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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.

AMMHU 2

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

INDEX

1. CERTIFICATE

2. ACKNOWLEDGEMENT

3. AIM

4. APPARATUS REQUIRED

5. PRINCIPLE / THEORY

(a) A Brief Introduction About Semiconductors

(b) A Brief Introduction About LDR

6. PROCEDURE

7. OBSERVATIONS

8. RESULT

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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

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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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

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.

AMMHU 5

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

AIM

AIM: To study the variation in the resistance of a given Light

Dependent Resistance, when

(i) Incandescent lamps of different powers are used to

illuminate the LDR kept at a fixed distance.

(ii) In the distance of LDR id varied from an incandescent

lamp of fixed power.

APPARATUS REQUIRED

A bulb holder attached to 220 volt power supply, Bulbs of

40W, 100W and 200W, an LDR (say NORP 12), one scale

and a multi-meter to measure resistance.

THEORY

(a) Brief Introduction To Semiconductors

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

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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

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

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.

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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

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

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.

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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

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.

(b) A Brief Introduction Light Dependent Resistance

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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

A photo-resistor (or light-dependent resistor, LDR,

or photocell) is a light-controlled variable resistor.

The resistance of a photo-resistor decreases with increasing

incident light intensity; in other words, it

exhibits photoconductivity. A photo-resistor can be applied in

light-sensitive detector circuits, and light- and dark-activated

switching circuits.

A photo-resistor is made of a high resistance semiconductor.

In the dark, a photo-resistor can have a resistance as high as

several mega ohms (MΩ), while in the light, a photo-resistor

can have a resistance as low as a few hundred ohms. If

incident light on a photo-resistor exceeds a

certain frequency, photons are absorbed by the semiconductor

give bound electrons enough energy to jump into

the conduction band. The resulting free electrons- holes pairs,

hence low resistance.

A photoelectric device can be either intrinsic or extrinsic. An

intrinsic semiconductor has its own charge carriers and is not

an efficient semiconductor, for example, silicon. In intrinsic

devices the only available electrons are in the valence band,

and hence the photon must have enough energy to excite the

electron across the entire band gap. Extrinsic devices have

impurities, also called dopants, and added whose ground state

energy is closer to the conduction band; since the electrons do

not have as far to jump, lower energy photons (that is, longer

wavelengths and lower frequencies) are sufficient to trigger

the device. If a sample of silicon has some of its atoms

replaced by phosphorus atoms (impurities), there will be extra

electrons available for conduction. This is an example of an

extrinsic semiconductor.

Photo resistors are less light-sensitive device than photodiodes

or phototransistors. The two latter components are

AMMHU 10

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

true semiconductor devices, while a photo-resistor is a passive

component and does not have a PN-junction. The photo-

resistivity of any photo-resistor may vary widely depending

on ambient temperature, making them unsuitable for

applications requiring precise measurement of or sensitivity to

light.

Photo-resistors also exhibit a certain degree

of latency between exposure to light and the subsequent

decrease in resistance, usually around 10 milliseconds. The

lag time when going from lit to dark environments is even

greater than, often as long as one second. This property makes

them unsuitable for sensing rapidly flashing lights, but is

sometimes used to smooth the response of audio signal

compression.

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LDR Symbol and picture

Applications

Photo-resistors come in many types. Inexpensive cadmium

sulphide cells can be found in many consumer items such as

camera light meters, clock radios, alarm devices (as the

detector for a light beam), nightlights, outdoor clocks, solar

street lamps and solar road studs, etc.

Photo-resistors can be placed in streetlights to control when

the light is on. Ambient light falling on the photo-resistor

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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

causes the streetlight to turn off. Thus energy is saved by

ensuring the light is only on during hours of darkness.

They are also used in some dynamic compressors together

with a small incandescent or neon lamp, or light-emitting

diode to control gain reduction. A common usage of this

application can be found in many guitar amplifiers that

incorporate an onboard tremolo effect, as the oscillating light

patterns control the level of signal running through the amp

circuit.

Lead sulphide (PbS) and indium anti-monide (InSb) LDRs

(light-dependent resistors) are used for the mid-infrared

spectral region. GeCu photoconductors are among the best

far-infrared detectors available, and are used for infrared

astronomy and infrared spectroscopy.

Procedure :— For Experiment 1

(i) Multimeter is set to measure resistance and connected

across the LDR.

(ii) A 40 watt bulb is fitted in the holder and power is

switched on. Now, measure the resistance of LDR keeping it

at a fixed distance (say 20 cm). Repeat for 100W and 200W

bulbs as well. For Experiment 2

(i) Multimeter is set to measure resistance and connected

across the LDR.

(ii) A 100 watt bulb is fitted in the holder and power is

switched on. Now, measure the resistance of LDR keeping it

at a fixed distance (say 20 cm). Now repeat procedure for

same bulb but keeping LDR at different distances.

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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

Observations:—

Table – 1

Distance of the source from the LDR =

S. No. Power of the lamp Resistance

1

2

3

Table – 2

Power of the lamp =

S. No. Distance of the source of

light from LDR

Resistance

1

2

3

4

5

6

Result-

(1) As Intensity increases resistance of the LDR decreases.

(2) As distance decreases resistance decreases.

AMMHU 14

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

Bibliography

1. NCERT

2. Google for searching circuits/images and graphs

3. Wikipedia

Note: —

(1) Hand written Investigatory Project Report

should be submitted. Work should be neat

and clean. Avoid unnecessary fancy

decoration of the project report.

(2) Learn theory, working and related viva

for board practical examination.

(3) Do alterations as per need. Take your

own observation and draw inferences.

Wish you all the best.

Raj Kumar Parashari

Head Science

JPHS, Jaipur