The
magic word semiconductor is composed of two words-Semi and Conductor. Semi
means not completely while conductor mean something, which can conduct
electricity. Everybody is familiar with "Electricity". It is present
everywhere; it runs many appliances in your home and outside the home like TV,
Bulb, Freeze, and Microwave Oven etc. In simple terms, the current must past
through wires so that the electricity can reach all these appliances. So a
conductor is nothing but a material having ability to conduct this electricity.
Semiconductors conduct electricity to some extent, less than the conductors,
how much do you think? Well, it depends on the type of material or it's mixture
and size. A semiconductor is a material that has intermediate conductivity between
a conductor and an insulator. It means that it has unique physical properties
somewhere in between a conductor like aluminum and an insulator like glass. In
a process called doping, small amounts of impurities are added to pure
semiconductors causing large changes in the conductivity of the material.
Examples include silicon, the basic material used in the integrated circuit,
and germanium, the semiconductor used for the first transistors.
Importance
To
understand the importance of semiconductors let's first understand the
difference between electricity and electronics. Both are concerned with
generating, transferring, and utilizing electrical energy. The chief difference
is that electricity is concerned with using that electrical energy in power
applications for heat, light, and motors while electronics is concerned with
power control and communications applications such as electronic thermostats,
electric motor speed control and radio. Engineering importance of
semiconductors results from the fact that they can be conductors as well as
insulators. Semiconductors are especially important because varying conditions
like temperature and impurity content can easily alter their conductivity. The
combination of different semiconductor types together generates devices with
special electrical properties, which allow control of electrical signals.
Semiconductors are employed in the manufacture of electronic devices and
integrated circuits. Imagine life without electronic devices. There would be no
radios, no TV's, no computers, no video games, and poor medical diagnostic
equipment.
Types of Semiconductor
Semiconductors
are mainly classified into two categories: Intrinsic and Extrinsic. An
intrinsic semiconductor material is chemically very pure and possesses poor conductivity.
It has equal numbers of negative carriers (electrons) and positive carriers
(holes). Whereas an extrinsic semiconductor is an improved intrinsic
semiconductor with a small amount of impurities added by a process, known as
doping, this alters the electrical properties of the semiconductor and improves
its conductivity. Introducing impurities into the semiconductor materials
(doping process) can control their conductivity. Doping process produces two
groups of semiconductors: the negative charge conductor (n-type) and the
positive charge conductor (p-type). Semiconductors are available as either
elements or compounds. Silicon and Germanium are the most common elemental
semiconductors. Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC,
GaN. Si and Ge both have a crystalline structure called the diamond lattice.
That is, each atom has its four nearest neighbors at the corners of a regular
tetrahedron with the atom itself being at the center. In addition to the pure
element semiconductors, many alloys and compounds are semiconductors. The
advantage of compound semiconductor is that they provide the device engineer
with a wide range of energy gaps and mobilities, so that materials are
available with properties that meet specific requirements. Some of these
semiconductors are therefore called wide band gap semiconductors.
Intrinsic Semiconductor
A
silicon crystal is different from an insulator because at any temperature above
absolute zero temperature, there is a finite probability that an electron in
the lattice will be knocked loose from its position, leaving behind an electron
deficiency called a "hole".
If
a voltage is applied, then both the electron and the hole can contribute to a
small current flow.
The
conductivity of a semiconductor can be modeled in terms of the band theory of
solids. The band model of a semiconductor suggests that at ordinary
temperatures there is a finite possibility that electrons can reach the
conduction band and contribute to electrical conduction.
The
term intrinsic here distinguishes between the properties of pure
"intrinsic" silicon and the dramatically different properties of
doped n-type or p-type semiconductors.
The
current which will flow in an intrinsic semiconductor consists of both electron
and hole current. That is, the electrons which have been freed from their
lattice positions into the conduction band can move through the material.
In
addition, other electrons can hop between lattice positions to fill the
vacancies left by the freed electrons. This additional mechanism is called hole
conduction because it is as if the holes are migrating across the material in
the direction opposite to the free electron movement.
The
current flow in an intrinsic semiconductor is influenced by the density of
energy states which in turn influences the electron density in the conduction
band. This current is highly temperature dependent.
Extrinsic Semiconductor
Doped
semiconductors (either n-type or p-type) are known as extrinsic semiconductors.
The activation energy for electrons to be donated by or accepted to impurity states
is usually so low that at room temperature the concentration of majority charge
carriers is similar to the concentration of impurities. It should be remembered
that in an extrinsic semiconductor there is an contribution to the total number
of charge carriers from intrinsic electrons and holes, but at room temperature
this contribution is often very small in comparison with the number of charge
carriers introduced by the controlled impurity doping of the semiconductor.
If
a very small number of atoms of a group V element such as phosphorus (P) are
added to the silicon as substitutional atoms in the lattice, additional valence
electrons are introduced into the material because each phosphorus atom has 5
valence electrons. These additional electrons are bound only weakly to their
parent impurity atoms (the bonding energies are of the order of hundredths of
an eV), and even at very low temperatures these electrons can be promoted into
the conduction band of the semiconductor. This is often represented
schematically in band diagrams by the addition of 'donor levels' just below the
bottom of the conduction band, as in the schematic below.
The
presence of the dotted line in this schematic does not mean that there now
exist allowed energy states within the band gap. The dotted line represents the
existence of additional electrons which may be easily excited into the
conduction band. Semiconductors that have been doped in this way will have a
surplus of electrons, and are called n-type semiconductors. In such
semiconductors, electrons are the majority carriers.
Conversely,
if a group III element, such as aluminium (Al), is used to substitute for some
of the atoms in silicon, there will be a deficit in the number of valence
electrons in the material. This introduces electron-accepting levels just above
the top of the valence band, and causes more holes to be introduced into the
valence band. Hence, the majority charge carriers are positive holes in this
case. Semiconductors doped in this way are termed p-type semiconductors.
How Semiconductors Works?
Most
of the semiconductor devices and chips are created with silicon. The commonly
heard expressions like "Silicon Valley" and the "Silicon Economy"
come from this fact. In the periodic table, you will find that silicon sits
next to aluminum, below carbon and above germanium. Carbon, silicon and
germanium have a unique property in their electron structure -- each has four
electrons in its outer orbital. This allows them to form nice crystals. The
four electrons form perfect covalent bonds with four neighboring atoms,
creating a lattice. In carbon, we know the crystalline form as diamond. In
silicon, the crystalline form is a silvery, metallic-looking substance. Metals
tend to be good conductors of electricity because they usually have "free
electrons" that can move easily between atoms, and electricity involves
the flow of electrons. While silicon crystals look metallic, they are not, in
fact, metals. All of the outer electrons in a silicon crystal are involved in
perfect covalent bonds, so they can't move around. A pure silicon crystal is
nearly an insulator -- very little electricity will flow through it. You can
change the behavior of silicon and turn it into a conductor by doping it. In
doping, you mix a small amount of an impurity into the silicon crystal. A
minute amount of either N-type or P-type doping turns a silicon crystal from a
good insulator into a viable (but not great) conductor -- hence the name
"semiconductor."
However
electrons are not the only players in the "conduction game"! Another
particle plays a major role in conduction in semiconductors. What is this
particle? That is also what happens when an electron in a semiconductor jumps
from the valence band to the conduction band. It is called a hole. Some
electrons and holes play an important role in electrical conduction in
semiconductors:
Electrons
have a negative charge. Holes have a positive charge. Electrons and holes are
not static: they can move. Holes move more slowly than electrons. When
electrons move in one direction, holes move in the opposite direction. This is
like cars parked along a street. If one car moves to an empty slot, the empty
slot moves the other way.
The
cars move to the right, the empty slot to the left. A solitary electron in the
presence of a solitary hole will recombine. Only electrons and holes which are
free, and hence have not recombined, play a role in electrical conduction.
Properties of Semiconductor
Semiconductors
have many useful properties that insulators and conductors do not possess.
These properties are based on the fact that an electron can jump from the
valence band to the conduction band and vice versa. Temperature can give this little
extra energy to an electron and make it jump to the conduction band thus
creating a hole in the valence band.
Light can also give this energy boost and create
what we call an electron-hole pair: a free electron and a free hole: this
phenomenon is called absorption. Photo conductivity is the increase of current
in a semiconductor due to the absorption of photons. Light has a dual nature:
it behaves as a wave and as a particle. The particle associated with light is
called a photon. Photons can have different energies.
When
light illuminates a semiconductor:
· the photons with the right energy are absorbed by the material
· the electrons from the valence band have enough energy to jump to the
conduction band
· the conductivity increases due to the higher number of electrons in the
conduction band.
Electroluminescence is the conversion of electrical energy into light. Let's
consider electrons in the conduction band. These electrons are in an excited
state: they have gained some energy to jump to the conduction band.
· they release the extra energy that they have
· this energy is emitted as a photon
Photons emitted by electroluminescence come out in random directions: this type
of light is called incoherent light. For instance light from a light bulb is
incoherent. Stimulated emission is a little bit like electroluminescence except
that it is not a spontaneous process: the excited electron is forced into
jumping back to the valence band and emitting a photon.
Such
electrons eventually fall back into the valence band in a lower energy
state.
