Vellore Institute of Technology BPHY101P Engineering Physics Lab

Manual
DEMONSTRATION OF WAVE NATURE OF
ELECTRONS THROUGH ELECTRON DIFFRACTION

Objective
To calculate the interplanar spacing of polycrystalline graphite from electron diffraction pattern and to
obtain de Broglie wavelength of electrons at different accelerating voltages.

Apparatus to be used

Electron diffraction tube, High voltage (up to 10 kV) power supply, Connecting wires, ruler.

Basic theory
In this experiment we form an electron diffraction pattern consisting of circular rings, after the electron gets
transmitted through a very thin polycrystalline graphite sheet. Figure shows sheet of graphite with
hexagonal arrangements of carbon atoms.

Consider this arrangement as two sets of inter-penetrating planes of atoms each with its own interplanar
distances 𝑑1 and 𝑑2 in order of Angstroms. These planes can be further considered as two sets of inter-
penetrating multiple slits. If electrons behave like waves and if they are allowed to pass through these slits,
they would get diffracted just as EM waves get diffracted (provided their wavelength is comparable to
interplanar distances).

The apparatus shown in the figure below depicts that electrons are produced at filament, accelerated and
passed through the thin graphite crystal. To accelerate electrons, a power supply is used. There are sets of
circular disks inside evacuated tube in which the right most is anode with graphite crystal and left most is
cathode. Remaining disks are to focus electrons. Electrons passing through the graphite hit the florescent
screen on the right end of the tube. As graphite has two different lattice spacing, two diffraction rings are
seen at each voltage.

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab
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Now, we can apply Bragg’s law to this case. For the first order diffraction
λ = d sinθ (1)

From theory, de Broglie wavelength of an electron accelerated through a potential V is

λ = 12.3 /√𝑉 Å (2)
From the geometry of the above figure (R = D/2),
sinθ = R/ √(R2+L2) (3)

Procedure
First, turn ON the voltage controller and make sure the initial voltage is set to zero. After that, turn the
voltage knob slowly so that we can see a set of two rings on the florescent screen. Since diffraction is
property of waves, we demonstrate that electrons too exhibit wave nature. By this demonstration,
interplanar distances can be also be found by measuring the diameter of the rings. For the inner ring,
first measure the inner diameter (𝐷1,in) and then the outer diameter 𝐷1,𝑜𝑢𝑡 using a ruler. Find out the
average diameter (𝐷1). Similarly for outer ring, measure the inner (𝐷2,in) and outer (𝐷2,𝑜𝑢𝑡) diameters and
find out the average (𝐷2). Now, Repeat these steps for different accelerating voltages 3.5 to 5 kV, at
voltage intervals of 0.5 kV.

Precautions
❖ Never accelerate beyond 5 kV.
❖ Never touch any controls on the power supply other than the “ON/OFF” switch and thevoltage
varying knob.
❖ Never apply force while measuring the ring diameters.
❖ Keep a ruler gently over the tube to measure the diameters of rings. Metalrulers are strictly prohibited.

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab
Manual
Observations
Distance between graphite sheet and screen (L) = 13.5 cm. For inner ring, d1 =
𝑉 1 𝐷1,𝑖𝑛 𝐷1,𝑜𝑢𝑡 𝐷1 𝜆𝑒𝑥𝑝
√𝑉 sinθ d1(Å)
(kV) (cm) (cm) (cm ) (nm)
(kV)-1/2

For outer ring, d2 =

𝑉 1 𝐷2,𝑖𝑛 𝐷2,𝑜𝑢𝑡 𝐷2 𝜆𝑒𝑥𝑝

√𝑉 sinθ d2(Å)
(kV) (cm) (cm) (cm ) (nm)
(kV)-1/2

Calculations
➢ By using the values from table, calculate sin θ, λ, d1 and d2 with the help of equations (3), (2) and
(1), respectively. And then calculate the average of d for inner and outer rings.

Results
Interplanar distances of the graphite are found to be and
Inferences/Conclusions
✓ …………………
✓ …………………
✓ …………………

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Questions on related concepts (Self-assessment)

Q1. Why cannot we explain diffraction by assuming particle nature of electrons?

Q2. What is/are the source of error in this experiment?
Q3. Why does the diameter change with voltage?

Q4. What does the inner and out circle indicate?

Q5. How can you get the interplanar spacing graphically?

Q6. Why do we have a circular pattern?

Further references

[1] https://www.youtube.com/watch?v=IYnU4T3jbgA

[2] https://www.youtube.com/watch?v=AM8LcaKxZGg

[3] https://www.youtube.com/watch?v=l2OXawoAD6M

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab
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Rubrics for the Continuous Assessment of Electron Diffraction Experiment

Allocated
Parameter LOW MEDIUM HIGH
Marks
Record writing: 02 The record The record has been Completed record
Aim, apparatus, has not been submitted in the lab has been submitted in
theory, formulae, submitted in session but was the lab session
procedure, the lab
incomplete
calculations, results, session
inferences
0 mark 01 mark 02 marks
Experimental 03 No Observation has been Completed the
details, observations/ done but no proper observation and
observations tabulation tabulation has been tabulation, and duly
completed
and tabulations has been verified by the
performed, faculty member.
or wrong
0 mark 01-02 marks 03 marks

Calculations 03 No Incomplete All calculations have

with proper calculations calculations/without been performed with
mention of have been mentioning of units the mention of units.
units, and performed Results have also
results been mentioned.
0 mark 01-02 marks 03 mark

02 The student Few questions were The student answered

did not answered by the all the questions
Viva-voce answer any student
viva-voce
questions
0 Marks 1 mark 02 marks

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

Determination of Planck’s constant and work function of a

metal using Photoelectric Effect
Objective
To determine Planck’s constant and work function of a given metal using the photoelectric effect.

Apparatus to be used
Photoelectric equipment, filters of different colors

Basic theory
It was observed as early as 1905 that most metals emit electrons when their surface is irradiated with
radiation. This phenomenon of emission of electrons from the metal surface exposed to the light of suitable
frequency is known as the photoelectric emission/photoelectric effect. The electrons emitted in this process
are known as photoelectrons, and the current constituted by these electrons is known as photoelectric
current. The basic experimental set up explaining the photoelectric effect is given below.

The detailed study of this effect has shown:

1. That the emission process depends strongly on the frequency of radiation.

2. For each metal, a critical frequency exists such that light of lower frequency cannot eject electrons,
whilst light of higher frequency always does irrespective of light intensity.
3. The emission of electrons occurs within a very short time interval after the arrival of the radiation
4. The number of electrons is directly proportional to the intensity of this radiation.

The experimental results obtained from this experiment are among the most substantial evidence which
prove that the electromagnetic radiation is quantized, and each quanta consisting of packets of energy,
where is the frequency of the radiation and is Planck’s constant. These quanta are called
photons.

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual
Further, it is assumed that electrons are bound inside the metal surface. The minimum energy required to
eject the electrons from the metal surface is known as the work function ( ) of the metal. The work
function can be expressed in terms of radiation frequency as:

(1)

where is the Planck's constant and is the threshold frequency (minimum frequency for photoelectric
effect). It then follows that if the frequency ( ) of the light (photon) is such that

, it will be possible to eject photoelectron, while if , it would be impossible. In the former

case, , the excess energy of light will appear as the kinetic energy of the ejected electron. According
to Einstein, the photoelectric equation must obey the following equation:

(2)

where is the energy of the incident photon, 𝐾𝐸 is the kinetic energy of the ejected electron
(photoelectron), and 𝑊 is the work function of the given metal. If we apply a retarding potential to stop the
flow of these photoelectrons completely, it is known as stopping potential, . The maximum kinetic energy
of the photoelectron is equal to charge of the electron (e) times the stopping potential, i.e. and the
Eq. (2) can be written as:
(3)

Further on rearranging Eq. (3), we obtain the expression of stopping potential:

(4)

(5)

(6)

The above equation represents a straight line . So, when we plot a graph as a function of
frequency ( ), the slope of the straight line will be and the intercept of the extrapolated point at
gives the work function of the given Metal. Further, the value of Planck’s constant can be established from
the obtained slope.

Work functions of certain metals are given as an example in the below table for reference:

Metals Work Functions (eV)

Platinum (Pt) 6.4
Silver (Ag) 4.7
Sodium (Na) 2.3
Potassium (K) 2.2
Caesium (Cs) 1.9

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual
Procedure

The structure of the experimental set-up and its basic functionalities are demonstrated as:

1. Vacuum Phototube. The sensitive component.

2. The removable forepart is used to install the colour filters and a focus lens fixed in the back end.
3. A scale of 40 cm in length. The centre of the vacuum phototube is used as the zero point.
4. Colour filter Set. Five pieces
5. Light Source, 12V/35W halogen tungsten lamp.
6. To move the light source to adjust the distance between the light Source and the vacuum phototube.
7. Digital Meter. Show current (μA), or voltage (V).
8. Display mode switch. For switching the display between voltage and current.
9. Current Multiplier.
10. Switch to adjust the appropriate intensity of incident light.
11. Accelerate voltage adjustor. Knob for adjusting accelerate voltage.
12. Voltage direction switch. Switch for choosing stopping potential.
13. Power switch.
14. Power indicator.

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual
For determination of Planck’s Constant and work function:

1. Adjust the distance between the Light Source enclosure and the Photodiode enclosure so that the
general spacing is between 20.0 cm to 40.0 cm. NOTE: The recommended distance is 25.0 cm. (3 &
6)
2. Turn ON the light source by pressing the power switch (13). Make sure the power indicator (14)
turns green LED On.
3. Allow the light source and the apparatus to warm up for 10 minutes.
4. Insert the red colour filter (635 nm) into the port (2), set the light intensity switch (10) at strong light
for an appropriate photocurrent, voltage direction switch (12) at ‘+‘, accelerating voltage knob (11)
at the minimum position and display mode switch (8) at current display.
5. Set the current multiplier switch (9) for a suitable amount of current on display.
6. Set the voltage direction switch (12) at ‘-‘, then increase the de-accelerating voltage using the knob
(11 )to decrease the photocurrent to zero.
7. Measure the de-accelerating voltage/stopping potential (Vs) corresponding to zero current of 635nm
wavelength by setting the switch (8) into Voltage display mode.
8. Repeat steps 4-6 for other colour filters of different wavelengths and measure the corresponding
stopping potential.
9. Once all measurements are done, remove the colour filters, Put back the blank cap to nozzle (3), Set
the voltage direction switch (12) at ‘+‘, the accelerating voltage knob (11) to zero, switch (8) to
current display mode, and TURN-OFF the power switch (13).
10. Return the colour filters.
11. Do the calculation and plotting figures from the obtained experimental data.

Observations
Sl. No. Incident Photon Frequency (Hz) Stopping Potential (Vs in
Wavelength (Filters) Volts)
1 Red (635 nm)

2 Orange (570 nm)

3 Yellow (540 nm)

4 Green (500 nm)

5 Blue (460 nm)

Model graph
1. Plot a graph of Stopping Potential (Vs) versus Frequency ( 1014 Hz).
2. Find the slope of the best-fit line through the data points on the graph.

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

Calculations
From the graph vs we can get the value of slope and the intercept

Substituting the values of and from the graph, the Planck’s Constant (h) can be calculated as, h =
… ................... Joule-sec.

Standard value of h0 = Joules-sec.

Again, from the same graph, the intercept at , can be calculated

Work function of metal, …………... eV

Compare your calculated value of Planck’s Constant, h to the standard value, h0 = Joules-
sec. The error % can be calculated as:

Results
1. Planck’s constant ‘h’ is found to be h = .................... J-sec
2. Work function of the given metal found to be, W = ............. eV

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual
Inferences/Conclusions
1. ……………………………………………..
2. ……………………………………………..
3. ……………………………………………..
Precautions
1. This instrument should be operated in a dry, cool indoor space.
2. The instrument should be kept in a dust- and moisture-proof environment; if there is dust on the
phototube, colour filter, lens, etc., clean it using absorbent cotton with a few drops of alcohol.
3. The colour filter should be stored in a dry and dust-proof environment.
4. Do not play with the knobs for random movements.
5. Do not put scratch marks on colour filters
6. While applying the negative potential, move the knob slowly and wait 2 secs after each move.
7. After finishing the experiment, remember to switch off the power (14) and cover the drawtube
(2) with the lens blank cover provided. Phototube is a light-sensitive device, and its sensitivity
decrease with exposure to light and due to aging.
Questions on related concepts (Self-assessment)
Q1. What are the applications of photoelectric effect?

Q2. What is the significance of work function?

Q3. Are all the metals useful for photoelectric effect? Justify your answer.

Q4. Why photoelectric effect cannot be explained by classical physics?

Q5. What will be the stopping potential if intensity is tripled?

Q6. Explain the relationship between the intensity of radiation and photoelectric current.

Q7. What is the difference between photoelectric current and photocurrent?

Q8. How does light intensity affect the Stopping Potential?

Q9. How does your calculated value of h compare to the accepted value?

Q10. What do you think may account for the difference – if any – between your calculated value of h
and the accepted value?

Further references
1. https://javalab.org/en/photoelectric_effect_2_en/ (Simulation)
2. https://applets.kcvs.ca/photoelectricEffect/PhotoElectric.html# (Simulation)
3. https://youtu.be/kS4ECdzONfE
4. https://youtu.be/5QRR0JIzSX4
5. https://drive.google.com/file/d/10pespgTuNxCA-186EMShDaMwiFjU57YB/view?usp=share_link (Video
Demonstration)

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

Rubrics for the Continuous Assessment of the Photoelectric Effect Experiment:

Allocated
Parameter LOW MEDIUM HIGH
Marks
Record writing: 02 The record The record has been Completed record
Aim, apparatus, has not been submitted in the lab has been submitted in
theory, formulae, submitted in session but was the lab session
procedure, the lab
incomplete
calculations, results, session
inferences
0 mark 01 mark 02 marks
Experimental 03 No Observation has been Completed the
details, observations/ done but no proper observation and
observations tabulation tabulation has been tabulation, and duly
completed.
and tabulations has been verified by the
performed, faculty member.
or wrong
0 mark 01-02 marks 03 marks

Graph & related 03 No graph has Graph with Completed the graph
calculations been drawn appropriate labeling with necessary
from graph, has been plotted but details and
no calculations from
with calculation from the
graph
appropriate graph has also been
units, & results done. Results are also
mentioned.
0 mark 01-02 marks 03 marks

02 The student Few questions were The student answered

did not answered by the all the questions
Viva-voce answer any student
viva-voce
questions
0 Marks 1 mark 02 marks

21
 vE [Type here] [Type here]

Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

TO STUDY THE CHARACTERISTICS OF

SOLAR CELL
OBJECTIVE:
To determine the fill factor and efficiency of a solar cell.

APPARATUS TO BE USED:
Solar Cell, Light Source (100 Watt), Ammeter, Voltmeter, Variable Load Circuit, Connecting
Wires

BASIC THEORY:
The Solar cell is a semiconductor device that converts solar energy into electrical energy. It is
a specially designed PN junction diode that converts sunlight into electrical power by a three-
step process: (i) Generation of carrier pairs (electron-hole pairs), (ii) Separation of electrons
and holes, (iii) Collection of separated carriers. When the PN junction is exposed to light,
electron-hole pairs are generated in the P and N regions. By diffusion in the material, the
electron and holes reach the junction. The barrier field separates the positive and negative
charge carriers at the junction. That is, under the action of the electric field, the electrons
(minority carriers) from the P region are swept into the N region. Similarly, the holes from
the N region are swept into the P region. The accumulation of charges on the two sides of the
junction produces an emf (Voltage), called a photo-emf/Photo-voltage. The photo emf or
voltage can be measured with a voltmeter, and this optical energy conversion is known as the
photovoltaic effect. Therefore, a solar cell is also called a photovoltaic cell. When an external
circuit (load) is connected across the solar cell terminals, the minority carriers return to their
original sides through the external circuit, causing the current to flow through the circuit.
Thus, the solar cell behaves as a battery with the N side as the negative terminal and the P
side as the positive terminal.

Light Energy
Metal Contact

Anti-Reflection
Coating External
Load

(a) (b)

Figure1: (a) Solar cell working principle (b) Equivalent circuit diagram of Solar cell experiment

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Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

The emf is generated by the solar cell in the open circuit, i.e., when no current is drawn from
it, and is denoted by VOC (V-open circuit). This is the maximum value of voltage that can be
generated by the solar cell. When an external (load of high resistance) is connected in the
circuit, a small current flows through it, and the corresponding voltage decreases. The voltage
goes on falling, and the current increases as the resistance in the external circuit is reduced.
When the load resistance is reduced to zero, the current rises to its maximum value, known as
short-circuit/saturation current, and is denoted as ISC; the voltage becomes zero in this case.
Figure 2 shows the I-V characteristic of a solar cell with its VOC and ISC.

Figure2: Current-Voltage(I-V) and Power-Voltage (P-V) characteristics of Solar cell

The product of open circuit voltage VOC and short circuit current ISC is the (ideal) power that
can be generated from a solar cell and it can be calculated as:

𝑃𝑜𝑤𝑒𝑟 = 𝑉0𝐶 × 𝐼𝑆𝐶

However, the maximum power (Pmax) that can be harvested from the solar is the area of the
largest rectangle that can be formed under the I-V curve (see Fig-2). It is calculated from the
corresponding Current (Imp) and Voltage (Vmp) at that condition as:

𝑃𝑚𝑎𝑥 = 𝑉𝑚𝑝 × 𝐼𝑚𝑝

The corresponding fill factor (FF) can be calculated by taking the ratio of the maximum
power to the ideal power as:

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Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

𝑉𝑚𝑝 × 𝐼𝑚𝑝
𝐹𝐹 =
𝑉0𝐶 × 𝐼𝑆𝑐

If AC is the area of the solar cell and  is the incident intensity, then the efficiency () of the
solar cell is calculated as:

𝑃𝑚𝑎𝑥
𝜂=
𝐴𝐶

PROCEDURE:

Procedure for I-V characteristics of Solar cell:

1. Place the solar cell and the light source (100-watt lamp) opposite to each other.

2. Connect the circuit as shown by dotted lines on the circuit board (See Fig. 1) using

connecting cables.

3. Switch ON the lamp to expose the light onto the Solar Cell.

4. Set a suitable distance between the solar cell and the lamp as mentioned in the solar

kit (Note: The gap between the solar panel edge to the tin-lamp box edge is the actual

distance)

5. Break the circuit by plugging out any cable to measure the open circuit voltage VOC

6. Set the load-resistance knob to Short-Circuit mode and measure the corresponding

short-circuit current ISC.

7. Vary the load resistance through the knob switch and note down the current and

voltage corresponds to each load resistance in the observation Table 1

8. Repeat it for different distances to minimise the error in the experiment.

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Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

OBSERVATION TABLE:
Voltmeter reading for open circuit and millimetre reading with zero resistance are

VOC: ; ISC: ;

Load
Resistance Distance (x) in mm ----------------- ; Intensity of
Light: --------------------
(Ohm)
Voltage (V) Current (mA) Power (mW)

MODEL GRAPH:
1. Plot a graph of current (I) versus Voltage (V).
2. Plot a graph of Power (P) versus Voltage (V).
3. From the plot, find the maximum power (Pmax) and the corresponding
Current (Imp) and Voltage (Vmp).
CALCULATIONS:
From the graph and observation table, the fill factor (FF) and the efficiency () can be
calculated by using the values of VOC, ISC, Vmp, Imp :

𝑉𝑚𝑝 × 𝐼𝑚𝑝
𝐹𝐹 =
𝑉0𝐶 × 𝐼𝑆𝑐

𝑃𝑚𝑎𝑥
𝜂=
𝐴𝐶

Compare your calculated value of efficiency () to the standard value, 0 = ? ?.

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Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

−
% Difference = | 0| × 100
0

RESULTS:
At a given distance, x= ...............mm
1. The fill factor of the give solar cell is found to be, FF = ....................
2. The efficiency of the give solar cell is found to be,  = …………….

INFERENCES/CONCLUSIONS:

1…………………………………………….

2…………………………………………….

3…………………………………………….

PRECAUTIONS:
1.The solar cell should be exposed to light before using it in the experiment.
2. Light from the lamp should fall normally on the cell.
3. Distance should be appropriately measured using a scale.
4.The load resistance should be used within a safe current limit.

QUESTIONS ON RELATED CONCEPTS:

1. What is the difference between a solar cell and a photodiode?

2. Why are solar cells known as photovoltaic cells?
3. What are the types of semiconductor materials used for solar cells?
4. What is Dark current?
5. What is the difference between solar photovoltaic and solar hot water systems?
6. What is the response time of a photocell?
7. What is the role of load resistance in solar cell experiment

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Vellore Institute of Technology BPHY101P Engineering Physics Lab Manual

Rubrics for the Continuous Assessment of the Solar Cell Experiment:

Allocated
Parameter LOW MEDIUM HIGH
Marks
Record writing: 02 The record The record has been Completed record
Aim, apparatus, has not been submitted in the lab has been submitted in
theory, formulae, submitted in session but was the lab session
procedure, the lab
incomplete
calculations, results, session
inferences
0 mark 01 mark 02 marks
Experimental 03 No Observation has been Completed the
details, observations/ done but no proper observation and
observations tabulation tabulation has been tabulation, and duly
completed.
and tabulations has been verified by the
performed, faculty member.
or wrong
0 mark 01-02 marks 03 marks

Graph & related 03 No graph has Graph with Completed the graph
calculations been drawn appropriate labeling with necessary
from graph, has been plotted but details and
no calculations from
with calculation from the
graph
appropriate graph has also been
units, & results done. Results are also
mentioned.
0 mark 01-02 marks 03 marks

02 The student Few questions were The student answered

did not answered by the all the questions
Viva-voce answer any student
viva-voce
questions
0 Marks 1 mark 02 marks

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab
Manual

TO STUDY THE CHARACTERISTICS OF PHOTODIODE

OBJECTIVE:
To study the I-V characteristics of photodiode at different illumination in reversed biased conditions

APPARATUS TO BE USED:
Photodiode Experiment set-up integrated with P-N photodiode, Light Source, Ammeter, Voltmeter, Voltage Supply

BASIC THEORY:
The A photodiode is a semiconductor device, typically a P-N junction diode, that converts light into current when
operated at reversed biased conditions. A P-N junction diode primarily operates in the forward direction, and its
reverse current is usually assumed to be zero. However, a photodiode differs from a standard PN junction diode as it is
photo-sensitive in reverse bias conditions. When the depletion region of the reversely biased PN junction is exposed to
light with sufficient energy, the photon passes the energy to the bound electrons on the covalent bond, causing some
electrons to break the covalent bond and become free, thus producing electron-hole pairs, called photogenerated
carriers. The strong barrier field quickly separates the generated electron-hole pairs due to the strong opposite
electrostatic force, causing them to move in opposite directions, i.e. the electron moves toward the cathode, and the
hole moves toward the anode. This current generated due to the motion of minority charge carriers is called
photocurrent, and it is directly proportional to the intensity of the falling light. When there is no incident light, the
reverse current is almost negligible and is called the dark current. The total current of the photodiode is the sum of
the photocurrent and the dark current. When the photodiode is exposed to light, the reverse current increases with the
light intensity. An increase in the amount of light intensity, expressed as irradiance (mW/cm 2), produces an increase in
the reverse current. Though fairly linearly proportional to the light intensity, it works as a photo-detection or
sensitivity.

Figure1: (a) Equivalent

circuit diagram (b)
working principle of
Photodiode experiment

Figure2: Voltage-Current(V-I) characteristics of photodiode in reverse biased conditions.

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Figure 2 shows V-I characteristics of a photodiode which is basically operated in the third quadrant of the Voltage (V)
– Current (I) plane implying operation with reverse voltage and monitoring reverse currents.

Typical application areas of silicon photodiodes include

• Spectroscopy
• Photography
• Optical position sensors
• Laser range finders
• Optical communications
• Medical imaging instruments, etc.

EXPERIMENTAL UNIT:
The experiment unit consists of studying the characteristics of four different sensors: LDR, photodiode,
phototransistor, and a small solar panel. Figure 3 shows the experimental unit. In addition, a 3W white LED is used to
vary the source light intensity to study photosensitivity. All the above are put inside a box on the top of the instrument
to avoid ambient light disturbing the measurements. This avoids the need for a dark room for this experiment. The box
includes Various controls and meters used in the experiment consisting of:
(a) The two 3½ DVM voltage and current measurements
(b) Illumination level control from 0 to 5
(c) Control of voltage applied to the three sensors
(d) Sensor selector
(e) Solar panel loading with variable voltage source
(f) Current range selection
As a result of the above, all circuit configurations are automatically selected for a chosen experiment. No external
connections are required to be made by the user, who can then concentrate on the conduct of the experiment only.

Figure 3: Photodiode experimental set-up unit.

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PROCEDURE:
Procedure for V-I characteristics of photodiode varying light Intensity:

1. Switch ON the unit. Check that the light source is facing the sensor

2. Choose the Photodiode sensor

3. Set the illumination level to 0 and read the current on an appropriate scale for voltage varying from 0 to

maximum. This is the dark current.

4. C Sl Photo Current (A) at different illumination level

No Voltage Illumination Illumination Illumination Illumination Illumination Illumination
h (V) Level-0 Level-1 Level-2 Level-3 Level-4 Level-5
1 0
a
2 -0.25
n 3 -0.5
4 -1
g
5 -2
e 6 -3
7 -4
8 -5
t 9 -6

he illumination level and measure the respective photocurrent

5. Make sure to wait 2-3 minutes after changing the intensity at different level.

6. Choose the appropriate current range from A to mA if the ammeter reach saturation

7. Now repeat the last step for all illumination levels and measured the photocurrent (A) of applied reversed

voltage (V) and complete the table given below

OBSERVATION TABLE:

MODEL GRAPH:
Plot Photodiode current versus voltage applied in the third quadrant for all illumination levels (the current meter shows
positive readings). Note that although the diode current is rather small, it is almost constant as the voltage is varied, at
a constant illumination. Photodiode therefore finds application in the measurement of illumination level i.e., in a
Luxmeter.

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab
Manual
RESULTS:
Photodiode characteristics have been studied in the above experiment and the dark current of the given photodiode is
found to be……….

INFERENCES/CONCLUSIONS:

1…………………………………………….

2…………………………………………….

3…………………………………………….

PRECAUTIONS:
1. The Photodiode should be exposed to no light before using it in the experiment.
2. Light from the lamp should fall normally on the photodiode.
3. The sensor knob should be in photodiode mode with a appropriate current range

QUESTIONS ON RELATED CONCEPTS:

1. What is the difference between a solar cell and a photodiode?

2. Why are Photodiode is known as photoconductive devices?
3. What are the types of semiconductor materials used for photodiodes?
4. What is Dark current?
5. What is the difference between solar photovoltaic and photoconductive?

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 Vellore Institute of Technology BPHY101P Engineering Physics Lab
Manual

Rubrics for the Continuous Assessment of Photodiode Characteristics

Experiment

Allocated
Parameter LOW MEDIUM HIGH
Marks
Record writing: 02 The record The record has been Completed record
Aim, apparatus, has not been submitted in the lab has been submitted in
theory, formulae, submitted in session but was the lab session
procedure, the lab
incomplete
calculations, results, session
inferences
0 mark 01 mark 02 marks
Experimental 03 No Observation has been Completed the
details, observations/ done but no proper observation and
observations tabulation tabulation has been tabulation, and duly
completed.
and tabulations has been verified by the
performed, faculty member.
or wrong
0 mark 01-02 marks 03 marks

Graph & 03 No graph has Graph without proper Completed the graph
estimation of been drawn labeling has been with necessary
dark current & plotted details of dark
results current. Results are
also mentioned.
0 mark 01-02 marks 03 marks

02 The student Few questions were The student answered

did not answered by the all the questions
Viva-voce answer any student
viva-voce
questions
0 Marks 1 mark 02 marks

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