Electronic_Devices_10th_CH01 - Read-Only.pptx

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Electronic Devices
10th
ed.
Chapter 1
Introduction to
Semiconductors

Electronic Devices
10th
ed.
◆ Describe the structure of an atom
◆ Discuss insulators, conductors, and semiconductors and
how they differ
◆ Describe how current is produced in a semiconductor
◆ Describe the properties of n-type and p-type
semiconductors
◆ Describe how a pn junction is formed
Objectives:

Electronic Devices
Bohr model
Shell 1
Shell 2
Nucleus
Energy
The Bohr model of the atom is that electrons can
circle the nucleus only in specific orbits, which
correspond to discrete energy levels called
shells.

Electronic Devices
Conductors
Materials can be classified by their ability to conduct
electricity. This ability is related to the valence electrons.
+29
Valence electrons are those
electrons that occupy the
outer shell.

Electronic Devices
Conductors
Materials can be classified by their ability to conduct
electricity. This ability is related to the valence electrons.
Copper is an example of an
excellent conductor. It has only
one electron in its valence band,
which can easily escape to the
conduction band, leaving behind a
positive ion (the core). Like all
metals, copper has many free
electrons which are loosely held
by the attraction of the positive
metal ions.
+29

Electronic Devices
Insulators
Insulators have tightly bound electrons with few
electrons available for conduction.
Nonmetals, such as glass, air, paper, and rubber are excellent
insulators and widely used in electronics. Even these materials can
break down and conduct
electricity if the voltage is
high enough such as in the
case of lightning, which
breaks down air.

Electronic Devices
Semiconductors
Semiconductors are between conductors and insulators
in their ability to conduct electricity.
+14
Core (+4)

Electronic Devices
Semiconductors
Silicon is an example of a single
element semiconductor. It has four
electrons in its valence band.
+14
Core (+4)

Electronic Devices
Semiconductors
Silicon is an example of a single
element semiconductor. It has four
electrons in its valence band.
Unlike metals, silicon forms strong covalent
bonds (shared electrons) with its neighbors.
Intrinsic silicon is a poor conductor because most
of the electrons are bound in the crystal and take
part in forming the bonds between atoms.
+4
+4
+4
+4
+4
+14
Core (+4)

Electronic Devices
Energy gap in semiconductors
In intrinsic silicon, a few electrons can jump the energy
gap between the valence and conduction band. Having
moved into the conduction band, a “hole” (vacancy) is
left in the crystal structure.
Conduction
band
Valence
band
Energy gap
Hole
Free
electron
Electron-hole pair
Heat
Energy

Electronic Devices
Charge movement in semiconductors
Within the crystalline structure, there are two types
of charge movement (current):
1) The conduction band electrons are free to move under the
influence of an electric field.
Si Si Si
Electrons

Electronic Devices
Charge movement in semiconductors
Within the crystalline structure, there are two types
of charge movement (current):
1) The conduction band electrons are free to move under the
influence of an electric field.
Si Si Si
2) The bound (valence) electrons move between atoms,
effectively moving holes from one atom to another as
illustrated. Holes act like positive charges, with their own
mobility.
Holes Electrons

Electronic Devices
Impurities in semiconductors
Certain impurities will change the conductivity of
silicon. An impurity such as Antimony has an electron
that is not part of the bonding electrons so is free. This
creates an n-material.
Free (conduction) electron
from Sb atom
Sb
Si
Si
Si
Si

Electronic Devices
from Sb atom
Sb
Si
Si
Si
Si
Where on the periodic table
would you expect to find another
element that could be used as an
impurity to create an n-material?

Electronic Devices
from Sb atom
Sb
Si
Si
Si
Si
Where on the periodic table
would you expect to find another
element that could be used as an
impurity to create an n-material?
Elements above or below Sb will have
the same valence electron structure.

Electronic Devices
Vacancies in the crystal structure
An impurity such as boron leaves a vacancy in the
valence band, creating a p-material. Both p- and n-
materials have energy levels that are different than
intrinsic silicon.
B
Si
Si
Si
Si

Electronic Devices
Formation of the pn junction
A p- and an n-material together form a pn junction.
p region n region
Depletion region
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
When the junction is formed, conduction electrons move to the p-
region, and fall into holes. Filling a hole makes a negative ion and
leaves behind a positive ion in the n-region. This creates a thin
region that is depleted of free charges at the boundary.

Electronic Devices
p region n region
Depletion region
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
What process stops the migration of
charge across the boundary?

Electronic Devices
p region n region
Depletion region
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
A potential is built up (called the
barrier potential) that prevents further
charge migration.
What process stops the migration of
charge across the boundary?

Electronic Devices
Energy diagram for a pn junction
The energy diagram for the n-region shows a lower
potential than for the p-region.
Conduction
band
Valence
band
0
Energy
p region n region
Depletion
region

Electronic Devices
Conduction
band
Valence
band
0
Energy
p region n region
Depletion
region
Why do you think that the
energy level in the n- region
is lower than the p-region?

Electronic Devices
Conduction
band
Valence
band
0
Energy
p region n region
Depletion
region
Why do you think that the
energy level in the n- region
is lower than the p-region?
The n-region tends to have filled valence
shells; conduction electrons are shielded by
these electrons, so they are further away
from the nucleus and have less energy.

Simple Example:
• Imagine you have two hills:
• Hill A (n-type material) is shorter.
• Hill B (p-type material) is taller.
• To get to the top of the hill (conduction band), you need to climb from
the bottom (valence band).
• Climbing Hill A (n-type) is easier because it's shorter.
• Climbing Hill B (p-type) is harder because it's taller.
• So, in an n-type material, electrons need less energy to move and
conduct electricity compared to a p-type material.

Electronic Devices
Diodes
A diode is a semiconductor device with a single pn
junction and metal connections to leads. It has the
ability to pass current in only one direction.
p n
Depletion
region

Electronic Devices
Forward bias
Forward bias is the condition which allows current in
the diode. The bias voltage must be greater than the
barrier potential.
R
IF
BIAS
V
–
+
–
+
+
–
F
V
B
0.7 V
C
A
0
0
Knee
VF
IF(mA)

Electronic Devices
Reverse bias
Reverse bias is the condition in which current is
blocked.
R
BIAS
BIAS
I = 0 A
– +
– +
V
V
0
0
Knee
VR
IR
VBR

Electronic Devices
Diode approximations
VR
IR
VF
IF
0.7 V
VR
IR
VF
IF
0.7 V
VR
IR
VF
IF
Ideal Practical Complete
Three diode approximations are:

Electronic Devices
Diode approximations
VR
IR
VF
IF
0.7 V
VR
IR
VF
IF
0.7 V
VR
IR
VF
IF
Ideal Practical Complete
In addition, the complete model includes the effect of a large
reverse resistance that accounts for a tiny current when reverse-
biased.
Three diode approximations are:

Electronic Devices
Example
Use the practical model to determine the current in the
circuit:
12 V
–
+
R
3.3 kW
VBIAS

Electronic Devices
Example
circuit:
12 V
–
+
R
3.3 kW
VBIAS
BIAS 0.7 V 12 V 0.7 V 11.3 V
R
V V
    

Electronic Devices
Example
circuit:
12 V
–
+
R
3.3 kW
VBIAS
BIAS 0.7 V 12 V 0.7 V 11.3 V
R
V V
    
R 11.3 V
3.3 k
V
I
R
  


Electronic Devices
Example
circuit:
12 V
–
+
R
3.3 kW
VBIAS
BIAS 0.7 V 12 V 0.7 V 11.3 V
R
V V
    
R 11.3 V
3.3 k
V
I
R
  

3.4 mA

Electronic Devices
Diode packages
Some common configurations are
A
K
K
K
A
K
A
A
K
A
K A
K
A
K
A
K
K
A
K
K
A

Electronic Devices
Selected key terms-1
Electron
Valence
Free Electron
Conductor
Insulator

Electronic Devices
Electron
Valence
Free Electron
Conductor
Insulator
The basic particle of negative electrical charge.

Electronic Devices
Electron
Valence
Free Electron
Conductor
Insulator
Related to the outer shell of an atom.

Electronic Devices
Electron
Valence
Free Electron
Conductor
Insulator
An electron that has acquired enough energy to
break away from the valence band of the parent
atom; also called a conduction electron.

Electronic Devices
Electron
Valence
Free Electron
Conductor
Insulator
A material that easily conducts electrical current.

Electronic Devices
Electron
Valence
Free Electron
Conductor
Insulator
A material that easily conducts electrical current.
A material that does not normally conduct
current.

Electronic Devices
Semiconductor
Crystal
Hole
Diode

Electronic Devices
Semiconductor
Crystal
Hole
Diode
A material that lies between conductors and
insulators in its conductive properties.

Electronic Devices
Semiconductor
Crystal
Hole
Diode
A solid material in which the atoms are
arranged in a symmetrical pattern.

Electronic Devices
Semiconductor
Crystal
Hole
Diode
The absence of an electron in the valence
band of an atom in a semiconductor crystal.

Electronic Devices
Semiconductor
Crystal
Hole
Diode
The absence of an electron in the valence
band of an atom in a semiconductor crystal.
A semiconductor device with a single pn
junction that conducts current in one direction
only.

Electronic Devices
Quiz: Q1
1. A semiconductor is a crystalline material with
a. many free electrons held by the attraction of
positive ions
b. strong covalent bonds between neighboring atoms
c. only one electron in its outer shell
d. a filled valence shell

Electronic Devices
Quiz: Q2
2. A metallic conductor has
a. many free electrons held by the attraction of
positive ions
b. covalent bonds between neighboring atoms
c. four electrons in its outer shell
d. a filled valence shell

Electronic Devices
Quiz: Q3
3. In a semiconductor, the concept of an energy gap is
used to show the difference between the energies of the
a. nucleus and outer shell electrons
b. nucleus and the free electrons
c. conduction band electrons and valence electrons
d. core electrons and valence electrons

Electronic Devices
Quiz: Q4
4. An impurity such as Antimony (Sb) has five electrons
in its outer shell. When silicon has Sb impurities,
a. an n material is formed
b. the crystal will be negatively charged
c. both of the above
d. none of the above

Electronic Devices
Quiz: Q5
5. Compared to a p-material, the energy levels in an n-
material are
a. the same
b. greater
c. lower

Electronic Devices
Quiz: Q6
6. When a pn junction is formed, electrons move across
the junction and fill holes in the p-region. The filled hole
is a
a. neutral atom
b. minority carrier
c. positive ion
d. negative ion

Electronic Devices
Quiz: Q7
7. The forward biased knee voltage in a semiconductor
diode is approximately equal to the
a. bias supply voltage
b. breakdown voltage
c. output voltage
d. barrier potential

Electronic Devices
Quiz: Q8
8. Using the ideal diode model, the current in the circuit
shown is
a. 0.73 mA
b. 0.80 mA
c. 0.87 mA
d. 1.2 mA

Electronic Devices
Quiz: Q9
9. Using the practical diode model, the current in the
circuit shown is
a. 0.73 mA
b. 0.80 mA
c. 0.87 mA
d. 1.2 mA
8.0 V
–
+
R
10 kW
VBIAS

Electronic Devices
Quiz: Q10
10. The diode model which includes the large reverse
resistance is the
a. ideal model
b. practical model
c. complete model
d. all of the above
8.0 V
–
+
R
10 kW
VBIAS

Quiz answers
Answers:
1. b
2. a
3. c
4. a
5. c
6. d
7. d
8. b
9. a
10. c

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