Principles of Technology
CH 14 SOLID
STATE PHYSICS 1
Name_______
KEY OBJECTIVES
At the conclusion of this chapter you will be able to:
• Define the term conductivity, and classify solids by their abilities to conduct electricity.
• Describe how the electron band theory of conduction distinguishes among conductors, insulators, semiconductors.
• Define the term intrinsic semiconductor; explain how temperature affects the conductivities of semiconductors.
• Define the term extrinsic semiconductor, and explain how the process of doping enhances the conductivity of
materials such as silicon and germanium.
• Define the terms donor element, N-type semiconductor acceptor element, positive hole, and P-type semiconductor
• Describe the operation of a junction diode
14.1 INTRODUCTION
Solid-state physics is the study of semiconductors (such as silicon and germanium) and the electronic devices in
which they are used. We study this topic because virtually every electronic device, from computers to talking
toys, is made with circuits containing semiconductor devices. To understand semiconductors, we need first to
understand how solid substances conduct electricity.
1.
Which statement is false?
a. Solid-state physics is the study of semiconductors and the electronic devices in which they are used.
b. We study solid-state physics because virtually every electronic device, from computers to talking
toys, is made with circuits containing semiconductor devices.
c. Silicon and germanium are typical semiconductors.
d. Semiconductors are lame and there is no need or practical application for them
14.2 CONDUCTION IN SOLIDS
A solid can be classified as a conductor, an insulator, or a semiconductor, depending on how well it conducts
electricity, that is, how well it carries an electric current. Conductors have low resistances and, consequently,
carry currents easily. Generally, metals such as silver and copper are the best conductors. Insulators have high
resistances and do not carry currents well. Nonmetals such as wood and glass are insulators. Semiconductors
vary widely in their abilities to conduct electricity, but these abilities are usually intermediate between those of
conductors and those of insulators. Silicon and germanium are typical semiconductors.
2.
Which statement is false?
a. A solid can be classified as a conductor, an insulator, or a semiconductor, depending on how well it
conducts electricity, that is, how well it carries an electric current.
b. Conductors have low resistances and carry currents easily. Metals (silver,copper) are the best conductors.
c. Insulators have high resistances and don’t carry currents well. Nonmetals (ex. wood, glass) are insulators.
d. In electric power lines the metal wires are used for insulation and the rubber is used for conduction.
In Chapter 10, we learned that a quantity known as resistivity (ρ) is used to compare the resistive properties of
substances. The reciprocal of resistivity is known as conductivity (σ) and we use this quantity to compare how
well substances conduct electricity. The table below lists the conductivities of a conductor, an insulator, and a
semiconductor.
The reciprocal of resistivity (ρ) is known as conductivity (σ)
1/ρ = σ
3.
Which material has an electrical conductivity less than deionized water?
a. platinum
b. germanium
c. sodium
d. sulfur
Substance
(Ω-1·m-1)
Electrical Substance Electrical
Conductivity (Ω-1·m-1) Conductivity
Substance
(Ω-1·m-1)
Electrical Substance Electrical
Conductivity (Ω-1·m-1) Conductivity
Annealed Copper 58.0 × 106
Aluminum 37.8 × 106
Silver
63.01 × 106
Sulfur
5.0 × 10-16
Germanium
Sea water
4.788
Copper
59.6 × 106
Silicon
2.5 × 10-4
Carbon
0.061
Gold
45.2 × 106
Boron
1.0 × 10-4
Sodium
21.0 x 106
Platinum
9.7 x 106
2.2
Deionized water 5.5 × 10-6
Drinking water
0.0005 to 0.05 Phosphorus 1.0 x 10-9
14.3 THEORIES OF CONDUCTION IN SOLIDS
Electron Sea Model
The simplest theory of conduction is known as the electron sea model. In this model, the ability of a solid to
conduct electricity depends on the number of valence electrons that can move freely through the solid. (Valence
electrons are the electrons that occupy the outermost energy level of an atom.) Although this model is useful in
explaining conduction in metals, it does not adequately explain the properties of semiconductors, particularly
why the conductivity of a semiconductor increases with a rise in temperature.
4.
Which statement is false?
a. Valence electrons are electrons that have no purpose and are not involved in chemical bonding or the
conduction of heat and electricity.
b. The simplest theory of conduction is known as the electron sea model.
c. In the electron sea model, the ability of a solid to conduct electricity depends on the number of valence
electrons that can move freely through the solid. Valence electrons are the electrons that occupy the
outermost energy level of an atom.
d. Although the electron sea model is useful in explaining conduction in metals, it does not adequately
explain the properties of semiconductors, particularly why the conductivity of a semiconductor increases
with a rise in temperature.
Electron Band Model
The second theory is known as the electron band model. In this model, the atoms of the solid form a series of
energy bands that may be populated by electrons. (: These bands are formed by the merging of individual
atomic orbitals that have similar energies:) For our purposes, the two most important bands are the valence band
and the conduction band.
A valence band contains the electrons in the outermost energy levels of the atoms comprising the solid.
However, these electrons must be “promoted” to the conduction band before they can serve as current carriers.
Between the valence band and the conduction band is an energy gap. The size of this gap determines how many
electrons will be promoted to the conduction band and, as a result, how well the solid will conduct electricity.
5.
a.
b.
c.
d.
Which statement is false?
The second theory is known as the electron band model. In this model, the atoms of the solid form a
series of energy bands that may be populated by electrons. The energy bands populated by electrons are
formed by the merging of individual atomic orbitals that have similar energies. For our purposes, the
two most important bands are the valence band and the conduction band.
A valence band contains the electrons in the outermost energy levels of the atoms comprising the solid
but these electrons must be “promoted” to the conduction band before they can serve as current carriers.
Between the valence band and the conduction band is an energy gap. The size of this gap determines
how many electrons will be promoted to the conduction band and, as a result, how well the solid will
conduct electricity.
The valence band and conduction band always overlap and are never separated by any gap of energy.
Electron Band Model
Conductors, semiconductors & insulators
In the conductor, the valence and conduction bands overlap, as
shown in the diagram. As a result, electrons flow easily through
the conductor. Metals with small numbers of valence electrons
(sodium, copper, silver) conduct especially well. When the
temperature is raised, the electrons collide with the atoms in the
conductor and kinetic energy is lost as heat. These collisions
reduce the current in the conductor (i.e., they raise its
resistance). In the insulator, the energy gap is large enough to
prevent a significant number of electrons from being promoted
to the conduction band. As a result, the resistance of an
insulator is very high, and this device carries only a minute
current under normal circumstances.
6.
Which statement is false?
a. In conductors the valence and conduction bands overlap so electrons flow easily through the conductor.
Metals with small numbers of valence electrons (sodium, copper, silver) conduct especially well.
b. When the temperature is raised, the electrons collide with the atoms in the conductor and kinetic energy
is lost as heat. These collisions reduce the current in the conductor (i.e., they raise its resistance).
c. In the insulator, the energy gap is large enough to prevent a significant number of electrons from being
promoted to the conduction band. As a result, the resistance of an insulator is very high, and this device
carries only a minute current under normal circumstances.
d. Rubber & plastic are better conductors than sodium or silver because they have more valence electrons.
Intrinsic semiconductors
An intrinsic semiconductor bands
The semiconductor has a small energy gap. At low temperatures, this device
behaves as an insulator. At higher temperatures, however, the increase in the
kinetic energy of the electrons allows some of them to be promoted
to the conduction band. As a result, the conductivity is increased dramatically.
A material that behaves this way is known as an intrinsic semiconductor,
and the diagram above illustrates this process.
7.
Which statement is false?
a. If a substance acts as an insulators or conductors can’t be changed regardless of energy or temperature.
b. The semiconductor has a small energy gap and at low temperatures, a semiconductor behaves as an
insulator.
c. At higher temperatures, however, the increase in the kinetic energy of the electrons allows some of them
to be promoted to the conduction band. As a result, the conductivity is increased dramatically.
d. A material that at low temperatures acts as an insulator and at higher temperatures acts as a conductor is
known as an intrinsic semiconductor.
Valence Band
In the conduction band, the promoted electrons act as negative charge carriers. The promotion creates an
electron deficiency in the valence band. This deficiency can serve as the equivalent of positive charge carriers,
and conduction can take place in both bands.
14.4 EXTRINSIC SEMICONDUCTORS: DOPING
Another way of increasing the conductivity of materials such as silicon and germanium is to add very small
amounts of other elements. This process is known as doping, and semiconductors made in this way are called
extrinsic semiconductors. The ratio of doping atoms to semiconductor atoms is on the order of 1 to
100,000,000. If more of the doping material were added, the conductivity of the silicon (or germanium) would
become too high for the element to function as a semiconductor. Two types of semiconductors can be made by
the doping process: N-type and P-type.
8.
a.
b.
c.
d.
Which statement is false?
In the conduction band, the promoted electrons act as negative charge carriers. The promotion creates an
electron deficiency in the valence band. The electron deficiency can serve as the equivalent of positive
charge carriers, and conduction can take place in both the valence and conduction bands.
Doping is increasing the conductivity of materials such as silicon and germanium is to add very small
amounts of other elements and semiconductors made in this way are called extrinsic semiconductors.
The ratio of doping atoms to semiconductor atoms is on the order of 1 to 100,000,000. If more of the
doping material were added, the conductivity of the silicon (or germanium) would become too high for
the element to function as a semiconductor.
26 types of semiconductors can be made by the doping process: A-type thru Z-type
N-Type Semiconductors
A silicon or germanium atom contains four valence electrons. If a doping atom contains five valence electrons,
as does arsenic or antimony, it will contribute an electron to the conduction band. For this reason, the doping
material is known as a donor element.
The doping atom remains in the crystal as a positive ion; therefore, most of the charge carriers in the
semiconductor are electrons. This type of material is known as an N-type semiconductor. The diagram below
illustrates how the current is carried by N-type material in the presence of a potential difference:
N-Type Semiconductor
P-Type Semiconductor
P-Type Semiconductors
If a doping atom contains three valence electrons, as does gallium or aluminum, it will accept an electron from
the conduction band. For this reason, the doping material is known as a acceptor element.
The doping atom remains in the crystal as a negative ion; therefore, an electron deficiency remains in the
conduction band. Since these holes behave as positive charges, most of the charge carriers in the semiconductor
can be considered to be positive holes. This type of material is known as a P-type semiconductor. The diagram
above illustrates how the current is carried by P-type material in the presence of a potential difference:
9.
Which statement is false?
a. A silicon or germanium atom contains 4 valence electrons. If a doping atom contains 5 valence electrons
(arsenic or antimony)it will contribute an electron to the conduction band and is the donor element.
b. If the doping atom remains in the crystal as a positive ion and most of the charge carriers in the
semiconductor are electrons, it is an N-type semiconductor. If a doping atom contains three valence
electrons, as does gallium or aluminum, it will accept an electron from the conduction band and is the
acceptor element.
c. The doping atom remains in the crystal as a negative ion, behave as positive charges, & are most of the
charge carriers in P-type semiconductor and an electron deficiency remains in the conduction band.
d. Donor elements become negative ions and acceptor elements become positive ions in semiconductors.
14.5 JUNCTION DIODES
The electrical symbol for a diode is as follows:
The junction diode is the simplest semiconductor device. A diode is a “one way valve,” which permits the
passage of current in only one direction. A junction diode is made by joining P-type and N-type
semiconductors; the boundary between the two materials is known as the P-N junction. The diode’s unique
characteristics are due to the presence of this boundary. The diagram below represents a junction diode.
In a junction diode, the anode comprises the P side of the diode and the cathode comprises the N side. The
arrow points in the direction in which the diode will allow the flow of positive charges.
In the narrow region at the P-N junction, the (negative) conduction electrons drift into the (positive) holes,
producing neutral atoms. The positive and negative ions that remain establish an electric field that acts as a
barrier to further migration. This electric field barrier is illustrated above.
10.
Which statement is false?
a. A diode is a “one way valve,” which permits the passage of current in only one direction. A junction
diode is made by joining P-type and N-type semiconductors; the boundary between the two materials is
known as the P-N junction. The diode’s unique characteristics are due to this boundary.
b. In a junction diode, the anode comprises the P side of the diode and the cathode comprises the N side.
The arrow points in the direction in which the diode will allow the flow of positive charges.
c. In the narrow region at the P-N junction, the (negative-) conduction electrons drift into the (positive+)
holes, producing neutral atoms. The + and - ions remaining form an electric field that acts as a barrier.
d. The junction diode is such a complex semiconductor device it is theorized it was invented by aliens.
SUMMARY
KEY IDEAS Semiconductors have unique electrical properties that make them useful for a wide range of
applications. The band theory explains why semiconductors possess these properties. There are two types of
semiconductors (N type and P-type), which are made by adding small amounts of impurities to semiconductor
elements such as germanium and silicon. The impurities have the property of donating or accepting electrons.
The simplest semiconductive device is the junction diode, which is made by bringing N-type and P-type
semiconductor material into contact. The diode allows charge to flow in one direction only.