A semiconductor is a crystalline material that is neither
a good conductor of electricity (like copper) nor a good insulator
(like rubber). It has intermediate values of electrical resistivity
(its electrical conductivity at room temperature is between
that of a conductor and that of an insulator). At high temperatures
its conductivity approaches that of a metal and at low temperatures
it acts as an insulator. The most common semiconductor materials
are silicon and germanium. Silicon is by far the more widely
used semiconductor for electronics, partly because it can be
used at much higher temperatures than germanium. These materials
can either be used in their pure “intrinsic” forms or can be
doped to create an excess or lack of electrons. Doping involves
adding elements into a semiconductor material during the manufacturing
process to increase its conductivity. The impurities added are
called dopants. Common dopants include arsenic, antimony, bismuth
and phosphorous. The type and level of doping determines whether
the semiconductor is N-type (negative), where the current is
conducted by excess free electrons or P-type (posi-tive), where
the current is conducted by electron vacancies. A semiconductor
device is any of a wide variety of devices that employ the electrical
properties of semiconductor materials to control the flow of
electrons. Such devices include diodes, transistors, logic devices,
integrated circuits, voltage regu-lators, operational amplifiers,
etc
A diode is a two-terminal electronic device
containing an anode and a cathode, that permits current flow
predominantly in only one direction. It has a low resistance
to electric current in one direction and a high resistance to
it in the reverse direction. This property makes a diode useful
as a rectifier, which can convert alternating current (AC) into
direct current (DC). When the voltage applied in the reverse
direction exceeds a certain value, a semiconductor diode “breaks
down” and conducts heavily in the direction of normally high
resistance. When the reverse voltage at which breakdown occurs
remains nearly constant for a wide range of currents, the phenomenon
is called avalanching. A diode using this property is called
a Zener diode. It can be used to regulate the voltage in a circuit.
Semiconductor diodes can be designed to have a variety of characteristics.
Light-sensitive or photosensitive diodes can be used to measure
illumination; the voltage drop across them depends on the amount
of light that strikes them. A light-emitting diode (LED) produces
light as current passes through it.
A logic circuit is an electric circuit whose output
depends upon the input in a way that can be expressed as a function
in symbolic logic; it has one or more binary inputs (capable
of assuming either of two states, e.g., “on” or “off”) and a
single binary output. Logic circuits that perform particular
functions are called gates.
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Basic logic circuits include the AND
gate, the OR gate, and the NOT gate, which perform the logical
functions AND, OR, and NOT. Logic circuits can be built from
any binary electric or electronic devices, including switches,
relays, electron tubes, solid-state diodes, and transistors;
the choice depends upon the application and design requirements.
A transistor is an active semiconductor
device, which possesses at least three terminals (typically,
a base,emitter, and collector). A small current in the centre
or base region can be used to control a larger current flowing
between the end regions (emitter and collector). It is characterised
by its ability to amplify current and is used in a wide variety
of equipment such as amplifiers, oscillators and switching circuits.
(It is a shortened form of transfer resistor.)
An electronic circuit built on a semiconductor
substrate, usually one of single-crystal silicon. The circuit,
often called a chip, is packaged in a hermetically sealed case
or a non-hermetic plastic capsule, with leads extending from
it for input, output, and power-supply connections and for other
connections that may be necessary when the device is put to
use. Integrated circuits can be classified into two groups based
on the type of transistors they contain. Bipolar integrated
circuits contain bipolar junction transistors as their principle
elements. Metal-oxide-semiconductor (MOS) integrated contain
MOS transistors as their principle elements. Some integrated
circuits contain both types of transistors. Integrated circuits
are also categorised according to the number of transistors
or other active circuit devices they contain. An IC is said
to use small-scale integration (SSI) if it contains fewer than
10 transistors. An IC that contains from 10 to 100 transistors
is said to use medium-scale integration. A large-scale integration
(LSI) IC contains from 100 to 1,000 transistors, and one that
uses very-large-scale integration (VLSI) contains more than
1,000 transistors. Some integrated circuits are analog devices;
an operational amplifier is an example. Other ICs, such as the
microprocessors used in computers, are digital devices. Some
hybrid integrated circuits contain both analog and digital circuitry;
a bilateral switch, which switches analog signals by means of
a digital control signal is an example of a hybrid IC. Integrated
circuit functions are virtually limitless. Improvements in IC
manufacturing have led to increasingly dense and capable integrated
circuits.
A circuit or device that produces a nearly
constant voltage output, even though the voltage input (line)
and current output (load) vary widely.
An amplifier characterised by very high
gain and input impedance, and very low output impedance. The
output voltage is proportional to its input voltage with gain |
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