole current
The electrons in the conduction band and the holes in
the valence band are the charge carriers. In other words,
current in the conduction band is by electrons; current
in the valence band is by holes.
When an electron jumps to the conduction band, valence
electrons move from hole-to-hole in the valence band,
effectively creating hole current shown by gray arrows.
Si Si Si
Free
electron
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Impurities
By adding certain impurities to pure (intrinsic) silicon,
more holes or more electrons can be produced within
the crystal.
To increase the number of holes, trivalent
impurities are added, forming ap-type
semiconductor. These are elements to the
left of Si on the Periodic Table.
To increase the number of conduction
band electrons, pentavalent impurities
are added, forming an n-type
semiconductor. These are elements to
the right of Si on the Periodic Table. Si
B
Al
Ga
P
As
Sb
Ge
C
Sn
N
III IV V
In
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Thepnjunction diode
When apnjunction is formed, electrons in the n-material
diffuse across the junction and recombine with holes in
thep-material. This action continues until the voltage of
the barrier repels further diffusion. Further diffusion
across the barrier requires the application of a voltage.
Thepnjunction is basically a diode,
which is a device that allows currentin only one direction. A few typical
diodes are shown.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Forward bias
When apnjunction is forward-biased, current is permitted.
The bias voltage pushes conduction-band electrons in the
n-region and holes in thep-region toward the junction
where they combine.
The barrier potential in the depletion
region must be overcome in order
for the external source to cause
current. For a silicon diode, this isabout 0.7 V.
p-region n-region
p n
+
R
VBIAS
The forward-bias causes the depletion region to be narrow.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Reverse bias
When apnjunction is reverse-biased, the bias voltage
moves conduction-band electrons and holes away from the
junction, so current is prevented.
The diode effectively acts as an
insulator. A relatively few electrons
manage to diffuse across the
junction, creating only a tiny reversecurrent.
p-region n-region
p n
+VBIAS
R
The reverse-bias causes the depletion region to widen.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Diode characteristics
The forward and reverse characteristics are shown on
a V-Icharacteristic curve.
In the forward bias region, current
increases dramatically after the
barrier potential (0.7 V for Si) is
reached. The voltage across the
diode remains approximately
equal to the barrier potential.
VR VF
IF
IR
Reversebias
Forward
bias
0.7 V
Barrierpotential
The reverse-biased diode
effectively acts as an insulator
until breakdown is reached.
VBR(breakdown)
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Diode models
The characteristic curve for a diode can be approximated
by various models of diode behavior. The model you will
use depends on your requirements.
The ideal model assumes the diode is
either an open or closed switch.
VR VF
IF
IR
Reversebias
Forward
bias
The complete model includes the
forward resistance of the diode.
The practical model includes the
barrier voltage in the approximation.0.7 V
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Half-wave Rectifier
Rectifiers are circuits that convert ac to dc. Special
diodes, called rectifier diodes, are designed to handle the
higher current requirements in these circuits.
The half-wave rectifier
converts ac to pulsating
dc by acting as a closed
switch during the
positive alteration.
The diode acts as an
open switch during the
negative alteration.
D
D
RL
RL
+ -
- +
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Full-wave Rectifier
The full-wave rectifier allows unidirectional current on
both alterations of the input. The center-tapped full-wave
rectifier uses two diodes and a center-tapped transformer.
F D1
D2RL
Vsec
2
Vsec2
The ac on each side of the center-tap is of the total secondary
voltage. Only one diode will be biased on at a time.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Bridge Rectifier
The bridge rectifier is a type of full-wave circuit that uses
four diodes. The bridge rectifier does not require a
center-tapped transformer.
F
D1
D2RL
At any instant, two of the diodes are conducting and two are off.
D3
D4
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Peak inverse voltage
Diodes must be able to withstand a reverse voltage when
they are reverse biased. This is called the peak inverse
voltage (PIV). The PIV depends on the type of rectifier
circuit and the maximum secondary voltage.
For example, in a full-wave circuit, if one diode is conducting
(assuming 0 V drop), the other diode has the secondary voltage
across it as you can see from applying KVL around the green path.
0 V
Vsec
Notice that Vp(sec)= 2Vp(out)forthe full-wave circuit because
the output is referenced to the
center tap.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Peak inverse voltage
For the bridge rectifier, KVL can be applied to a loop that
includes two of the diodes. Assume the top diode is
conducting (ideally, 0 V) and the lower diode is off. The
secondary voltage will appear across the non-conductingdiode in the loop.
0 V
Notice that Vp(sec)= Vp(out)for the bridge because the output is
across the entire secondary.
Vsec
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Power supplies
By adding a filter and regulator to the basic rectifier, a
basic power supply is formed.
7805
FD1
D2C1
D3
D4 C2
Typically, a large electrolytic capacitor is used as a filter before the
regulator, with a smaller one following the regulator to complete
filtering action.
1000 mF 1 mF
IC regulator
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Special-purpose diodes
Special purpose diodes include
Zener diodesused for establishing a reference voltage
Varactor diodesused as variable capacitors
Light-emitting diodesused in displays
Photodiodesused as light sensors
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Troubleshooting power supplies
Begin troubleshooting by analyzing the symptoms and how
it failed. Try to focus on the most likely causes of failure.
7805
FD1
D2C1
D3
D4 C21000 mF 1 mF
IC regulator
A power supply has no output, but was working until a
newly manufactured PC board was connected to it. (a) Analyze
possible failures. (b) Form a plan for troubleshooting.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Troubleshooting power supplies
7805
FD1
D2C1
D3
D4 C21000 mF 1 mF
IC regulator
The supply had been working, so the problem is not
likely to be an incorrect part or wiring problem. The failure was
linked to the fact that a new PC board was connected to it,
which points to a possible overloading problem. If the load wastoo much for the supply, it is likely a fuse would have blown, or
a part would likely have overheated, accounting for the lack of
output.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Troubleshooting power supplies
1.Disconnect power and check the fuse. If it is bad, replace it.
Before reapplying power, remove the load, open the power supply
case, and look for evidence of overheating (such as discolored
parts or boards). If no evidence of overheating proceed.
2.Check the new pc board (the load) for a short or overloading of
the power supply that would cause the fuse to blow. Look for
evidence of overheating.
3.Verify operation of the supply with measurements (see next
slide).
Based on the analysis, a sample plan is as
follows. (It can be modified as circumstances warrant.)
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Summary
Troubleshooting power supplies
Reapply power to the supply but with no load. If the output is
okay, put a resistive test load on the power supply and measure
the output to verify it is operational. If the output is correct, theproblem is probably with the new pc board. If not, you will need
to further refine the analysis and plan, looking for an internal
problem.
The analysis showed that a
likely cause of failure was due to an overload. For the
measurement step, it may be as simple as replacing the
fuse and confirming that the supply works. After
replacing the fuse:
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Majori ty carr ier
Minority carrier
PN junction
Diode
The most numerous charge carrier in a dopedsemiconductor material (either free electrons
or holes.
Selected Key Terms
The boundary between n-type andp-type
semiconductive materials.
An electronic device that permits current in
only one direction.
The least numerous charge carrier in a dopedsemiconductor material (either free electrons
or holes.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Barrier
potential
Forward bias
Reverse bias
Full-wave
rectifier
A circuit that converts an alternating sine-
wave into a pulsating dc consisting of both
halves of a sine wave for each input cycle.
The condition in which a diode conducts
current.
The inherent voltage across the depletionregion of apnjunction diode.
Selected Key Terms
The condition in which a diode prevents
current.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Bridge recti f ier
Zener diode
Varactor
Photodiode A diode whose reverse resistance changeswith incident light.
A type of diode that operates in reverse
breakdown (called zener breakdown) toprovide a voltage reference.
A type of full-wave rectifier consisting ofdiodes arranged in a four corner configuration.
Selected Key Terms
A diode used as a voltage-variable capacitor.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
1. An energy level in a semiconductor crystal in which
electrons are mobile is called the
a. barrier potential.
b. energy band.
c. conduction band.
d. valence band.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
2. A intrinsic silicon crystal is
a. a poor conductor of electricity.
b. an n-type of material.
c. ap-type of material.
d. an excellent conductor of electricity.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
3. A small portion of the Periodic Table is shown. The
elements highlighted in yellow are
a. majority carriers.
b. minority carriers.
c. trivalent elements.
d. pentavalent elements.
Si
B
Al
Ga
P
As
Sb
Ge
C
Sn
N
III IV V
In
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
4. At room temperature, free electrons in ap-material
a. are the majority carrier.
b. are the minority carrier.
c. are in the valence band.
d. do not exist.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
5. The breakdown voltage for a silicon diode is reached
when
a. the forward bias is 0.7 V.
b. the forward current is greater than 1 A.
c. the reverse bias is 0.7 V.
d. none of the above.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
6. The circuit shown is a
a. half-wave rectifier.
b. full-wave rectifier.
c. bridge rectifier.
d. zener regulator.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
7. PIV stands for
a. Positive Ion Value.
b. Programmable Input Varactor.
c. Peak Inverse Voltage.
d. Primary Input Voltage.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
8. A type of diode used a a voltage-variable capacitor is a
a. varactor.
b. zener.
c. rectifier.
d. LED.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
9. If one of the four diodes in a bridge rectifier is open, the
output will
a. be zero.
b. have as many pulses as normal.
c. have as many pulses as normal.
d. be unaffected.
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Chapter 16
Electronics FundamentalsCircuits, Devices and App lications - Floyd
Copyright 2007 Prentice-Hall
Quiz
10. When troubleshooting a power supply that has a
bridge rectifier, begin by
a. replacing the bridge rectifier.
b. replacing the transformer.
c. making measurements.
d. analyzing the symptoms and how it failed.
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Chapter 16
Electronics Fundamentals Copyright 2007 Prentice Hall
Quiz
Answers:
1. c
2. a
3. c
4. b
5. d
6. b
7. c
8. a
9. b
10. d