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ELECTRIC FIELD: ELECTRON MOVEMENT IN UNIFORM ELECTRIC
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An electron, left, accelerates uniformly toward the positive plate in a uniform
electric field. A horizontally moving electron, right, follows a parabolic path in
the field.
Single Particle Motion in Electric and Magnetic Fields
The following photographs were made in the Physics 180E undergraduate laboratory
course in plasma physics by Professor Reiner L. Stenzel. They are designed to
demonstrate the single particle motion in electric and magnetic fields. A weak electron
beam of typically 100 eV, 1-10 mA is injected from an oxide-coated cathode into a low
pressure (0.2 mTorr) argon gas. The beam partially ionizes the gas. The background
plasma has a potential close to that of the grounded chamber wall. The beam electrons are
accelerated by an electric field mainly concentrated in the cathode sheath. The trajectory
of the beam electrons is visible due to light excitation when the beam electrons collide
with argon atoms. No visible light is produced when the electron energy is below
typically 10 eV. Several basic phenomena can be inferred from the following pictures.
These were taken with a Rolleiflex camera, 400 ASA, black and white film, with up to
one minute exposure time.
Fig. 1. A 100 eV electron beam is injected into a
neutral gas without applying external electric or
magnetic fields. The beam light starts very close to
the cathode indicating that the electrons acquire
their full energy in a very thin cathode sheath. Thus,
the self-produced plasma acts as an anode. The
initial beam convergence indicates concave
equipotential surfaces near the cathode, presumably
due to a radial density gradient. The beam scatters
by collisions and beam-plasma instabilities.
(Larger image: 114,808 bytes, 913×461 pixels).
Fig. 2. The beam is injected against a negatively
biased grid. When the bias voltage is slightly larger
than the cathode voltage most of the beam is
reflected but a few electrons are transmitted. Note
that the equipotential surface near the grid is not
plane but convex. This causes an angular spread of
the reflected electrons. Electrons in the center of
the beam are normal to the sheath and can pass the
grid when their energy exceeds the potential energy
of the sheath.
(Larger image: 111,597 bytes, 927×475 pixels).
Fig. 3. Beam injection against a very negatively
biased grid which reflects all electrons. Note dark
sheath near grid where the electron energy is
decreased below 10 eV. The electrons stagnate in
the dark sheath and form a convex equipotential
surface. This causes the reflected beam to be highly
divergent.
(Larger image: 249,846 bytes, 836×732 pixels).
Fig. 4. A 100 eV electron beam injected
perpendicular to a dc magnetic field. The sense of
the cyclotron orbit implies that the magnetic field
points into the plane. From the beam energy and the
cyclotron radius the field strength can be calculated.
Note that the beam light weakens with propagation
distance
(Larger image: 229,104 bytes, 767×809 pixels).
Fig. 5. A low energy beam is injected across a
magnetic field as in Fig. 1. The cyclotron radius is
decreased. Note the dark gap between the cathode
and beam onset. In this sheath region the electrons
still have insufficient energy for light excitation.
(Larger image: 124,601 bytes, 575×609 pixels).
Fig. 6. A low energy beam is injected against a
decelerating electric field. As the beam energy falls
below 10 eV the light emission stops. The beam and
current continue to flow through the dark region.
(Larger image: 95,390 bytes, 663×669 pixels).
Fig. 7. At high neutral pressures the injected
electron beam rapidly spreads. The electron mean
free path can be inferred from the beam decay.
(Larger image: 206,348 bytes, 799×767 pixels).
Fig. 8. Electron motion in crossed electric and
magnetic fields. The trajectory is a cycloid, i.e., a
superposition of a circular motion and a constant
drift to the right. The cyclotron orbit implies a
magnetic field direction into the plane and the E×B
drift implies that the electric field points downward.
From the known beam energy the field strengths
can be obtained from cyclotron radius and guiding
center drift.
(Larger image: 147,526 bytes, 863×601 pixels).
Fig. 9. Mirror reflection of a weak electron beam in
a nonuniform magnetic field. The beam is injected
oblique to the field lines which converge to the
right. Due to the adiabatic invariants (energy and
magnetic moment) the parallel energy is converted
into perpendicular energy at which point the
particles reflect. Alternatively, the parallel motion is
decelerated by the repelling force of an increasing
magnetic field.
(Larger image: 173,651 bytes, 850×941 pixels).
Fig. 10. Mirror reflection of a stronger electron
beam in a magnetic field which converges to the
right. Note that the guiding center (axis of spiral) of
the reflected beam does not coincide with that of
the incident. This is due to the gradient and
curvature drift in a nonuniform field.
(Larger image: 215,620 bytes, 1015×761 pixels).
Fig 11. Multiple reflections of an oblique electron
beam in a mirror magnetic field. The cyclotron
motion is the fastest periodic motion, followed by
the slower bounce motion between the mirror
points. Gradient and curvature drifts cause a third,
slowest rotation azimuthally around the mirror axis.
This is the classical charged particle motion in the
ionosphere in Earth's magnetic dipole field.
Collisions and other scattering processes diffuse the
beam.
(Larger image: 188,031 bytes, 817×1015 pixels).
Fig.12. Electron beam injected into a nonuniform
magnetic field with a null line. The beam meanders
along a magnetic null line pointing diagonally down
to the right. Note the reversal of the cyclotron
rotation as the magnetic field reverses to either side
of the neutral line. Such meandering and figureeight trajectories have been studied theoretically in
the problem of magnetic reconnection occuring in
the magnetosphere and on the sun.
(Larger image: 190,012 bytes, 761×893 pixels).
The History of the Cathode Ray Tube
Electronic television is based on the development of the cathode ray tube - CRT which is the picture tube found in modern television sets. A cathode ray tube or CRT
is a specialized vacuum tube in which images are produced when an electron beam
strikes a phosphorescent surface. Television sets, computers, automated teller
machines, video game machines, video cameras, monitors, oscilloscopes and radar
displays all contain cathode-ray tubes. Phosphor screens using multiple beams of
electrons have allowed CRTs to display millions of colors.
The first cathode ray tube scanning device was invented by
the German scientist Karl Ferdinand Braun in 1897. Braun
introduced a CRT with a fluorescent screen, known as the
cathode ray oscilloscope. The screen would emit a visible
light when struck by a beam of electrons. In 1907, the
Russian scientist Boris Rosing (see Zworykin) used a CRT in
the receiver of a television system that, at the camera end,
made use of mirror-drum scanning. Rosing transmitted crude geometrical patterns
onto the television screen and was the first inventor to do so using a CRT. The first
practical signal generating tubes were invented by Vladimir K. Zworykin and Philo T.
Farnsworth. Zworykin invented the iconoscope, which became the imaging
iconoscope. Farnsworth invented the image dissector.
electronic glassware
History and Physics Instruments
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J.J. Thomson in the Cavendish Lab
Cathode ray Tubes
From the beginning to the end....
THE EARLY DAYS OF THE CRT
Deflection of a Moving Electron in
Magnetic Field
In this movie you see an electron moving and then entering a magnetic field (Bfield). The direction of the B-field is into the screen. Notice that the force
created by the magnetic field ends up always pointing to the center of the
curve. This force is considered to be a centripetal force (see more in the motion
in a plane unit ) because it causes the electron to travel in a circular path.
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If the charge on the electron were greater, the force would be greater.
If the speed of the electron were greater, the force would be greater.
If the magnetic field strength were greater, the force on the electron
would be greater.
If the mass of the electron were greater, it would have no effect on the
force, but the circular path would be larger.
The screen of your computer (unless it is an LCD) and your television screen
both use magnetic fields to deflect electrons fired from an "electron gun". The
moving electrons are directed toward different areas of the screen by magnetic
fields created by electromagnets. Wherever the electrons strike the screen, they
cause phosphorus to give off light. If millions of electrons are directed to the
screen in the right places, the little bursts of light leave an impression our eyes
which forms an image. An instant later (1/24th of a second) a million more
electrons strike the screen and form a new image. Our mind pieces them
together to create the impression of smooth motion.
History of semiconductor device development
1900s
Semiconductors had been used in the electronics field for some time before the invention
of the transistor. Around the turn of the 20th century they were quite common as
detectors in radios, used in a device called a "cat's whisker". These detectors were
somewhat troublesome, however, requiring the operator to move a small tungsten
filament (the whisker) around the surface of a galena (lead sulfide) or carborundum
(silicon carbide) crystal until it suddenly started working. Then, over a period of a few
hours or days, the cat's whisker would slowly stop working and the process would have
to be repeated. At the time their operation was completely mysterious. After the
introduction of the more reliable and amplified vacuum tube based radios, the cat's
whisker systems quickly disappeared. The "cat's whisker" is a primitive example of a
special type of diode still popular today, called a Schottky diode.
World War II
During World War II, radar research quickly pushed radar receivers to operate at ever
higher frequencies and the traditional tube based radio receivers no longer worked well.
The introduction of the cavity magnetron from Britain to the United States in 1940 during
the Tizard Mission resulted in a pressing need for a practical high-frequency amplifier.
On a whim, Russell Ohl of Bell Laboratories decided to try a cat's whisker. By this point
they had not been in use for a number of years, and no one at the labs had one. After
hunting one down at a used radio store in Manhattan, he found that it worked much better
than tube-based systems.
Ohl investigated why the cat's whisker functioned so well. He spent most of 1939 trying
to grow more pure versions of the crystals. He soon found that with higher quality
crystals their finicky behaviour went away, but so did their ability to operate as a radio
detector. One day he found one of his purest crystals nevertheless worked well, and
interestingly, it had a clearly visible crack near the middle. However as he moved about
the room trying to test it, the detector would mysteriously work, and then stop again.
After some study he found that the behaviour was controlled by the light in the room–
more light caused more conductance in the crystal. He invited several other people to see
this crystal, and Walter Brattain immediately realized there was some sort of junction at
the crack.
Further research cleared up the remaining mystery. The crystal had cracked because
either side contained very slightly different amounts of the impurities Ohl could not
remove–about 0.2%. One side of the crystal had impurities that added extra electrons (the
carriers of electrical current) and made it a "conductor". The other had impurities that
wanted to bind to these electrons, making it (what he called) an "insulator". Because the
two parts of the crystal were in contact with each other, the electrons could be pushed out
of the conductive side which had extra electrons (soon to be known as the emitter) and
replaced by new ones being provided (from a battery, for instance) where they would
flow into the insulating portion and be collected by the whisker filament (named the
collector). However, when the voltage was reversed the electrons being pushed into the
collector would quickly fill up the "holes" (the electron-needy impurities), and
conduction would stop almost instantly. This junction of the two crystals (or parts of one
crystal) created a solid-state diode, and the concept soon became known as
semiconduction. The mechanism of action when the diode is off has to do with the
separation of charge carriers around the junction. This is called a "depletion region".
Semiconductor device fundamentals
The main reason semiconductor materials are so useful is that the behaviour of a
semiconductor can be easily manipulated by the addition of impurities, known as
doping. Semiconductor conductivity can be controlled by introduction of an
electric field, by exposure to light, and even pressure and heat; thus,
semiconductors can make excellent sensors.
Current conduction in a semiconductor occurs via mobile or "free" electrons and
holes, collectively known as charge carriers. Doping a semiconductor such as
silicon with a small amount of impurity atoms, such as phosphorus or boron,
greatly increases the number of free electrons or holes within the semiconductor.
When a doped semiconductor contains excess holes it is called "p-type", and
when it contains excess free electrons it is known as "n-type", where p (positive
for holes) or n (negative for electrons) is the sign of the charge of the majority
mobile charge carriers. The junctions which form where n-type and p-type
semiconductors join together are called p-n junctions.
SEMICONDUCTOR :
Definition
Semiconductors are materials which are neither conductors or insulators, having
conductivities intermediate to those of conductors like copper and insulators like
wood or plastic. Common semiconductors are Silicon and Germanium.
The reason semiconductors are important is that with some engineering they
can sometimes both conduct and insulate depending on their connections. Thus
they serve as the basis for switching and amplification, the fundamental actions
of computer elements.
Semiconductor device fabrication
NASA's Glenn Research Center cleanroom.
Semiconductor device fabrication is the process used to create chips, the integrated
circuits that are present in everyday electrical and electronic devices. It is a multiple-step
sequence of photographic and chemical processing steps during which electronic circuits
are gradually created on a wafer made of pure semiconducting material. Silicon is the
most commonly used semiconductor material today, along with various compound
semiconductors.
The entire manufacturing process from start to packaged chips ready for shipment takes
six to eight weeks and is performed in highly specialized facilities referred to as fabs.
Semiconductor device applications
All transistor types can be used as the building blocks of logic gates, which are
fundamental in the design of digital circuits. In digital circuits like microprocessors,
transistors act as on-off switches; in the MOSFET, for instance, the voltage applied to the
gate determines whether the switch is on or off.
Transistors used for analog circuits do not act as on-off switches; rather, they respond to a
continuous range of inputs with a continuous range of outputs. Common analog circuits
include amplifiers and oscillators.
Circuits that interface or translate between digital circuits and analog circuits are known
as mixed-signal circuits.
Power semiconductor devices are discrete devices or integrated circuits intended for high
current or high voltage applications. Power integrated circuits combine IC technology
with power semiconductor technology, these are sometimes referred to as "smart" power
devices. Several companies specialize in manufacturing power semiconductors.
Semiconductor device materials
By far, silicon (Si) is the most widely used material in semiconductor devices. Its
combination of low raw material cost, relatively simple processing, and a useful
temperature range make it currently the best compromise among the various competing
materials.
Germanium (Ge) was a widely used early semiconductor material but its thermal
sensitivity makes it less useful than silicon. Today, germanium is often alloyed with
silicon for use in very-high-speed SiGe devices; IBM is a major producer of such
devices.
Gallium arsenide (GaAs) is also widely used in high-speed devices but so far, it has been
difficult to form large-diameter boules of this material, limiting the wafer diameter to
sizes significantly smaller than silicon wafers thus making mass production of GaAs
devices significantly more expensive than silicon.
Other less common materials are also in use or under investigation.
Silicon carbide (SiC) has found some application as the raw material for blue lightemitting diodes (LEDs) and is being investigated for use in semiconductor devices that
could withstand very high operating temperatures and environments with the presence of
significant levels of ionizing radiation. IMPATT diodes have also been fabricated from
SiC.
Various indium compounds (indium arsenide, indium antimonide, and indium phosphide)
are also being used in LEDs and solid state laser diodes. Selenium sulfide is being studied
in the manufacture of photovoltaic solar cells.
List of common semiconductor devices
Two-terminal devices:
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Avalanche diode (avalanche breakdown diode)
DIAC
Diode (rectifier diode)
Gunn diode
IMPATT diode
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Laser diode
Light-emitting diode (LED)
Photocell
PIN diode
Schottky diode
Solar cell
Tunnel diode
VCSEL
VECSEL
Zener diode
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Bipolar transistor
Darlington transistor
Field effect transistor
IGBT (Insulated Gate Bipolar Transistor)
SCR (Silicon Controlled Rectifier)
Thyristor
Triac
Unijunction transistor
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Hall effect sensor (magnetic field sensor)
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Charge-coupled device (CCD)
Microprocessor
Random Access Memory (RAM)
Read-only memory (ROM)
Three-terminal devices:
Four-terminal devices:
Multi-terminal devices:
DOPING:
Doping refers to the addition of impurities to a semiconductor. The addition of
impurities adds charge carrying elements to the semiconductor.
The two classes of doping are p-type and n-type which refer to the introduction
of positive and negative charge carriers. For instance if one introduces a
Phosphorus atom into a silicon lattice, the phosphous atom would prefer to shed
one of the electrons in its outer shell in order to fit in with the silicon lattice. This
electron is then available to slide through the material, carrying current. This is an
example of n-type doping.
By doping the same lattice with Boron, the Boron site wishes to suck an electron
out of the silicon lattice to fit neatly into the structure. The site which is now
missing an electron represents a positive charge, and therefore the doping is ptype. Movement of this site constitutes a current.
Doping becomes important when p-doped and n-doped materials are connected.