Electronic Devices

Ninth Edition

Floyd

Chapter 1

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Bohr Atom

The Bohr model of the atom is that electrons can circle

the nucleus only in specific orbits, which correspond to
Energy
discrete energy levels called shells.
The atomic number is the
number of protons in the nucleus.

The outermost occupied shell is

called the valence shell and
electrons that occupy this shell are
called valence electrons. Nucleus Shell 1

Shell 2

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Conductors
Materials can be classified by their ability to conduct
electricity. This ability is related to the valence electrons.
Core (+1)
Copper is an example of an
excellent conductor. It has only
one electron in its valence band,
which can easily escape to the
+29
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.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

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.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Semiconductors
Semiconductors are between conductors and insulators
in their ability to conduct electricity.
Core (+4)
Silicon is an example of a single
element semiconductor. It has four
+14
electrons in its valence band.

+4

Unlike metals, silicon forms strong covalent

bonds (shared electrons) with its neighbors. +4 +4 +4

Intrinsic silicon is a poor conductor because most

of the electrons are bound in the crystal and take
+4
part in forming the bonds between atoms.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

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 Free
band electron
Energy gap Heat
Energy
Valence Hole
band
Electron-hole pair

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

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.
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

Si Si Si

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

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
Si from Sb atom

Where on the periodic table

would you expect to find another
Si Sb Si
element that could be used as an
impurity to create an n-material?
Elements above or below Sb will have Si

the same valence electron structure.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Semiconductors

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.
Si Hole from
B atom

Si B Si

Si

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

The pn junction
A p- and an n-material together form a pn junction.
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.
Depletion region
p region n region
What process stops the migration of – +
–
charge across the boundary? –
+
+
– +
A potential is built up (called the – +
– +
barrier potential) that prevents further – +

charge migration. – +

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

The pn junction

The energy diagram for the n-region shows a lower

potential than for the p-region. Energy

Conduction
band
Why do you think that the
Valence
energy level in the n- region band
is lower than the p-region?
0
n region
The n-region tends to have filled valence p region Depletion
shells; conduction electrons are shielded by region

these electrons, so they are further away

from the nucleus and have less energy.
Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Forward bias

Forward bias is the condition which allows current in

the diode. The bias voltage must be greater than the
barrier potential.
I F (mA)

V
+ F–
C
IF
–
R
+ VBIAS
Knee
+ – A B
VF
0
0 0.7 V

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Reverse bias

Reverse bias is the condition in which current is

blocked.
VBR 0
VR 0
VBIAS Knee
– +

I=0A
R
VBIAS
– +
IR

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Approximations

Three diode approximations are:

IF IF IF

VR VF VR VF VR VF
0.7 V 0.7 V

IR IR IR

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.
Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Example

Use the practical model to determine the current in the

circuit:
R

3.3 kW
+
VBIAS 12 V
–

VR  VBIAS  0.7 V  12 V  0.7 V  11.3 V

VR 11.3 V
I   3.4 mA
R 3.3 kW

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Summary

Typical diode packages

Some common configurations are

K
K A
K
A
A K
A K
A
K

K
A

K A
K K

A A
K
K
A

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Selected Key Terms

Electron The basic particle of negative electrical charge.

Valence Related to the outer shell of an atom.

Free Electron An electron that has acquired enough energy to

break away from the valence band of the parent
atom; also called a conduction electron.

Conductor A material that easily conducts electrical current.

Insulator A material that does not normally conduct

current.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Selected Key Terms

Semiconductor A material that lies between conductors and

insulators in its conductive properties.

Crystal A solid material in which the atoms are

arranged in a symmetrical pattern.

Hole The absence of an electron in the valence

band of an atom in a semiconductor crystal.

Diode A semiconductor device with a single pn

junction that conducts current in one direction
only.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Selected Key Terms

PN junction The boundary between two different types of

semiconductive material.

Barrier The amount of voltage required to produce

Potential full conduction across the pn junction in
forward bias.

Forward bias The condition in which a diode conducts

current.

Reverse bias The condition in which a diode prevents

current.

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

5. Compared to a p-material, the energy levels in an n-

material are
a. the same
b. greater
c. lower

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

8. Using the ideal diode model, the current in the circuit

shown is
R
a. 0.73 mA
10 kW
b. 0.80 mA +
VBIAS 8.0 V
–
c. 0.87 mA
d. 1.2 mA

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

9. Using the practical diode model, the current in the

circuit shown is
R
a. 0.73 mA
10 kW
b. 0.80 mA +
VBIAS 8.0 V
–
c. 0.87 mA
d. 1.2 mA

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

10. The diode model which includes the large reverse

resistance is the
R
a. ideal model
10 kW
b. practical model +
VBIAS 8.0 V
–
c. complete model
d. all of the above

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.
 Quiz

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

Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458.
Thomas L. Floyd All rights reserved.