days with the rise of very large scale integration(VLSI) technology the
transistors, which are the building blocks of memory chips ,can be made as tiny
as in the scale of nano meter which allows us to fit millions of them into a
small area. And the building blocks of these transistors are semiconductors.
find wide range of application in modern day gadgets. In fact if you observe
carefully you are surrounded by semiconductors. The cell phone you use or the
laptop has millions of tiny transistors embedded in them in order to empower
them with various functionalities. The LED which is aggressively replacing many
of the existing lighting technologies is a semiconductor device.
Semiconductor devices have number of
applications. Some of the important applications of semiconductor devices are
described in this section.
Junction Diode as a Rectifier
The function of a
rectifier circuit is that to convert a.c signal to d.c signals using p-n
junction diodes. The peculiarity of a p-n junction diode is that it conduct
only in one direction. If a diode is in forward biased condition, it will
conduct, otherwise it will not. If a pulsating a.c signal is applied to the
diode, only during positive half cycle it will conduct. Thus p-n junction
diode subjected to an a.c voltage acts as a rectifier converting alternating
voltage to a pulsating d.c voltage.
as an Amplifier
Transistor is a three terminal device:
Base, emitter and collector, can be operated in three configurations common
base, common emitter and common collector. According to configuration it can be
used for voltage as well as current amplification. The input signal of a small
amplitude is applied at the base to get the magnified output signal at the
collector. Thus provides an amplification of the signal. The amplification in
the transistor is achieved by passing input current signal from a region of low
resistance to a region of high resistance. This concept of transfer of
resistance has given the name TRANSfer- resISTOR (TRANSISTOR).
As shown in figure, in
this configuration input is applied between base and emitter, and output is
taken from collector and emitter. Here, emitter of the transistor is common to
both, input and output circuits and hence the name common emitter
configuration. Common emitter configurations for both n-p-n and p-n-p
transistors are shown in figure and 1 and 2 respectively.
Transistor as a Switching Device
One of the very
important applications of a BJT (bipolar junction transistor) is its use as a
switching device for the computer logic circuits. The circuit shown in the
figure can be used as a switch by proper selection of resistances. The
transistor is made to operate in the two extreme modes, that is it operates
either in the cut off or saturation mode. Consider an input signal shown in
figure b applied at the input of the circuit in the figure a. When input Vin =
-V1, transistor is designed to operate in cut off state by proper
choice of RB, RC. Now IC is close to
zero, therefore the output voltage V0 = VCC as
shown in figure c. For Vin = V2, transistor goes
into saturation or is driven 'ON'. When a transistor is biased in saturation
region, we know that VCE = VCEsat. So that the
output voltage V0 = VCEsat = 0.2 V. Thus, the
output voltage is either high (= VCC for transistor OFF) or low
(= 0 V for transistor ON). So, the circuit is indeed working as a switch.
Logic Gates and Their Realization
Logic gates are the
basic elements that make up a digital system. The electronic gaate is a circuit
that is able to operate on a number of binary inputs in order to perform a
particular logical function. The types of gates available are the AND, OR, NOT,
NAND, NOR, XOR and XNOR. The gate is a digital circuit with one or more input
voltages but only one output voltage. By connecting the different gates in
different ways, we can build circuits that perform arithmetic and other
functions associated with the human brain because they simulate mental
processes. The operation of a logic gate can be easily understood with the help
of truth table. A truth table is a table that shows all the input-output
possibilities of a logic circuit; that is the truth table indicates the outputs
for different possibilities of the inputs.
READING TASK 2: INTRINSIC
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.
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 that electrons can reach the conduction band and contribute to electrical
term intrinsic here distinguishes between the properties of pure
"intrinsic" silicon and the dramatically different properties
of doped n-typeor p-type semiconductors.
Both electrons and holes contribute to current
flow in an intrinsic semiconductor.
The current which will flow in
an intrinsic semiconductor consists
of both electron and hole current. That is, the
electrons which have been freed from their lattice positions into the conduction band can move through the
addition, other electrons can hop between lattice positions to fill the
vacancies left by the freed electrons. This additional mechanism is called
hole conduction because it is as if the holes are migrating across the
material in the direction opposite to the free electron movement.
The current flow in an intrinsic semiconductor is influenced by
the density of energy states which
in turn influences the electron density in the conduction band.
This current is highly temperature dependent.
Electrons and Holes
In an intrinsic semiconductor like silicon at temperatures above
absolute zero, there will be some electrons which are excited across the band gapinto the conduction band and
which can produce current. When the electron in pure silicon crosses the gap,
it leaves behind an electron vacancy or "hole" in the regular silicon lattice. Under the influence of an
external voltage, both the electron and the hole can move across the material.
In an n-type semiconductor, the
dopant contributes extra electrons, dramatically increasing the conductivity.
In a p-typesemiconductor, the dopant
produces extra vacancies or holes, which likewise increase the conductivity. It
is however the behavior of the p-n junction which is the key to the
enormous variety of solid-state electronic devices.
Скачано с www.znanio.ru
READING TASK 1: APPLICATIONS OF
As shown in figure, in this configuration input is applied between base and emitter, and output is taken from collector and emitter
The conductivity of a semiconductor can be modeled in terms of the band theory of solids
The current flow in an intrinsic semiconductor is influenced by the density of energy states which in turn influences the electron density in the conduction…