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TOPIC 5: ELECTRONIC | PHYSICS FORM 4

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PHYSICS FORM FOUR NOTES  ALL TOPICS

Semi Conductors

The Concept of Energy Band in Solids

Explain the concept of energy bands in solids

In solid-state physics, the electronic band structure (or simply band structure) of a solid describes those ranges of energy that an electron within the solid may have (called energy bandsallowed bands, or simply bands) and ranges of energy that it may not have (called band gaps or forbidden bands).

Band theory derives these bands and band gaps by examining the allowed quantum mechanical wave functions for an electron in a large, periodic lattice of atoms or molecules. Band theory has been successfully used to explain many physical properties of solids, such as electrical resistivity and optical absorption, and forms the foundation of the understanding of all solid-state devices (transistors, solar cells, etc.).

Difference between Conductors, Semiconductors and Insulators
Distinguish between conductors, semiconductors and insulators
Insulators
An electrical insulator
is a material whose internal electric charges do not flow freely, and
therefore make it impossible to conduct an electric current under the
influence of an electric field. This contrasts with other materials,
semiconductors and conductors, which conduct electric current more
easily.
The
property that distinguishes an insulator is its resistivity; insulators
have higher resistivity than semiconductors or conductors.
A
perfect insulator does not exist, because even insulators contain small
numbers of mobile charges (charge carriers) which can carry current. In
addition, all insulators become electrically conductive when a
sufficiently large voltage is applied that the electric field tears
electrons away from the atoms. This is known as the breakdown voltage of
an insulator.
Some
materials such as glass, paper and Teflon, which have high resistivity,
are very good electrical insulators. A much larger class of materials,
even though they may have lower bulk resistivity, are still good enough
to prevent significant current from flowing at normally used voltages,
and thus are employed as insulation for electrical wiring and cables.
Examples include rubber-like polymers and most plastics.
Conductors
conductor
is an object or type of material that allows the flow of electrical
current in one or more directions. For example, a wire is an electrical
conductor that can carry electricity along its length.
In
metals such as copper or aluminum, the movable charged particles are
electrons. Positive charges may also be mobile, such as the cationic
electrolyte(s) of a battery, or the mobile protons of the proton
conductor of a fuel cell. Insulators are non-conducting materials with
few mobile charges and support only insignificant electric currents.
Semiconductors
semiconductor
material has an electrical conductivity value falling between that of a
conductor, such as copper, and an insulator, such as glass.
Semiconductors are the foundation of modern electronics. Semiconducting
materials exist in two types: elemental materials andcompound materials.
The
modern understanding of the properties of a semiconductor relies on
quantum physics to explain the movement of electrons and holes in a
crystal lattice. The unique arrangement of the crystal lattice makes
silicon and germanium the most commonly used elements in the preparation
of semiconducting materials.
An
increased knowledge of semiconductor materials and fabrication
processes has made possible continuing increases in the complexity and
speed of microprocessors and memory devices. Some of the information on
this page may be outdated within a year because new discoveries are made
in the field frequently.
Examples of semiconductors are Silicon, Germanium.
The Effects of Temperature on the Conductivity of Conductors, Semiconductors and Insulators
Describe the effect of temperature on the conductivity of conductors, semiconductors and insulators
The
conductivity of pure defect free metal decreases with increase in
temperature .With increased temperature in a metal, thermal energy
causes atoms in metal to vibrate, in this excited state atoms interact
with and scatter electrons.
Thus decreasing the mean free path, and hence the mobility of electrons too decreases, and resistivity increases.
Since, resistivity = 1/conductivity
The
electrical conductivity of a semiconductor will increase exponentially
with an increase in temperature, as temperature increases the electrons
in the valance band will gain energy and go into the higher energy
levels in the conduction band where they become charge carriers.
The
increase in conduction can also be explained, I guess,due to the
formation of Cooper pairs and hence the creation of Phonon field.
Types of Semiconductors
Identify types of Semiconductors
There are two types of semiconductors
  • Intrinsic semiconductors
  • Extrinsic semiconductors
Intrinsic semiconductors
An
intrinsic semiconductor material is chemically very pure and possesses
poor conductivity. It has equal numbers of negative carriers (electrons)
and positive carriers (holes). Examples are Silicon and Germanium.
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.”
Silcon 1450007954429
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 thatelectrons can reach the conduction
band and contribute to electrical conduction. The term intrinsic
heredistinguishes between the properties of pure “intrinsic” silicon and
the dramatically different properties ofdoped n-type or p-type
semiconductors.
The
current flow in an intrinsic semiconductor is influenced by the density
of energy states which in turn influencesthe electron density in the
conduction band. This current is highly temperature dependent. The
electrical conductivityof intrinsic semiconductors increase with
increasing temperature.
Extrinsic semiconductors
Extrinsic
semiconductor is an improved intrinsic semiconductor with a small
amount of impurities added by a process,known as doping, which 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).
  • The positive charge conductor (p-type).
Semiconductors
are available as either elements or compounds. Silicon and Germanium
are the most commonelemental semiconductors. Compound Semiconductors
include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. Si and Geboth have a
crystalline structure called the diamond lattice. That is, each atom has
its four nearestneighbors 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 gapsand
mobilities, so that materials are available with properties that meet
specific requirements. Some of thesesemiconductors are therefore called
wide band gap semiconductors.
The Mechanism of Doping Intrinsic Semiconductors
Describe the mechanism of doping intrinsic semiconductors
The
addition of a small percentage of foreign atoms in the regular crystal
lattice of silicon or germanium produces dramatic changes in their
electrical properties, producing n-type and p-type semiconductors.
Pentavalent impurities
The
addition of pentavalent impurities such as antimony,arsenic or
phosphorous contributes free electrons, greatly increasing the
conductivity of the intrinsic semiconductor. Phosphorous may be added by
diffusion of phosphine gas (PH3).(5 valence electrons) produce n-type
semiconductors by contributing extra electrons.
Conduction 1450008533014
N Type 1450008603994
Trivalent impurities
(3 valence electrons) produce p-type semiconductors by producing a “hole” or electron deficiency.
N-Type Semiconductor
The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons,greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion ofphosphine gas (PH3).
N Semi 1450008825451
P-Type Semiconductor
The
addition of trivalent impurities such as boron, aluminum or gallium to
an intrinsic semiconductor creates deficiencies of valence
electrons,called “holes”. It is typical to use B2H6 diborane gas to diffuse boron into the silicon material.
P Type 1450009083883
P-n junctions
P-n junctions are formed by joining n-type and p-type semiconductor materials.
Pnsil 1450009296093
Since
the n-type region has a high electron concentration and the p type a
high hole concentration, electrons diffuse from the n-type side to the
p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side.
If
the electrons and holes were not charged, this diffusion process would
continue until the concentration of electrons and holes on the two sides
were the same, as happens if two gasses come into contact with each
other. However, in a p-n junction, when the electrons and holes
move to the other side of the junction, they leave behind exposed
charges on dopant atom sites, which are fixed in the crystal lattice and
are unable to move.
On the n-type side, positive ion cores are exposed. On the p-type side, negative ion cores are exposed. An electric field Ê forms between the positive ion cores in the n-type material and negative ion cores in the p-type
material. This region is called the “depletion region” since the
electric field quickly sweeps free carriers out, hence the region is
depleted of free carriers.

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