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DEPARTMENT OF PHYICS

MOTHER TERESA SR. SEC. CO-ED SCHOOL

BHOPAL

MOTHER TERESA SR. SEC. CO-ED SCHOOL

KOLAR ROAD BHOAPLDEPARTMENT OF PHYSICS

PROJECT REPORT

INVESTIGATORY PROJECT ON

PN JUNCTION AND

DIODES

PROJECT REPORT

Submitted by

Govind Patel

Submitted toMrs Ratna Hajela

RegisterationNo:

In the accomplishment of this project

successfully, many people have best

owned upon me their blessings and the

heart pledged support, this time I am

utilizing to thank all the people who

have been concerned with project.

Primarily I would thank god for being

able to complete this project with

success. Then I would like to thank my

principal Mr. James MJ and physics

teacher Mrs Ratna hajela, whose

valuable guidance has been the ones

that helped me patch this project and

make it full proof success her

suggestions and her instructions has

served as the major contributor towards

the completion of the project. Then I

would like to thank my parents and

friends who have helped me with their

valuable suggestions and guidance has

been helpful in various phases of the

completion of the project. Last but not

the least I would like to thank my

classmates who have helped me a lot.

ACKNOWLEDGEMENT

CERTIFICATE

This is to certify that project

work titled A STUDY ON PN

JUNCTIONS AND DIODES being

submitted by Govind Patel a

student of class XII-A has

successfully completed the research

on the below mentioned project

under the guidance of Mrs Ratna

Hajela ( Subject Teacher ) during

the year 2016-17 in partial

fulfillment of physics practical

examination conducted by AISSCE

Signature of external

examiner

Signature of

physics teacher

INDEX

• PN Junction

• Diode Equation

• Zener Diodes

Semiconductor: An Introduction

• Conductors: Allow Electric

current to flow through them

• Insulators: Do not Allow Electric current to flow through them

• Semiconductors: Materials whose conductivity lies in between of Conductors and Semiconductor

Insulators, Semiconductors,and Metals: Comparison

This separation of the valence and conduction bands determines the electrical properties of the material

Insulators have a large energy gap electrons can’t jump from valence to

conduction bands no current flows

Conductors (metals) have a very small (or nonexistent) energy gap electrons easily jump to conduction

bands due to thermal excitation current flows easily

Semiconductors have a moderate energy gap only a few electrons can jump to the

conduction bandleaving “holes”

only a little current can flow

P-N JUNCTION

Also known as a diode

One of the basics of semiconductor

technology -

Created by placing n-type and p-

type material in close contact

Diffusion - mobile charges (holes) in

p-type combine with mobile

charges (electrons) in n-type

P-N JUNCTION

Region of charges left behind

(dopants fixed in crystal lattice)

Group III in p-type (one less

proton than Si- negative charge)

Group IV in n-type (one more

proton than Si - positive charge)

Region is totally depleted of mobile

charges - “depletion region”

Electric field forms due to fixed

charges in the depletion region

Depletion region has high

resistance due to lack of mobile

charges

THE P-N JUNCTION

Direction of

Current

DEPLETION LAYER FORMATION

The “potential” or voltage across

the silicon changes in the

depletion region and goes from +

in the n region to – in the p region

Biasing the P-N Diode

Forward

Bias

Applies -

voltage to the

n region and

+ voltage to

the p region

CURRENT!

Reverse Bias

Applies +

voltage to n

region and –

voltage to p

region

NO

CURRENT

DIODES CAN BE

CONSIDERED AS

SWITCH

P-N Junction – Reverse Bias

positive voltage placed on n-type material

electrons in n-type move closer to positive terminal, holes in p-type move closer to negative terminal

width of depletion region increasesallowed current is essentially zero (small

“drift” current)

No current Flow

Depletion layer width

Increses

P-N Junction – Forward Bias

positive voltage placed on p-type materialholes in p-type move away from positive

terminal, electrons in n-type move further from negative terminal

depletion region becomes smaller -resistance of device decreases

voltage increased until critical voltage is reached, depletion region disappears, current can flow freely

P-N Junction - V-I characteristics

Voltage-Current relationship for a p-n junction (diode)

Current-Voltage Characteristics

THE IDEAL DIODE

Positive voltage yields

finite current

Negative voltage

yields zero currentREAL DIODE

Ideal diode equation

where:I = the net current flowing through the

diode;I0 = "dark saturation current", the diode

leakage current density in the absenceof light;V = applied voltage across the terminals

of the diode;q = absolute value of electron charge;k = Boltzmann's constant; andT = absolute temperature (K).

The diode equation gives an expression for the current through a diode as a function of voltage.

The Ideal Diode Law, expressed as:

Semiconductor diode -opened region

The p-side is the cathode, the n-side is the anode

The dropped voltage, VD is measured from the cathode to the anode

Opened: VD VF:

VD = VF

ID = circuit limited, in our model the VD

cannot exceed VF

Semiconductor diode -closed region

• Closed: VF < VD 0:

– VD is determined by the circuit, ID = 0 mA

• Typical values of VF: 0.5 ¸ 0.7 V

Cut-off: 0 < VD < VF:

ID 0 mA

Semiconductor diode - cut-off region

A Zener diode allows current to flow from its

anode to its cathode like a normal semiconductor

diode, but it also permits current to flow in the

reverse direction when its "Zener voltage" is

reached. Zener diodes have a highly doped p-n

junction. Normal diodes will also break down with

a reverse voltage but the voltage and sharpness of

the knee are not as well defined as for a Zener

diode. Also normal diodes are not designed to

operate in the breakdown region, but Zener

diodes can reliably operate in this region.

The device was named after Clarance Melvin

Zener, who discovered the Zener effect. Zener

reverse breakdown is due to electron quantum

tunnelling caused by a high strength electric

field. However, many diodes described as "Zener"

diodes rely instead on avalanche breakdown.

Both breakdown types are used in Zener diodes

with the Zener effect predominating under 5.6

V andavalanche breakdown above.

Zener diodes are widely used in electronic

equipment of all kinds and are one of the basic

building blocks of electronic circuits. They are

used to generate low power stabilized supply rails

from a higher voltage and to provide reference

voltages for circuits, especially stabilized power

supplies. They are also used to protect circuits

from over-voltage, especiallyelectrostatic

discharge

Zener diode

Zener diode shown with typical packages.Reverse current is

shown.

Zener diodes are widely used as voltage references and

as shunt regulators to regulate the voltage across small circuits. When

connected in parallel with a variable voltage source so that it is reverse

biased, a Zener diode conducts when the voltage reaches the diode's

reverse breakdown voltage. From that point on, the relatively low

impedance of the diode keeps the voltage across the diode at that

value.[8]

In this circuit, a typical voltage reference or regulator, an input voltage,

UIN, is regulated down to a stable output voltage UOUT. The breakdown

voltage of diode D is stable over a wide current range and holds

UOUT relatively constant even though the input voltage may fluctuate

over a fairly wide range. Because of the low impedance of the diode

when operated like this, resistor R is used to limit current through the

circuit.

In the case of this simple reference, the current flowing in the diode is

determined using Ohm's law and the known voltage drop across the

resistor R; The value of R must satisfy two conditions :

1.R must be small enough that the current through D keeps D in

reverse breakdown. The value of this current is given in the data sheet

for D. For example, the common BZX79C5V6[9] device, a 5.6 V 0.5 W

Zener diode, has a recommended reverse current of 5 mA. If

insufficient current exists through D, then UOUT is unregulated and less

than the nominal breakdown voltage (this differs to voltage-regulator

tubes where the output voltage will be higher than nominal and could

rise as high as UIN). When calculating R, allowance must be made for

any current through the external load, not shown in this diagram,

connected across UOUT.

2.R must be large enough that the current through D does not destroy

the device.

2.If the current through D is ID, its breakdown voltage VB and its

maximum power dissipation PMAX correlate as such: .

A load may be placed across the diode in this reference circuit,

and as long as the Zener stays in reverse breakdown, the diode

provides a stable voltage source to the load. Zener diodes in this

configuration are often used as stable references for more

advanced voltage regulator circuits.

Shunt regulators are simple, but the requirements that the ballast

resistor be small enough to avoid excessive voltage drop during

worst-case operation (low input voltage concurrent with high load

current) tends to leave a lot of current flowing in the diode much of

the time, making for a fairly wasteful regulator with high quiescent

power dissipation, only suitable for smaller loads.

These devices are also encountered, typically in series with a

base-emitter junction, in transistor stages where selective choice

of a device centered around the avalanche or Zener point can be

used to introduce compensating temperature co-efficient balancing

of the transistor p–n junction. An example of this kind of use would

be a DC error amplifier used in aregulated power supply circuit

feedback loop system.

Zener diodes are also used in surge protectors to limit transient

voltage spikes.

Another application of the Zener diode is the use of noise caused

by its avalan che breakdown in a random number generator.

NCERT textbook class 12 NCERT physics lab Manuel INTERNET www.yahoo.com www.scribd.com www.google.com