Transcript Introduction and Digital Images

Today
• Course overview and information
09/16/2010
© 2010 NTUST
Silicon and Germanium Atoms
•Two types of semiconductors are silicon (Si) and germanium (Ge)
•Both the Si and Ge atoms have four valence electrons
•Si has 14 protons in its nucleus and Ge has 32
Energy Band
Semiconductors
Semiconductors are crystalline materials that are
characterized by specific energy bands for electrons.
Energy
Between the bands are gaps;
these gaps represent energies
Conduction band
that electrons cannot posses.
The last energy band is the
conduction band, where electrons
are mobile.
The next to the last band is the
valence band, which is the energy
level associated with electrons
involved in bonding.
Energy gap
Valence band
Energy gap
Second band
Energy gap
First band
Nucleus
Atomic Bonding
Atomic Bonding
Electron-Hole Pair
Electron and Hole
At room temperature, some electrons have enough
energy to jump into the conduction band.
After jumping the gap, these electrons are free to drift throughout
the material and form electron current when a voltage is applied.
Electronhole pair
Energy
For every electron
in the conduction
band, a hole is left
behind in the
valence band.
Conduction band
Energy gap
Valence band
Heat
energy
Electron and Hole
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
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 conduction
band electrons, pentavalent impurities
are added, forming an n-type
semiconductor. These are elements to
the right of Si on the Periodic Table.
To increase the number of holes, trivalent
impurities are added, forming a p-type
semiconductor. These are elements to the
left of Si on the Periodic Table.
III IV V
B C N
Al Si P
Ga Ge As
In Sn Sb
Doping
Doping
N-Type Semiconductor
N-Type Semiconductor
•To increase number of free electrons in intrinsic
silicon pentavalent atoms are added
•These are atoms with five valence electrons
•Each pentavalent atom forms covalent bonds
with four adjacent silicon atoms
N-Type Semiconductor
•Four of a pentavalent atoms’s valence electrons are used to form the
covalent bonds with silicon atoms, leaving extra electron
•This extra electron becomes a free electron because it is not attached
to any atom.
•Since most of the current carriers are electrons, silicon doped in this
way is an n-type semiconductor.
•The n stands for the negative charge on an electron
P-Type Semiconductor
•To increase number of holes in intrinsic silicon
trivalent atoms are added
•These are atoms with three valence electrons
•Each trivalent atom forms covalent bonds with
four adjacent silicon atoms
P-Type Semiconductor
•Since four electrons are required, a hole is formed with each trivalent
atom.
•Holes can be thought of as positive charges
•Since most of the current carriers are holes, silicon doped in this way
is an p-type semiconductor.
•The p stands for the positive charge on an electron
Distinction between Conductor,
Semiconductor and Insulator
Diode
Diode
The PN Junction Diode
When a pn junction is formed, electrons in the n-material
diffuse across the junction and recombine with holes in
the p-material. This action continues until the voltage of
the barrier repels further diffusion. Further diffusion
across the barrier requires the application of a voltage.
The pn junction is basically a diode,
which is a device that allows current
in only one direction. A few typical
diodes are shown.
Forward Bias
When a pn junction is forward-biased, current is permitted.
The bias voltage pushes conduction-band electrons in the
n-region and holes in the p-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 is
about 0.7 V.
p-region nregion
p
n
R
-
+
VBIAS
The forward-bias causes the depletion region to be narrow.
Forward Bias
Forward Bias
Formation of the Depletion Region
Formation of the Depletion Region
Reverse Bias
When a pn junction is reverse-biased, the bias voltage
moves conduction-band electrons and holes away from the
junction, so current is prevented.
p-region n-region
The diode effectively acts as an
insulator. A relatively few electrons
manage to diffuse across the junction,
creating only a tiny reverse current.
p
n
R
-
+
VBIAS
The reverse-bias causes the depletion region to widen.
Reverse Bias
Diode Characteristics
The forward and reverse characteristics are shown on
a V-I characteristic 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.
The reverse-biased diode
effectively acts as an insulator
until breakdown is reached.
IF
VBR (breakdown)
Forward
bias
VR
0.7 V
Reverse
bias
Barrier
potential
IR
VF
Diode Characteristic Curve
Diode Characteristic Curve
Diode Symbol
Ideal Diode Model
Practical Diode Mode
Practical Diode Model
Diode Models
The characteristic curve for a diode can be approximated
by various models of diode behavior. The model you will
IF
use depends on your requirements.
The ideal model assumes the diode is
either an open or closed switch.
The practical model includes the VR
barrier voltage in the approximation.
Forward
bias
0.7 V
Reverse
bias
The complete model includes the
forward resistance of the diode.
IR
VF
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
RL
D
- +
RL
Half-wave Rectifier
Examples
Determine the peak output voltage and the average
value of the output voltage of the rectifier
Vout  Vin - 0.7  5 - 0.7  4.3
VAVG 
Vout

 1.37V
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.
The ac on each side of the center-tap is ½ of the total secondary
voltage. Only one diode will be biased on at a time.
D1
F
Vsec
2
Vsec
2
D2
RL
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.
At any instant, two of the diodes are conducting and two are off.
F
D3
D2
D1
D4
RL
Peak Inverse Voltage
Peak
inverse
Diodes
must voltage
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.
Notice that Vp(sec) = 2Vp(out) for
the full-wave circuit because
the output is referenced to the
center tap.
0V
Vsec
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-conducting
diode in the loop.
Notice that Vp(sec) = Vp(out) for the bridge because the output is
across the entire secondary.
0V
Vsec
Examples
Example
(a).
Determine the peak output voltage for the bridge
recitfier
(b). What minimum PIV rating is required for the
diodes
Vout  Vin  n  25V
PIV  Vout  25V
Special-purpose Diodes
Special-purpose diodes
Special purpose diodes include
Zener diodes – used for establishing a reference voltage
Varactor diodes – used as variable capacitors
Light-emitting diodes – used in displays
Photodiodes – used as light sensors
Selected Key Terms
Majority carrier The most numerous charge carrier in a doped
semiconductor material (either free electrons or
holes.
Minority carrier The least numerous charge carrier in a doped
semiconductor material (either free electrons or
holes.
PN junction The boundary between n-type and p-type
semiconductive materials.
Diode An electronic device that permits current in only
one direction.
Selected Key Terms
Barrier potential The inherent voltage across the depletion region of a
pn junction diode.
Forward bias The condition in which a diode conducts current.
Reverse bias The condition in which a diode prevents current.
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.
Selected Key Terms
Bridge rectifier A type of full-wave rectifier consisting of
diodes arranged in a four corner
configuration.
Zener diode A type of diode that operates in reverse
breakdown (called zener breakdown) to
provide a voltage reference.
Varactor A diode used as a voltage-variable capacitor.
Photodiode A diode whose reverse resistance changes
with incident light.
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.
Quiz
2. A intrinsic silicon crystal is
a. a poor conductor of electricity.
b. an n-type of material.
c. a p-type of material.
d. an excellent conductor of electricity.
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.
III IV V
B C N
Al Si P
Ga Ge As
In Sn Sb
Quiz
4. At room temperature, free electrons in a p-material
a. are the majority carrier.
b. are the minority carrier.
c. are in the valence band.
d. do not exist.
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.
Quiz
6. The circuit shown is a
a. half-wave rectifier.
b. full-wave rectifier.
c. bridge rectifier.
d. zener regulator.
Quiz
7. PIV stands for
a. Positive Ion Value.
b. Programmable Input Varactor.
c. Peak Inverse Voltage.
d. Primary Input Voltage.
Quiz
8. A type of diode used a a voltage-variable capacitor is
a
a. varactor.
b. zener.
c. rectifier.
d. LED.
Quiz
9. If one of the four diodes in a bridge rectifier is open,
the output will
a. be zero.
b. have ½ as many pulses as normal.
c. have ¼ as many pulses as normal.
d. be unaffected.
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.
Quiz
Answers:
1. c
6. b
2. a
7. c
3. c
8. a
4. b
9. b
5. d
10. d