SCHOOL NAME SCHOOL NAME SCHOOL NAME Constant112.pdf · A categorized blogging website and an Online Resource Centre For CBSE, RBSE, and Other State Boards — Raj Kumar Parashari,

AMMHU 1

www.ammhu.com A categorized blogging website and an Online Resource Centre For CBSE, RBSE, and Other State Boards — Raj Kumar Parashari, Head Science @ JPHS, Jaipur-302021

SCHOOL NAME SCHOOL NAME SCHOOL

NAME

ADDRESS WITH PINCODE AND PHONE NUMBER

Submitted by Guided by

Student Name [ ]

Class With Section Teacher Name

Class Roll No. Designation

Board Roll No.

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INDEX

1. CERTIFICATE

2. ACKNOWLEDGEMENT

3. AIM

4. APPARATUS REQUIRED

5. PRINCIPLE / THEORY

(a) A Brief Introduction About Semiconductors

(b) A Brief Introduction About LEDs

(c) Principle Used For The Determination Of Plank’s

Constant

6. PROCEDURE

7. OBSERVATIONS With CALCULATIONS

8. RESULT

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CERTIFICATE

This is to certify that _______________________

student of ______ school name _______, class XII Sc

__ has carried out his work for the investigatory

project entitled ______________________-

_________________ under my guidance and

supervision.

Signature of the Teacher

Designation

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ACKNOWLEDGEMENT I __________________________________ a student of

____________________________________,_____________

_ would like to pay my sincere gratitude to my Physics

teacher Mr. __________________________ for helping and

guiding me throughout the completion of the project

report. I would like to thank Mr./Mrs. _______________

_______________________ our school principal/Director

for making school such a wonderful place of learning. I also

would like to thank Mr. ________________ our lab assistant

for the timely assistant in completion of the project.

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AIM

To determine the value of Plank’s constant with the help of

LEDs of different colours

APPARATUS REQUIRED

Red, Blue and green LEDs at least one of each type, a

voltmeter, a rheostat, a battery and an ammeter.

THEORY

(a) Brief Introduction To Semiconductors & PN Diodes

Semiconductors are of two types intrinsic semiconductors and

extrinsic semiconductors

Intrinsic semiconductors are pure pieces of silicon or

germanium without a single foreign particle. It’s electrical

conductivity is totally governed by the thermally excited

electrons and consequently created holes.

In a crystal of germanium, each Ge atom is has four electrons

in its outer orbit so bonded by four neighbouring Ge atoms.

At low temperatures all bonds are intact. As temperature

increases, the thermal energy of the valence electrons

increases. Some of the electron may break free from the

covalent bond and becomes free to conduct. Each of these free

electrons leaves behind vacancy of effective positive charge

called as a hole. Each hole behaves like an apparent free

particle with positive charge. In intrinsic semiconductor, as

each electron creates a hole, so the number density of free

electrons (ne) is equal to number density of holes (nh) i.e. ne =

nh = ni

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Under the application of electric field, holes move in the

direction of the field (due to jumping of bound electrons in

opposite direction from one atom to another) so they act as

positive charge carries and result in a hole current. In

thermally generated free elections give rise to an independent

current, Ie

Total current I = Electron current + Hole current = Ie + Ih

Extrinsic semiconductor are obtained by doping a pure

semiconductor with acceptor or donor impurity atoms. In the

process conductivity is considerably increased. Extrinsic

semiconductors are of 2 types:

N-type semiconductors are Obtained by doping Ge or Si

with pentavalent dopants such as phosphorous P, arsenic As

or antimony Sb.

P-type semiconductors are Obtained by doping Ge or Si with

trivalent or acceptor dopants such as boron B, aluminum Al,

gallium Ga or indium In.

N-Type Semiconductors

The pentavalent impurity atoms substitute the tetravalent Si

(or Ge) atom. It uses 4 of its 5 valance electrons in forming

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four covalent bonds with neighbouring Si atoms, while 5th

electron is loosely bound to the impurity atom. A very small

amount of ionization energy (0.05 eV) for Si or 0.01 eV of

Ge) is required to detach this electron. At room temperature,

thermal energy is enough to set free this electron. The dopant

atom is converted into an ionized + vely charged core. As the

pentavalent impurity donates one extra electron it is called a

donor atom. Such semiconductors have free electrons

contribute by donors and generated by thermal process while

holes are only due to thermal generation.

Hence electrons are majority charge carriers while holes are

minority charge curriers. (ne >> nh) Most current is carried by

negatively charges electrons, so the semiconductor doped with

donor type impurities is called n-type semiconductor.

P-Type Semiconductors

Such a semiconductor is obtained by doping the tetravalent

semiconductor Si (or Ge) with trivalent impurities such as B,

Al, Ga. The impurity uses its three valence electrons in

forming covalent bonds with three neighbouring Si atoms and

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one covalent bond with neighbouring Si is left incomplete due

to deficiency of one electron.

An electron from neighbouring Si-Si covalent bond slides into

this vacant band creating a vacancy or a hole in that bond.

This hole is now available for conduction. The trivalent

impurity atom becomes negatively charged when all its

valence bonds get filled. The trivalent impurity is called an

acceptor atom because it creates a hole which can accept an

electron from the neighbouring bond. These holes are created

due to acceptor atom and thermal generation while electrons

are only due to thermal generation. Hence holes are the

majority change carrier and electrons are minority charge

carrier. Most current is carried by holes and hence these

semiconductors are called p-type semiconductors.

Doping

The process of deliberate addition of a desirable impurity to a

pure semiconductor so as to increase its conductivity is called

doping. The impurity atoms added are called dopants and the

semiconductors doped with impurity are called doped

semiconductors.

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Essential Requirements for a doping process:

1. The semiconductor material should be of very high

purity.

2. The dopant atom should neatly replace semiconductor

atom

3. Size of dopant atom should be almost the same as that of

a semiconductor atom. For this third & fifth group atoms are

must suitable.

4. Dopant should not distort the crystal lattice.

5. Concentration of dopant atoms should be small about 1

ppm ( part per million).

Methods of Doping

1. By adding the impurity atom to any extremely pure

sample of molten semiconductor

2. By heating the crystalline semiconductor in an

atmosphere containing dopant atoms or their molecules so that

the dopant atoms diffuse into the semiconductor.

3. By bombarding the semiconductor with ions of dopant

atoms.

P-N Junctions

It is a single crystal of Ge or Si doped in such a manner that

one portion of it is p-type semiconductor and other part as n-

type semiconductor.

We take thin p-type silicon semiconductor wafer and dope in

to it a small quality of penta-valent impurity. A part of the p-

type semiconductor is converted into n-type. The wafer now

contains p-region and n-region with metallurgical junction

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between two junctions. The device so formed is called as PN

diode.

Formation of depletion region and potential barrier in p-n

junction:

As soon as a p-n junction is formed, majority charge carriers

begin to diffuse from region of higher concentration to region

of lower concentration. Thus the electrons from the n-region

diffuse into the p-region and where they combine with holes

with holes and get neutralized. Similarly, the holes from the p-

region diffuse into n-region where they combine with the

electrons and get neutralized. This is called electron-hole

recombination.

The p region near the junction is left with immobile –ve cores

and n region near the junction is left with +ve cores. The

small region in the vicinity of junction which is depleted of

free charge carriers and has only immobile ions is called

depletion layer. The accumulation of negative charges in the

p-region and positive charges in the n-region sets up potential

difference across the junction. This acts as a barrier and is

called barrier potential VB which opposes the further diffusion

of elections and holes across the junction. The barrier

potential sets up a barrier field in the direction from n-region

to p-region.

The barrier potential VB depends on (i) nature of

semiconductor (ii) temperature (iii) amount of dopants.

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Circuit symbol for p-n junction

The direction of arrow is from P region to n-region. The arrow

indicates the direction in which the conventional current can

flow easily. P side is known as anode and n-side is known as

cathode.

Working of a p-n junction

(a) In case of Forward Biasing

(i) Forward biasing If the positive terminal of a battery is

connected to the P side and the negative terminal of the n side,

then the PN -junction is said to be forward biased.

(i) The effective barrier potential decreases to (VB - V) and

hence energy barrier across the junction decreases.

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(ii) The majority charge carriers from p side and electrons

from n side begin to flow towards the junction.

(iii) The diffusion of electrodes and holes into the depletion

layer decreases its width and

(iv) The effective resistance across the p-n junction

decreases.

When V exceeds VB, the majority charge carriers start flowing

easily across the junction and set up a large current ( mA)

called forward current.

(b) In case of Reverse biasing :

If the positive terminal of a battery is connected to the N-

segment and negative terminal to P-segment, the p-n junction

is said to be reverse biased.

The applied voltage and barrier potential are in same direction

:

(i) Barrier potential increases to VB +V. Hence energy

barrier increase.

(ii) Majority of charge move away from junction, widening

the depletion layer.

(iii) Resistance of p-n junction becomes very large

(iv) no current flows across the junction due to majority

charge carriers.

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However at room temperature, there are always present some

minority charge carriers like holes in n-region and electron in

p-region. The reverse bias pushes them towards junction ,

which contributes to weak reverse saturation current.

V-I characteristics of a p-n junction diode

A graph showing the variation of current flowing through a p-

n junction with the voltage applied across it is called voltage

current.

OR

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(b) A Brief Introduction About Light Emitting Diode

It is a heavily doped forward biased p-n junction which

spontaneously converts the biasing electrical energy into

optical energy like infrared and visible light.

In a LED, the shorter lead responds to N-or cathode and

longer lead responds to P - or anode side.

A p-n junction made from a translucent semiconductor like

gallium arsenide or indium phosphide is provided with

metalized contact. When it is forward biased through a series

resistance R (to limit the current), light photons are emitted

from non metalized surface of a n region.

When the p-n junction is forward biased, electrons are sent

from N-region to P-region and holes are sent from p region to

n region. Near the junction, the concentration of minority

carriers increases as compared to equilibrium concentration on

either side excess minority carriers combine with majority

carriers. On recombination the energy is released in form of

photons. Photons with energy equal or slightly less than the

band gap are emitted. When the forward bias of the diode is

small, the intensity of emitted light is small. As forward

current increases, intensity of light increases and reaches a

maximum.

1. The colour of light emitted from LED depends on its

bond gap energy.

2. Intensity of light is determined by forward current

through the PN- junction

Choice of Semiconductor material used in LED

The wavelength of visible light ranges from 0.4m to 0.7m.

For semiconductor to emit visible light-min band gap must be

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1.8eV. Compound semiconductor Gallium-Arsenide

phosphide (GaAs1–xPx) is used to make LEDs of different

color.

Advantage of LEDs over conventional incandescent lamps

1. Low operational voltage and less power consumption.

2. Low action and no warm up time required

3. Light emitted is nearly monochromic

4. Long life

5. Fast ON/OFF switching capacity.

Uses of LEDs

1. Infrared LEDs are used in burglar alarm systems

2. Optical communication

3. Image scanning circuits for picture phone

4. Remote controls

Principle Used For The Determination Of Plank’s

Constant:

Given LED is forward biased and applied potential is

increased gradually. Initially current remains zero, for a

particular value of applied potential current breaks (and LED

glows weakly) called as threshold voltage. As applied

potential is further increased current grows rapidly. If V be the

threshold potential, it means that corresponding to it electrons

gains just enough energy overcome barrier potential and

recombine with the hole. Energy released during

recombination is equal or slightly less than the band gap

energy, so h c

E eV

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The Plank constant can be calculated using eV

hc

Procedure :—

(i) Use the forward bias circuit with LED as PN diode.

(ii) Now increase potential across it gradually and record that

value of voltmeter for which LED starts glowing weakly /

ammeter gives some noticeable reading.

Observations With Calculations :—

(a) Least Count of milli-voltmeter = 20 milli-volt

(b) Wavelength of Red LED = 6.33 × 10–7 m,

Wavelength of Green LED = 5.55 × 10–7 m,

Wavelength of Blue LED = 4.7 × 10–7 m,

Reading of the voltmeter for which red LED just starts

glowing

1. 880 milli-volt 2. 860 milli-volt

3. 880 milli-volt 4. 880 milli-volt

Average value of voltmeter reading

V = 875 volt

So, the Plank constant will be

eVh

c

19 7

34

8

1.6 10 0.875 6.33 10h 2.95 10 J s

3 10

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Reading of the voltmeter for which green LED just starts

glowing

1. ------- milli-volt 2. -------- milli-volt

3. -------- milli-volt 4. -------- milli-volt


V = ------ volt


eVh

c

h J s

Reading of the voltmeter for which blue LED just starts

glowing

1. ------- milli-volt 2. -------- milli-volt

3. -------- milli-volt 4. -------- milli-volt


V = ------ volt

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eVh

c

h J s

Now the average value of Plank’s constant will be

1 1 1h h hh J s

3

Result-

The average value of Plank’s constant is ___________

Bibliography

1. NCERT

2. Google for searching circuits/images and graphs

3.

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Note: — Hand written Investigatory Project

Report should be submitted. Work should be

neat and clean. Avoid unnecessary fancy

decoration of the project report.

Note: — Learn theory, working and related

viva for board practical examination.

Wish you all the best.

Raj Kumar Parashari

Head Science

JPHS, Jaipur

Documents

SCHOOL NAME SCHOOL NAME SCHOOL NAME Constant112.pdf · A categorized blogging website and an Online Resource Centre For CBSE, RBSE, and Other State Boards — Raj Kumar Parashari,