Transcript CHAPTER 16

CHAPTER 16
INTRODUCTION TO
SEMICONDUCTORS
ATOMIC STRUCTURE
AND SEMICONDUCTORS
• The basic structure of semiconductors
– Silicon and Germanium Atoms
ATOMIC BONDING
• The atoms within the crystal structure are
held together by covalent bonds
• This sharing of valence electrons produces
the covalent bonds that hold the atoms
together
CONDUCTION IN
SEMICONDUCTORS
• An energy band
diagram for silicon
crystal occurs only at
a temperature of
absolute 0 K
Conduction Electrons and Holes
• An intrinsic (pure) silicon crystal at room
temperature has sufficient heat energy for
some valence electrons to jump the gap
from the valence band into the conduction
band, which become free electrons
• When an electron jumps to the conduction
band, a vacancy is left in the valence band
within the crystal, called a hole.
Creation of electron-hole
Electron-hole pairs
• Recombination occurs when a conductionband electron loses energy and fall back
into a hole in the valence band
Electron and Hole Current
• When a voltage is applied across a piece
of silicon, the movement of free electrons
is called electron current. The current
which flow opposite with electron current is
called hole current.
Hole current in intrinsic silicon
Comparison of Semiconductors to
Conductors and Insulators
• Pure semiconductive
materials are neither
insulators nor good
conductors because
current in a material
depends directly on
the number of free
electrons
N-TYPE AND P-TYPE
SEMICONDUCTORS
• The conductivities of silicon and
germanium can be increased and
controlled by the addition of impurities to
the intrinsic (pure) semiconductive
material called doping
• The two categories of impurities are ntype and p-type
N-TYPE SEMICONDUCTOR
• To increase the
number of
conduction-band
electron in intrinsic
silicon, pentavalent
impurity atom with
five valence electrons
(such as arsenic (As),
phosphorus (P), and
antimony (Sb) are
added.
n-type
Majority and Minority Carriers of
N-type Semiconductor
• The electrons are called the majority
carries in n-type material ( the n stand for
the negative charge on an electron)
• Holes which are not produced by the
addition of the pentavalent impurity atoms
are called minority carries
P-TYPE SEMICONDUCTOR
• Trivalent impurity atom
(three valence
electrons, such as
aluminum (Al), Boron
(B), and gallium (Ga))
are added to increase
the number of holes in
intrinsic silicon
• Atoms with three
valence electrons are
known acceptor atoms
because they leave a
hole in the
semiconductor’s crystal
structure
p-type
Majority and Minority Carriers of
P-type Semiconductor
• The holes are the majority carries in p-type
material and Electron in p-type material
are the minority carries
THE PN JUNCTION
• The junction of silicon which it has doped
on one half with a trivalent impurity and
the other half with a pentavalent impurity is
called the pn junction
• The pn junction is the feature that allows
diodes , transistor, and other devices to
work
Formation of the Depletion Region
• The area on both sides of the junction is called
depletion region
• The existence of the positive and negative ions on
the opposite sides of the junction creates a barrier
potential (VB) that is the amount of voltage required
to move electrons through the electric field
Energy Diagram of the PN Junction
BIASING THE PN JUNCTION
• Forward Bias
– Forward bias is the condition that permits
current through a pn junction
– The negative terminal of the VBIAS source is
connected to the n region, and the positive
terminal is connected to the p region
The Effect of the Barrier Potential
on Forward Bias
BIASING THE PN JUNCTION
• Reverse Bias
– Reverse bias is the condition that prevents
current through the pn junction
– Reverse current is a very small current
produced by minority carries during reverse
bias
Energy Diagram for Reverse Bias
• When a pn junction is reverse-biased, the nregion conduction band remain at an energy
level that prevents the free electrons from
crossing into the p-region
• There are a few free minority electrons in the pregion conduction band that flow down the
‘energy hill’ into the n-region, and they combine
with minority hole in the valence band
Reverse Breakdown
• If the external reverse-bias voltage is increased
to a large enough value, reverse breakdown
occurs
• When one minority conduction-band electron
goes toward the positive end of the pn junction,
during its travel, it collides with an atom and
imparts enough to knock a valence electron into
the conduction band
• The rapid multiplication of conduction-band
electrons, known as an avalanche effect
DIODE CHARACTERISTICS
• Diode Characteristic
Curve
– Forward bias
• As the forward
voltage approaches
the value of the
barrier potential (0.7 V
for silicon and 0.3 V
for germanium), the
current begins to
increase
DIODE CHARACTERISTICS
• Diode Characteristic
Curve
– Reverse bias
• As the voltage (VR)
increases to the left,
the current remains
near zero until the
breakdown voltage
(VBR) is reached
• When breakdown
occurs, there is a
large reverse current
that can destroy the
diode
Diode symbol
Diode Approximations
• The ideal diode model
Diode Approximations
• The practical diode
model
Diode Approximations
• The complex diode
model
Typical Diode package