By RITESH SAINI ECE
Diodes
RESISTORS
The electronic
component known as the resistor is best described as electrical friction.
Pretend, for a moment, that electricity travels through hollow pipes like
water. Assume two pipes are filled with water and one pipe has very rough
walls. It would be easy to say that it is more difficult to push the water through
the rough-walled pipe than through a pipe with smooth walls. The pipe with
rough walls could be described as having more resistance to movement than the
smooth one.
Pioneers in the
field of electronics thought electricity was some type of invisible fluid that
could flow through certain materials easily, but had difficulty flowing through
other materials. In a way they were correct since the movement of electrons
through a material cannot be seen by the human eye, even with the best
microscopes made. There is a similarity between the movement of electrons in wires
and the movement of water in the pipes. For example, if the pressure on one end
of a water pipe is increased, the amount of water that will pass through the
pipe will also increase. The pressure on the other end of the pipe will be
indirectly related to the resistance the pipe has to the flow of water. In other
words, the pressure at the other end of the pipe will decrease if the
resistance of the pipe increases.
Electrons flow
through materials when a pressure (called voltage in electronics) is placed on
one end of the material forcing the electrons to “react” with each other until
the ones on the other end of the material move out. Some materials hold on to
their electrons more than others making it more difficult for the electrons to
move. These materials have a higher resistance to the flow of electricity
(called current in electronics) than the ones that allow electrons to move
easily. Therefore, early experimenters called the materials insulators if they had very
high resistance to electron flow and conductors
if
they had very little resistance to electron flow. Later materials that offered
a medium amount of resistance were classified as semiconductors.
When a person
designs a circuit in electronics, it is often necessary to limit the amount of
electrons or current that will move through that circuit each second. This is
similar to the way a faucet limits the amount of water that will enter a glass
each second. It would be very difficult to fill a glass without breaking it if
the faucet had only two states, wide open or off. By using the proper value of
resistance in an electronic circuit designers can limit the pressure placed on
a device and thus prevent it from being damaged or destroyed.
SUMMARY: The resistor is an electronic component that has
electrical friction. This friction opposes the flow of electrons and thus
reduces the voltage (pressure) placed on other electronic components by
restricting the amount of current that can pass through it.
Capacitors
A capacitor (originally known as condenser) is a passive two-terminal electrical
component used to
store energy in an electric field. The forms of practical capacitors vary widely, but all
contain at least two electrical
conductors separated by
a dielectric (insulator);
for example, one common construction consists of metal foils separated by a
thin layer of insulating film. Capacitors are widely used as parts of electrical
circuits in many
common electrical devices.
When there
is a potential
difference (voltage)
across the conductors, a static electric field develops
across the dielectric, causing positive charge to collect on one plate and
negative charge on the other plate. Energy is stored in
the electrostatic field. An ideal capacitor is characterized by a single
constant value, capacitance, measured in farads. This is
the ratio of theelectric
charge on each
conductor to the potential difference between them.
The
capacitance is greatest when there is a narrow separation between large areas
of conductor, hence capacitor conductors are often called "plates,"
referring to an early means of construction. In practice, the dielectric
between the plates passes a small amount of leakage
current and also has
an electric field strength limit, resulting in a breakdown voltage, while the conductors and leads introduce
an undesired inductance and resistance.
Capacitors
are widely used in electronic circuits for blocking direct current while
allowing alternating
current to pass, in
filter networks, for smoothing the output of power supplies, in theresonant
circuits that tune
radios to particular frequencies, in electric power transmission systems for stabilizing
voltage and power flow, and for many other purposes.
Overview
Charge separation in a
parallel-plate capacitor causes an internal electric field. A dielectric
(orange) reduces the field and increases the capacitance.
A capacitor consists of two conductors separated by a
non-conductive region. The non-conductive region is called the dielectric. In simpler terms, the
dielectric is just an electrical insulator. Examples of dielectric
media are glass, air, paper, vacuum, and even a semiconductor depletion
region chemically
identical to the conductors. A capacitor is assumed to be self-contained and
isolated, with no net electric charge and no influence from
any external electric field. The conductors thus hold equal and opposite
charges on their facing surfaces, and the dielectric develops an electric field.
In SI units, a capacitance
of one farad means that one coulomb of charge on each
conductor causes a voltage of one volt across the device.
The capacitor is a
reasonably general model for electric fields within electric circuits. An ideal
capacitor is wholly characterized by a constant capacitance C,
defined as the ratio of charge ±Q on each conductor to
the voltage V between them:
C=Q/V
Sometimes charge build-up
affects the capacitor mechanically, causing its capacitance to vary. In this
case, capacitance is defined in terms of incremental changes:
C=dq/dv
Inductors
An inductor (also choke, coil or reactor)
is a passive two-terminal electrical component that
stores energy in
its magnetic
field. For comparison, a capacitor stores
energy in an electric
field, and a resistor does
not store energy but rather dissipates energy as heat.
Any
conductor has inductance. An inductor is typically made of a wire or other
conductor wound into a coil, to increase the magnetic field.
When the
current flowing through an inductor changes, creating a time-varying magnetic
field inside the coil, a voltage is induced, according to Faraday's law of electromagnetic induction, which by Lenz's
law opposes the change in current that created it. Inductors
are one of the basic components used in electronics where current and voltage
change with time, due to the ability of inductors to delay and reshape
alternating currents.
Overview
Inductance (L)
results from the magnetic
field forming
around a current-carrying conductor which
tends to resist changes in the current. Electric
current through the
conductor creates a magnetic
flux proportional to the current. A change in this current
creates a corresponding change in magnetic flux which, in turn, by Faraday's law generates
an electromotive force (EMF)
that opposes this change in current. Inductance is a measure of the amount of
EMF generated per unit change in current. For example, an inductor with an
inductance of 1 henry produces an EMF of 1 volt when the
current through the inductor changes at the rate of 1 ampere per second. The
number of loops, the size of each loop, and the material it is wrapped around
all affect the inductance. For example, the magnetic flux linking these turns
can be increased by coiling the conductor around a material with a high permeability such
as iron.
Diodes
that conduct in the reverse direction when the reverse bias voltage exceeds the
breakdown voltage. These are electrically very similar to Zener diodes, and are
often mistakenly called Zener diodes, but break down by a different mechanism,
the avalanche effect. This occurs when the reverse electric field
across the p–n junction causes a wave of ionization, reminiscent of an
avalanche, leading to a large current. Avalanche diodes are designed to break
down at a well-defined reverse voltage without being destroyed. The difference
between the avalanche diode (which has a reverse breakdown above about
6.2 V) and the Zener is that the channel length of the former exceeds the
mean free path of the electrons, so there are collisions between them on the
way out. The only practical difference is that the two types have temperature
coefficients of opposite polarities.
These
are a type of point-contact diode. The cat’s whisker diode consists of a thin
or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of coal. The wire forms the anode and the crystal
forms the cathode. Cat’s whisker diodes were also called crystal diodes and
found application in crystal radio
receivers. Cat’s whisker diodes are generally obsolete, but may be
available from a few manufacturers.[citation
needed]
These
are actually a JFET with the gate shorted to the
source, and function like a two-terminal current-limiter analog to the Zener
diode, which is limiting voltage. They allow a current through them to rise to
a certain value, and then level off at a specific value. Also called CLDs, constant-current
diodes, diode-connected transistors, or current-regulating
diodes.
These
have a region of operation showing negative
resistance caused by quantum tunneling,allowing
amplification of signals and very simple bistable circuits. Due to the high
carrier concentration, tunnel diodes are very fast, may be used at low (mK)
temperatures, high magnetic fields, and in high radiation environments. Because of
these properties, they are often used in spacecraft.
These
are similar to tunnel diodes in that they are made of materials such as GaAs or
InP that exhibit a region of negative
differential resistance. With appropriate biasing, dipole domains
form and travel across the diode, allowing high frequency microwave oscillators to
be built.
Light-emitting diodes (LEDs)
In a
diode formed from a direct band-gap semiconductor,
such as gallium arsenide,
carriers that cross the junction emit photons when they recombine with the
majority carrier on the other side. Depending on the material,wavelengths (or
colors)[16] from the infrared to the near ultraviolet may be
produced.[17] The forward potential of these
diodes depends on the wavelength of the emitted photons: 2.1 V corresponds
to red, 4.0 V to violet. The first LEDs were red and yellow, and
higher-frequency diodes have been developed over time. All LEDs produce
incoherent, narrow-spectrum light; "white" LEDs are actually
combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating.
LEDs can also be used as low-efficiency photodiodes in signal applications. An
LED may be paired with a photodiode or phototransistor in the same package, to
form an opto-isolator.
When
an LED-like structure is contained in a resonant cavity formed
by polishing the parallel end faces, a laser can be formed. Laser diodes are
commonly used in optical storage devices
and for high speed optical
communication.
This
term is used both for conventional p–n diodes used to monitor temperature due
to their varying forward voltage with temperature, and for Peltier heat
pumps for thermoelectric
heating and cooling.. Peltier heat pumps may be made from
semiconductor, though they do not have any rectifying junctions, they use the
differing behaviour of charge carriers in N and P type semiconductor to move
heat.
All
semiconductors are subject to optical charge carrier generation.
This is typically an undesired effect, so most semiconductors are packaged in
light blocking material. Photodiodes are intended to sense light(photodetector), so they
are packaged in materials that allow light to pass, and are usually PIN (the
kind of diode most sensitive to light).[18] A photodiode can be used in solar cells, in photometry,
or in optical
communications. Multiple photodiodes may be packaged in a single
device, either as a linear array or as a two-dimensional array. These arrays
should not be confused with charge-coupled
devices.
A PIN
diode has a central un-doped, or intrinsic, layer, forming a
p-type/intrinsic/n-type structure. They are used as radio
frequency switches and attenuators. They are also used as large volume ionizing
radiation detectors and asphotodetectors.
PIN diodes are also used in power electronics, as
their central layer can withstand high voltages. Furthermore, the PIN structure
can be found in many power semiconductor devices, such as IGBTs, power MOSFETs, and thyristors.
Schottky diodes
are constructed from a metal to semiconductor contact. They have a lower
forward voltage drop than p–n junction diodes. Their forward voltage drop at
forward currents of about 1 mA is in the range 0.15 V to 0.45 V,
which makes them useful in voltage clamping
applications and prevention of transistor saturation. They can
also be used as low loss rectifiers,
although their reverse leakage current is in general higher than that of other
diodes. Schottky diodes are majority carrier devices
and so do not suffer from minority carrier storage problems that slow down many
other diodes — so they have a faster reverse recovery than p–n junction diodes.
They also tend to have much lower junction capacitance than p–n diodes, which
provides for high switching speeds and their use in high-speed circuitry and RF
devices such as switched-mode power supply, mixers, and detectors.
Super
barrier diodes
Super
barrier diodes are rectifier diodes that incorporate the low forward voltage
drop of the Schottky diode with the surge-handling capability and low reverse
leakage current of a normal p–n junction diode.
Gold-doped
diodes
As a
dopant, gold (or platinum)
acts as recombination centers, which helps a fast recombination of minority
carriers. This allows the diode to operate at signal frequencies, at the
expense of a higher forward voltage drop. Gold-doped diodes are faster than
other p–n diodes (but not as fast as Schottky diodes). They also have less
reverse-current leakage than Schottky diodes (but not as good as other p–n
diodes). A typical example is the 1N914.
Snap-off or Step recovery
diodes
The
term step recovery relates to the form of the reverse recovery
characteristic of these devices. After a forward current has been passing in an SRD and
the current is interrupted or reversed, the reverse conduction will cease very
abruptly (as in a step waveform). SRDs can, therefore, provide very fast
voltage transitions by the very sudden disappearance of the charge carriers.
Stabistors or Forward Reference Diodes
The
term stabistor refers to a special type of diodes featuring
extremely stable forward voltage characteristics.
These devices are specially designed for low-voltage stabilization applications
requiring a guaranteed voltage over a wide current range and highly stable over
temperature.
These
are avalanche diodes designed specifically to protect other semiconductor
devices from high-voltage transients.Their p–n junctions have a much larger
cross-sectional area than those of a normal diode, allowing them to conduct
large currents to ground without sustaining damage.
Varicap or varactor diodes
These
are used as voltage-controlled capacitors. These are
important in PLL (phase-locked
loop) and FLL (frequency-locked
loop) circuits, allowing tuning circuits, such as those in
television receivers, to lock quickly. They also enabled tunable oscillators in
early discrete tuning of radios, where a cheap and stable, but fixed-frequency,
crystal oscillator provided the reference frequency for a voltage-controlled oscillator.
Diodes
that can be made to conduct backward. This effect, called Zener breakdown, occurs at
a precisely defined voltage, allowing the diode to be used as a precision
voltage reference. In practical voltage reference circuits, Zener and switching
diodes are connected in series and opposite directions to balance the
temperature coefficient to near-zero. Some devices labeled as high-voltage
Zener diodes are actually avalanche diodes (see above). Two (equivalent) Zeners
in series and in reverse order, in the same package, constitute a transient
absorber (or Transorb,
a registered trademark). The Zener diode is named for Dr. Clarence Melvin
Zener of Carnegie Mellon University, inventor of the device.
No comments:
Post a Comment