Transcript NTUST-EE-2013S

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
Semiconductors
• Semiconductors are crystalline materials that are
characterized by specific energy bands for electrons.
Energy
• Between the bands are gaps; these
gaps represent energies that
Conduction band
electrons cannot posses.
Energy gap
• 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.
Valence band
Energy gap
Second band
Energy gap
First band
Nucleus
Only occurred in 0 K
Atomic Bonding
Atomic Bonding
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.
ElectronEnergy
• For every electron in
the conduction band, a
hole is left behind in
the valence band.
hole pair
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
Free
electron
electrons move from hole-to-hole in the valence band,
effectively creating “hole current” shown by gray arrows.
Si
Si
Si
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
III IV V
band electrons, pentavalent impurities
B C N
are added, forming an n-type
Al Si P
semiconductor. These are elements to
the right of Si on the Periodic Table.
Ga Ge As
• To increase the number of holes,
In Sn Sb
trivalent impurities are added, forming
a p-type semiconductor. These are
elements to the left of Si on the
Periodic Table.
N-Type Semiconductor
• To
increaseSemiconductor
number of free electrons in intrinsic silicon pentavalent
N-Type
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 pmaterial. 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.
p-region nregion
p
n
• The barrier potential in the
depletion region must be overcome
R
in order for the external source to
cause current. For a silicon diode,
+
VBIAS
this is about 0.7 V.
• 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
p
n
insulator. A relatively few electrons
R
manage to diffuse across the
junction, creating only a tiny
+
VBIAS
reverse current.
• 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.
IF
• In the forward bias region,
current increases dramatically
Forward
after the barrier potential (0.7 V
bias
V (breakdown)
for Si) is reached. The voltage
VR
0.7 V
across the diode remains
Reverse
Barrier
approximately equal to the
bias
potential
barrier potential.
• The reverse-biased diode
IR
effectively acts as an insulator
until breakdown is reached.
BR
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 use
IF
depends on your requirements.
• The ideal model assumes the diode is
either an open or closed switch.
VR
• The practical model includes the
barrier voltage in the approximation.
• The complete model includes the
forward resistance of the diode.
Forward
bias
0.7 V
Reverse
bias
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
D1
F
time.
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
Peakmust
inverse
voltage
• Diodes
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