TOPIC 1
Solid State Principles
“The history of semiconductor physics is not one of grand heroic theories, but one of painstaking
intelligent labor. Not strokes of genius producing lofty edifices, but great ingenuity and endless
undulation of hope and despair. Not sweeping generalizations, but careful judgment of the border
between perseverance and obstinacy. Thus the history of solid-state physics in general, and of
semiconductors in particular, is not so much about great men and women and their glorious deeds,
as about the unsung heroes of thousands of clever ideas and skillful experiments—reflection of an
age of organization rather than of individuality.”
-Ernest Braun
LEARNING OUTCOMES:
At the end of this lesson, you are expected to:
1. Explain how a semiconductor is made to be of positive or negative in
producing a semiconductor junction
PRE-TASK 1.1: Read the text below
Introduction to Electronics
Electronics is a branch of physics concerned with the design of circuits and the study of
electrons under a variety of conditions. It deals with circuits that are made with parts
called components and connecting wires that control the flow of electricity and direct it to
do useful things. The science behind Electronics comes from the study of physics and gets
applied in real-life ways through the field of electrical engineering. Electronics also describes
the design, function, and utilization of electronic devices and systems.
Electrons, a component of atoms, and their use—known as electronics—play an important
role in many pieces of household equipment. Basic electronics comprises the minimal
“electronics components” that make up a part of everyday electronics equipment. These
electronic components include resistors, transistors, capacitors, diodes, inductors and
transformers. Powered by a battery, they are designed to work under certain physics laws
and principles. Basic electronics also concerns the measurement of voltage, current
(electron flow) and resistance in the assembled working "circuit."
This course introduces students to the fundamentals of electrical and electronic engineering.
It provides the students with an understanding of basic electrical quantities, circuit elements
and circuit analysis techniques. It also provides an understanding of the principles and
operation of diodes, transistors and other electronic components. This course also includes
the use of Programmable Logic Controllers and wiring diagrams of Motor Control Circuits.
DISCUSSION
SEMICONDUCTORS
Semiconductors are any of a class of crystalline solids intermediate in electrical
conductivity between a conductor and an insulator.
Semiconductors are employed in the manufacture of various kinds of electronic
devices, including diodes, transistors, and integrated circuits. Such devices have
found wide application because of their compactness, reliability,
power efficiency, and low cost.
As discrete components, they have found use in power devices, optical sensors,
and light emitters, including solid-state lasers.
They have a wide range of current- and voltage-handling capabilities and, more
important, lend themselves to integration into complex but readily
manufacturable microelectronic circuits. They are, and will be in the foreseeable
future, the key elements for the majority of electronic systems, serving
communications, signal processing, computing, and control applications in both
the consumer and industrial markets.
Semiconductor Materials
Solid-state materials are commonly grouped into three classes: insulators,
semiconductors, and conductors. (At low temperatures some conductors,
semiconductors, and insulators may become superconductors.)
Insulators, such as fused quartz and glass, have very low conductivities, on the order
of 10−18 to 10−10 siemens per centimetre; and conductors, such as aluminum, have
high conductivities, typically from 104 to 106 siemens per centimetre.
The study of semiconductor materials began in the early 19th century. The
elemental semiconductors are those composed of single species of atoms, such
as silicon (Si), germanium (Ge), and tin (Sn) in column IV and selenium (Se)
and tellurium (Te) in column VI of the periodic table.
There are, however, numerous compound semiconductors, which are composed of
two or more elements. Gallium arsenide (GaAs), for example, is a binary III-V
compound, which is a combination of gallium (Ga) from column III and arsenic (As)
from column V. Ternary compounds can be formed by elements from three different
columns—for instance, mercury indium telluride (HgIn2Te4), a II-III-VI compound.
They also can be formed by elements from two columns, such as aluminum gallium
arsenide (AlxGa1 − xAs), which is a ternary III-V compound, where both Al and Ga are
from column III and the subscript x is related to the composition of the two
elements from 100 percent Al (x = 1) to 100 percent Ga (x = 0). Pure silicon is the
most important material for integrated circuit applications, and III-V binary and
ternary compounds are most significant for light emission.
Electronic Properties
The semiconductor materials described here are single crystals; i.e., the atoms are
arranged in a three-dimensional periodic fashion. Part A of the figure shows a
simplified two-dimensional representation of an intrinsic (pure) silicon crystal that
contains negligible impurities. Each silicon atom in the crystal is surrounded by four
of its nearest neighbours. Each atom has four electrons in its outer orbit and shares
these electrons with its four neighbours. Each shared electron
pair constitutes a covalent bond.
The force of attraction between the electrons and both nuclei holds the two atoms
together. For isolated atoms (e.g., in a gas rather than a crystal), the electrons can
have only discrete energy levels. However, when a large number of atoms are
brought together to form a crystal, the interaction between the atoms causes the
discrete energy levels to spread out into energy bands. When there is no thermal
vibration (i.e., at low temperature), the electrons in an insulator or semiconductor
crystal will completely fill a number of energy bands, leaving the rest of the energy
bands empty. The highest filled band is called the valence band. The next band is the
conduction band, which is separated from the valence band by an energy gap (much
larger gaps in crystalline insulators than in semiconductors). This energy gap, also
called a bandgap, is a region that designates energies that the electrons in the
crystal cannot possess. Most of the important semiconductors have bandgaps in the
range 0.25 to 2.5 electron volts (eV). The bandgap of silicon, for example, is 1.12 eV,
and that of gallium arsenide is 1.42 eV. In contrast, the bandgap of diamond, a good
crystalline insulator, is 5.5 eV.
Semiconductor bonds. Three bond pictures of a semiconductor.
At low temperatures the electrons in a semiconductor are bound in their respective
bands in the crystal; consequently, they are not available for electrical conduction.
At higher temperatures thermal vibration may break some of the covalent bonds to
yield free electrons that can participate in current conduction. Once an electron
moves away from a covalent bond, there is an electron vacancy associated with that
bond. This vacancy may be filled by a neighbouring electron, which results in a shift
of the vacancy location from one crystal site to another. This vacancy may be
regarded as a fictitious particle, dubbed a “hole,” that carries a positive charge and
moves in a direction opposite to that of an electron. When an electric field is applied
to the semiconductor, both the free electrons (now residing in the conduction band)
and the holes (left behind in the valence band) move through the crystal, producing
an electric current.
The electrical conductivity of a material depends on the number of free electrons
and holes (charge carriers) per unit volume and on the rate at which these carriers
move under the influence of an electric field. In an intrinsic semiconductor there
exists an equal number of free electrons and holes. The electrons and holes,
however, have different mobilities; that is, they move with different velocities in an
electric field. For example, for intrinsic silicon at room temperature, the
electron mobility is 1,500 square centimetres per volt-second (cm 2/V·s)—i.e., an
electron will move at a velocity of 1,500 centimetres per second under an electric
field of one volt per centimetre—while the hole mobility is 500 cm 2/V·s. The electron
and hole mobilities in a particular semiconductor generally decrease with increasing
temperature.
Electron hole: movement
Movement of an electron hole in a crystal lattice.
Encyclopædia Britannica, Inc.
Electrical conduction in intrinsic semiconductors is quite poor at room temperature.
To produce higher conduction, one can intentionally introduce impurities (typically to a
concentration of one part per million host atoms). This is called doping, a process that
increases conductivity despite some loss of mobility.
For example, if a silicon atom is replaced by an atom with five outer electrons, such
as arsenic (see part B of the figure), four of the electrons form covalent bonds with the
four neighbouring silicon atoms. The fifth electron becomes a conduction electron that is
donated to the conduction band. The silicon becomes an n-type semiconductor because of
the addition of the electron. The arsenic atom is the donor.
Similarly, part C of the figure shows that, if an atom with three outer electrons, such
as boron, is substituted for a silicon atom, an additional electron is accepted to form four
covalent bonds around the boron atom, and a positively charged hole is created in the
valence band. This creates a p-type semiconductor, with the boron constituting an
acceptor.
The P-N Junction
If an abrupt change in impurity type from acceptors (p-type) to donors (n-type)
occurs within a single crystal structure, a p-n junction is formed. On the p side, the
holes constitute the dominant carriers and so are called majority carriers. A few
thermally generated electrons will also exist in the p side; these are termed minority
carriers. On the n side, the electrons are the majority carriers, while the holes are
the minority carriers. Near the junction is a region having no free charge carriers.
This region, called the depletion layer, behaves as an insulator.
p-n junction
characteristics
(A) Current-voltage
characteristics of a
typical silicon p-
n junction. (B) Forward-
bias and (C) reverse-
bias conditions. (D)
The symbol for a p-
n junction.
Encyclopædia
Britannica, Inc.
The most important characteristic of p-n junctions is that they rectify. Part A of the figure
shows the current-voltage characteristics of a typical silicon p-n junction. When a forward
bias is applied to the p-n junction (i.e., a positive voltage applied to the p-side with respect
to the n-side, as shown in part B of the figure), the majority charge carriers move across
the junction so that a large current can flow.
However, when a reverse bias is applied (as in part C of the figure), the charge carriers
introduced by the impurities move in opposite directions away from the junction, and only
a small leakage current flows. As the reverse bias is increased, the leakage current remains
very small until a critical voltage is reached, at which point the current suddenly increases.
This sudden increase in current is referred to as the junction breakdown, usually a
nondestructive phenomenon if the resulting power dissipation is limited to a safe value.
The applied forward voltage is typically less than one volt, but the reverse critical voltage,
called the breakdown voltage, can vary from less than one volt to many thousands of volts,
depending on the impurity concentration of the junction and other device parameters.
Although other junction types have been invented (including p-n-p and n-p-n), p-n junctions
remain fundamental to semiconductor devices.
Summary:
1. Semiconductor material has an electrical conductivity value falling between that of a
conductor and an insulator.
2. Their resistance decreases as their temperature increases, which is behavior opposite to
that of a metal.
3. Their conducting properties may be altered in useful ways by the deliberate, controlled
introduction of impurities (“DOPING”) into the crystal structure.
4. Elemental semiconductors – Si and Ge (column IV of periodic table) –composed of single
species of atoms. Compound semiconductors – combinations of atoms of column III and
column V and some atoms from column II and VI. (combination of two atoms results in
binary compounds). There are also three-element (ternary) compounds (GaAsP) and four-
elements (quaternary) compounds such as InGaAsP.
5. A perfect semiconductor crystal with no impurities or lattice defects is called an intrinsic
semiconductor.
6. Extrinsic Semiconductors - By doping, a crystal can be altered so that it has a
predominance of either electrons or holes. Thus there are two types of doped
semiconductors, n-type (mostly electrons) and p-type (mostly holes).
7. PN Junction is constructed when a p-type silicon and n-type silicon make contact with
each other. At this coupling point, free electrons (n-type) and holes (p-type) cancel each
other and form a "depletion zone" that acts as a non-conductive barrier. The PN junction
is one of the primary building blocks of semiconductors.
References:
1. https://simple.wikipedia.org/wiki/Electronics
2. https://www.britannica.com/science/semiconductor
