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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 bands, allowed 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
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
property that distinguishes an insulator is its resistivity; insulators
have higher resistivity than semiconductors or conductors.
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.
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.
A 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.
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.
A 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.
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.
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
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
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.
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
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.
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.”
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.
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
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
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
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).
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.
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
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
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.
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).
P-Type Semiconductor
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-n junctions
P-n junctions are formed by joining n-type and p-type semiconductor materials.
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.
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.


Introduction to Astronomy
The Concept of Astronomy
Explain the concept of astronomy
is a branch of science which deals with the study of origin,evolution,
composition, distance and the motion of all bodies and scattered matter
in the universe.
  • Universe is the totality of space and time together with matter and energy.
  • Astronomers are the people who deals astronomy.
The Importance of Astronomy in Everyday Life
Explain the importance of astronomy in everyday life
Importance of astronomy include:
  • It was the earliest method of measuring time.
  • It was used to develop calendars that made it possible to predict the seasons.
  • It is used in navigation.-Helps us to understand the earth and the life it supports originated from and how it evolved.
  • It presents a new frontier for exploration.
Solar System
Difference between a Star and a Planet
Distinguish between a star and a planet
solar system is made up of the sun and the celestial objects bound to
it by gravity.These objects include the eight planets and their known
moons and billions of small bodies that include
asteroids,comets,meteoroids and interplanetary dust.
Stars and planets
A star is a large celestial body made up of hot gases known as plasma.
Plasma is an ionized gas in which a certain proportion of electrons are free rather than bound to an atom or molecule.
sun is a large star. The sun is also the closest star to the earth.
Astronomical unit is the distance between the earth and the sun which is
used to measure distances across the solar system (its value is
approximately 149.60 million kilometers).
A Galaxy is a giant collection of stars,gas and dust.
stars in the universe are in the galaxies. Nearly all of the stars
visible in the night sky are within our own galaxy, sometimes called the
Milky Way Galaxy.
is a major (large) object which is in orbit around a star.There are
eight planets which are Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune.
Characteristics of a planet
  • It is a celestial body that orbits a star.
  • It is massive enough so that its own gravity cause it to assume a spherical shape.
  • It has cleared the neighbourhood around its orbit.
is not considered as a planet because it resides in an area of space
populated by numerous other objects.It is now designated a dwarf
planet.The dwarf planet does not meet the third characteristic i.e has
not cleared the neighbourhood around its orbit.
Differences between stars and planets
Stars Planets
Emit their own light Do not emit their own light
Twinkle at night Do not twinkle at night
Appear to be moving from east to west. Planets move around the sun from west to east.
Their temperatures are usually very high Their temperature depends on their distances from the sun
Countless in number There are eight in the solar system
Very big in size but they appear small because they are very far away Very small in size as compared to stars.
(minor planets) are small solar system bodies in orbit around the
sun,especially in the inner solar system.Asteroids are smaller than
planets but larger than a speck of dust.
comet is a solid body orbiting the sun typically composed of rock dust
or ice. Most comets were formed from condensed interstellar gas and dust
clouds in the early stages of the creation of the universe.
The Force of Gravitation which Maintains Celestial Bodies in their Orbits
Explain the force of gravitation which maintains celestial bodies in their Orbits
force is the attractive force existing between any two objects that
have mass. It pulls objects together and acts on all matter on the
universe, hence it is sometimes referred to as universal gravitation.
Newton’s law of universal gravitation
states that: ‘Every single point mass attracts every other point by a
force directed along the line joining the two masses.’The force is
proportional to the product of the two masses and inversely proportional
to the square of the distance between the point masses.
  • F is the magnitude of the attractive force between the two point masses.
  • G is the universal gravitation constant.m₁ is the mass of the first point mass.
  • m₂ is the mass of the second point mass.
  • r is the distance between the centers of the two point masses.
force is actually very weak force.The huge gravitational force of the
nearest star,the sun,holds together the eight planets of the solar
system. The planets move round the space at speeds that just balance the
sun’s gravitational pull, so they are locked into a permanent path
(orbit) around the sun.
satellites (moons) orbits planets while artificial satellites orbit the
earth in the same way as the moon orbits the planets.
is the gravitational force that occurs between the earth and the other
objects.It pulls the objects towards the center of the earth.It holds us
on the ground and causes objects to fall back to the ground after being
thrown uo in the air.
earth gravitational pull extends out into space in all directions. The
further you move away from the center of the earth,the weaker the force
becomes. The measure of the force of gravity on an object on the earth’s
surface is the weight of that object and is measured in newton (N).
The weight of an object changes depending on its location in the universe.
The Surface Features and Temperature of the Moon
Describe the surface features and temperature of the moon
moon of the earth is the sixth largest in the solar system. It has a
diameter of 3,476km and a mass of 7.35 x 10²²kg. Like the earth, the
moon has an iron core surrounded by a rocky mantle and crust.Unlike the
earth, no part of the moon’s iron core is molten so it does not have a
magnetic field.Surface gravity on the moon is 1/6 that of the earth.
moon revolves in a anticlockwise direction around the earth in an
elliptical orbit. The moon’s orbit is tilted at 5° relative to the
earth’s orbit around the sun.The distance between the earth and the moon
varies from perigee (nearest the earth) where it is 356,000km to apogee
(furthest from the earth) where it is 406,000km. The average distance
is 384,000km.
take the moon 27.3 earth days to complete one orbit, a period of time
called the Sidereal month.The moon also rotates about its axis at a rate
equal to its rate of revolution. The result of this is that one side of
the moon face the earth. The side which faces the earth is called the
near side while the side which faces away is called the far side.
spinning of the earth causes the moon to rise and set each day ,just
like the sun. However, because of moons’ orbital motion around the earth
,it(the moon) rises about 50 minutes later each day .As a result, the
moon can be seen at different times of the day and night during a month.
The temperatures on the surface of the moon are on average 107°C during the day and 53°C during the night.
Surface features of the moon.
There are two primary types of terrain on the moon. These are;
  1. Heavily cratered very old lunar highlands.
  2. Relatively smooth and younger Maria.
From the surface of the earth,the moon’s surface appears to have bright and dark regions when viewed with the unaided eye.
bright areas are the lunar highlands that have many craters and covered
with a highly reflective layer of fine dust. The highlands are
geologically the oldest parts of the moon’s surface.
dark areas are low areas similar to ocean basins on the earth. They are
with dark solidified lava and are less cratered than the highlands.
Galileo called these areas matia, Italian word for seas, because their
dark smooth surface appears to be large bodies of water.
maria which makes 16% of the moon’s surface , are huge impact craters
that were later flooded with molten lava. Most of the maria is covered
with regolith,a mixture of fine dust and rocky debris produced by meteor
The Causes of Ocean Tides
Explain the causes of ocean tides
are periodic rises and falls of large bodies of water caused mainly by
gravitational interaction between the earth and the moon.
earth and the moon are attracted each other, just like magnets are
attracted to each other. The moon rises to pull at anything on the earth
to bring it closer. But the earth is able to hold onto everything
except the water. Since the water always moving, the earth can’t hold
onto it and the moon is able to pull at it. This results into ocean
day, there are two high tides and two low tides. The ocean constantly
moves from high tide to low tide, and then back to high tide. There is a
time interval of about 12 hours and 25 minutes between the two high
How tides occur
attraction of the moon causes the oceans to bulge out in the direction
of the moon. Another bulge occurs on the opposite side since the earth
is also being pulled towards the moon (and away from the water on the
far side).
levels fluctuate daily as the sun, moon and earth interact .As the moon
travels around the earth, and as they together travel around the sun,
the combined gravitational forces cause the world ocean water levels to
rise and fall. Since the earth is rotating while this is happening, two
tides occur each day.
Types of tides
There are two types of tides:
Spring tides
occur during the full moon and the new moon. During this time, the
earth,the sun and the moon are in a line. The gravitational forces of
the moon and the sun both contribute to the tides.
these times, the high tides are very high and the low tides are very
low. These are known as spring high tines and spring low tides
tides are especially strong tides.Proxigen spring tide is a rare
unusually high tide. It occurs when the moon is both unusually close to
the earth (at its closest perigee, called the proxigee) and in the new
moon phase(when the moon is between the earth and the sun).
The proxigen spring spring tide occurs at most once every 1.5 years.
Neap tide
the sun and the moon are not aligned, the gravitational forces cancel
each other out, and the tides are not very high or very low. These are
called neap tides.
occur during quarter moons. During this time, the gravitational forces
of the moon and the sun are perpendicular to one another (with respect
to the earth).
causes the bulges to cancel each other. The result is a smaller
difference between high and low tide and is known as a neap tide. Neap
tides are especially weak tides.






  1. Some people who have enough experiences might not find any difficulty in designing electronic circuit. However, for people who are in beginner level, making proper and appropriate design might be something difficult. IC


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