Download Action Of pn Junction

INTRODUCTIONSemiconductors possess conductivity between those of metals and
insulators. They find a variety of uses. One such example is an electronic
musical doorbell.
EXPERIMENT
AIM To make an electronic musical doorbell.
COMPONENT REQUIRED –
1, One IC chip – UM 66 to 25
2, One power transistor – SL100 (n-p-n)
3, One resistor of 1 k (0.25W)
4, One 3 speaker of impedance 4 
5, A full wave rectifier (battery eliminator) with output of 3v
6, A push button.
THEORY –
An ordinary electric door bell is based on the principle of an
electromagnet. When current passes through the coil wound around a soft
iron core, it gets magnetised. The magnetised core then attracts a hammer,
which strikes against a metallic gong to produce the sound. Such an electric
bell suffers from the following drawbacks.
1, when current passes through the coil, eddy currents are produced in the
soft iron core, which in turn heats up the coil. As a result, the coil of the bell
gets burnt, if the current is passed for a long time.
2) Such a bell gets burnt due to excessive current passing through its coil
due to voltage fluctuation.
3) The sound produced by the bell is not soothing to the ears.
Since an electric musical bell operates on 3 v d.c. Supply the
small voltage fluctuations do not hamper its performance. Moreover as the
name suggest, a musical doorbell produces sound, which is quite pleasing to
the ears.
BRIEF DISCRIPTION OF CIRCUIT DIAGRAMFig. Shows the circuit diagram for a musical doorbell.
The positive terminal of the full wave rectifier is connected to the pin 2 of
the IC chip UM 66 to 25 through a push button, while the negative terminal
is connected to the pin 3 of the IC. The musical notes of the sound produced
by the IC are obtained at pin 1 of the IC, which are fed to the base of the
power transistors SL 100 through a ressistance of 1 k . The emitter of the
transistor is also connected to the negative terminal of the full wave rectifier
the speaker of impedance 4  is connected across the collector of the
transistor. And the positive terminal of the d.c. Supply from the rectifier.
The circuit is based on the simple application of IC chip UM 66
to 25. The whole circuit is wired primarily on the IC chip, which comprises
of a ROM memory of 64 notes. It has a built in oscillator and pre-amplifier.
Since the signal generated by IC is very weak, the power transistor SL 100
has been use to amplifier the signal. The amplified signal, than drive the
speaker, which produces a musical sound.
FORMATION OF p-n JUNCTIONA p-n junction is basic semiconductor device.
When a p- type crystal is placed in contact with n- type crystal
so as to form one piece. This assemble so obtained is called p-n junction or
junction diode or crystal diode. The surface of contact of p- and n- type
crystals is called junction. In the p-section holes are the majority carriers;
while in n-section the majority carriers are electrons. Due to high
concentration of different types of charge carriers in the two sections, holes
from p- region diffuse into n- region and electron from n- region diffuse to
p- region. In both cases when an electron meets a hole, the two cancel the
effect on each other and as a result a thin layer at the junctions becomes
devoid of charge carriers. This is called Depletion Layer. The thickness of
the depletion layer is of the order of 10–6 m.
The potential difference developed across the junction due to
migration of majority charge carriers is called potential barrier.
Thus the formation of p-n junction results in a very strong
electric field across the junction.
FORWARD BIASINGWhen an external d.c. source is connected to the junction
diode with p-section connected to positive pole and n-section to the negative
pole, the junction diode is said to be forward biased.
Action of p-n junctionWhen the p-n junction is forward biased, the
positive holes in the p- section are repelled by positive pole of the battery
towards the p-n junction. Simultaneously, the negative electrons in the nsection are repelled by negative pole of the battery towards the junction.
However, the movement of electron and holes across the junction is opposed
by the factious battery voltage (= 0.3 v to 0.7 v) developed across the
junction. Just near the p- n junction electrons and holes combine and cease
to exist as mobile charge carriers after the potential barrier is overcome by
the applied potential.
REVERSE BIASING –
When a battery is connected to junction diode with psection connected to negative pole and n- section connected to the positive
pole, the junction diode is said to be reverse biased.
Action Of p-n Junction –
When the p-n junction is reverse biased, the holes in the
p- section get attracted towards the negative terminal of battery and
therefore, the holes move away from the junction. At the same time the
electron in the n- section get attracted toward the positive terminal and move
away from the junction.
A small electron hole combination current called reverse
current is maintained by the minority carriers.
The junction diode is represented by the symbol as
shown. The arrow head represents the p- section of the junction diode and it
points in the direction in which the hole current or conventional current will
flow , when junction diode is foreword biased . The electron current or the
electronic current will flow in opposite direction.
ZENER DIODE –
The specially designed junction diodes, which can operate the
reverse breakdown voltage region continuously, without being damaged, are
called Zener Diodes.
A zener diode is represented by the symbol as shown.
TRANSISTORSA transistor is formed, when a thin layer of one type of extrinsic
semiconductor is sandwiched between two thick layers of the other type of
extrinsic semiconductor.
Thus, a transistor is a semiconductor device having three
sections and two junctions. The three sections are combined, so that the two
at extreme ends have the same type of majority carriers; while the section
that separates them, has the majority carriers of opposite nature. The
semiconductor device so obtained is called n-p-n or p-n-p transistor.
Therefore a transistor can be n-p-n or p-n-p type. In other words, in an n-p-n
transistor, the p- section is sandwiched between two n- sections. On the
other hand, in a p-n-p transistor, the n-section is sandwiched between two p
sections. The three section of the transistor are called emitter (E), base (B)
and collector (C) symbol for transistor.
Symbol For TransistorsIn the symbol for a transistor, the normal
points hole currents conventional current. Therefore, the emitter in n-p-n
transistor is represented by an arrow pointing away from the base, while the
emitter in p-n-p transistor is represented by an arrow pointing towards the
base. The symbols for n-p-n and p-n-p transistors are respectively shown.
When a transistor is used in a circuit , the base- emitter
junction is always forward biased and the base- collector junction is always
reverse biased.
Action Of TransistorThe action of both the type of transistors i.e. n-p-n and p-n-p is
similar, except that the majority and minority carriers in the cases are of
apposite nature.
A) Action Of n-p-n transistorFig. shows the proper biasing of an n-p-n transistor.
Connecting it to negative pole of the battery Vee forward biases the ntype emitter and n- type collector is reverse biased by connecting it to the
positive pole of the battery Vcc (collector, base both)
B) Action of p-n-p TransistorThe p- type emitter of p-n-p transistor is forward biased by connecting
it to positive pole of emitter base battery Vee and the p- type collector is
reverse biased by connecting it to the negative pole of the collector base
battery Vcc as shown.
INTEGRATED CIRCUITS (IC)An integrated circuit consists of a silicon chip
incorporating a large no of microelectronic circuits. In other words an IC
consists of whole system rather than separate electronic components.
Fig. (a) shows a thin slice of silicon crystal about 0.5 cm in thickness.
It is called a silicon wafer. It may have diameter ranging between 0.25 cm to
10-cm. Fig (b) shows a small part of such a silicon wafer of dimensions 50
mil  50 mil. This small portion of the wafer is called a silicon chip. The
various components such as resistors, inductor, capacitor, diodes, transistors,
logic gates etc. can be grown over such one silicon chip. The components
are connected internally to produce a desired circuit. Fig(c) shows the
mounting of the silicon chip into a casing. The pins are connected internally
to the integrated circuit and the other ends of the pins are used to make
external connections.
Depending on the no of components fabricated on a chip, the integrated
circuits are classified as below:
i)
Medium scale IC (MSI) – Such ICS have about 100 circuits
components.
ii)
Large scale IC (LSI) – They have more than 1000 circuits
components.
An IS is a multi terminal device. They are of
diff. shapes & sizes as shown. A commonly used IC may posses 8,10, or 16
terminals.
ICs have following advantages over the conventional electronic
circuits:
a) They are highly reliable.
b) They require very small space as compared to conventional electronic
circuits
c) They have lower total cost.
JUNCTION DIODE AS A RECTIFIERAn electronic device which converts a.c. power
into d.c. power is called a rectifier.
The junction diode offers a low resistance path, when
forward biased, and a high ressistance path, when reverse biased.
FULL WAVE RECTIFIERA rectifier, which rectifies both halves of each a.c.
input cycle, is called a full wave rectifier.
PrincipleIt also works on the principle that a junction diode offers low
ressistance during forward bias and high ressistance during reverse
biased. Here, two junction diodes are connected in such a manner that if
one diode gets forward biased but when the next opposite half cycle
comes, the first diode gets reverse biased and the second, forward biased.
Thus, output obtained during both the half cycle of the a.c. input.
ArrangementThe a.c. supply is fed across the primary coil P of a stepdown transformer. The two ends of the secondary coil S of the
transformer are connected to the P- section of the junction diodes D1
&D2. A load ressistance RL is connected across the n-section of the two
diodes and the central tapping of the secondary coil. The D.C. Output
will be obtained across the load ressistance RL.
TheorySuppose that during first half of the input cycle, upper end of
the coil S is at positive potential and t6he lower end is at negative
potential. The junction diode D1 will get forward biased, while the
diode D2 reverse biased. The conventional current due to the diode D1
will flow along the path of full arrows.
When the second half of the input cycle comes, the
situation will be exactly reversed. Now, the junction diode D2 will
conduct and the conventional current will flow along the path of the
dotted arrows. Since current during both the half cycle, flows from
right to left through the load ressistance RL, the output during both the
half cycles will be of the same nature. The right end of the load
ressistance w.r.t its left ends.
Thus, in a full wave rectifier the output is
continues but pulsating in nature.
CONCLUSION -
Semiconductors devices are being use to most widely in the
digital communication, nano technology etc. with their help communication
luxury, entertainment, health care, that is have been benefited. They are the
most important aspects for the modern life style and have been a real boon
for the digital industry.
INDEX
Introduction
Experiment
P-n Junction
Transistor
Rectifier
IC’s