diode (lab report)

7/29/2019 Diode (Lab Report)

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Title: Diodes

Introduction:

A diode is a dispositive made of a semiconductor material, which has two

terminals or electrodes (diode) that act like an on-off switch. It acts as a short circuitand allows the current flow when the diode is on or forward-biased. However,

diode behaves like an open circuit and blocks the flow of current if the diode is off

or reversed-biased. Hence, in order to make the diode on, the potential or voltage

applied must match the polarity of the diode (forward-biased). To turn off the diode,

invert the polarity of the diode (reversed biased). Of course this is the theoretical

behaviour of an ideal diode, but it can be seen as a good approximation for a real

diode.

Diode is used in various electronic applications. For example, due to the

characteristic of diode, it can be used in AC-DC rectification and voltage multiplier.

Besides, it is found very useful to make FM and AM detector.

Light-Emitting Diode or LED which is one of the types of diode, is commonly

used in electronic devices due to its properties. LED is a semiconductor light source.

When current flow through the LED, it will produce certain wavelength of photon and

this light is used in giving indication and not illumination.

LEDs present many advantages over incandescent light sources including

lower energy consumption, longer lifetime, improved robustness, smaller size, faster

switching, and greater durability and reliability. However, they are relativelyexpensive and require more precise current and heat management than traditional

light sources.

They also enjoy use in applications as diverse as replacements for traditional

light sources in automotive lighting (particularly indicators) and in traffic signals. The

compact size of LEDs has allowed new text and video displays and sensors to be

developed, while their high switching rates are useful in advanced communications

technology.

Objectives:

The objectives of this experiment are:

i) To show the difference between Pn Diodes, Zener Diodes and Light Emitting

Diodes.

ii) To fathom the behavious of pn diodes under forwards biased and reversed

biased conditions.

iii) To demonstrate the characteristics of Zener Diode at reversed biased condition.

iv) To show the relationship between forward current and LED intensity.


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Theory:

The Operation of PN diode

1. When applying positive potential to the p-type material and negative potential to

the n-type material of the PN junction diode, this will provide a forward-biasedcondition for the diode.

2. This forward bias potential will pressure the electrons in n-type material and

holes in p-type material to recombine with the ions near the boundary and reduce

the width of the depletion region.

3. The resulting minority-carrier flow of electron from p-type material to the n-type

material (and holes from n-type material to p-type material) has not changed in

magnitude, but reduction in depletion region width has result in heavy majority

flow across the junction.

4. As the applied bias increase in magnitude, the depletion region will continue to

decrease in width until a flood of electron can pass through the junction, resulting

in exponential rise in current.

5. Hence, the diode allows the current flows and the diode is on.

6. As for reverse-bias condition, it will increase the width of the depletion region

and form a very strong barrier and hence block the flow of majority carrier. No

current can flow through the diode and the diode is off.

The Operation of the Light-Emitting Diode (LED)

1. When applying positive potential to the p-type material and negative potential to

the n-type material of the PN junction diode, this will provide a forward-biased

condition for the diode.

2. This forward bias potential will pressure the electrons in n-type material and holes

in p-type material to recombine with the ions near the boundary.

3. Electrons are able to recombine with electron holes within the device.

4. Electroluminescence occurs and the LED will release energy in term of photon.

5. The colour of the light emitted is determined by the energy gap of the

semiconductor.


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Procedures:

(A) The Junction Diode

Figure 1

1. The circuit in the Figure 1 is set up as shown above to measure the forward

characteristics of silicon diode.

2. The current is increased at intervals from 0 to 10mA by varying the power supply

and the potential across the diode, VF is measured.

3. The graph of VF versus IF is then plotted on a graph.

4. The diode is now connected in the reversed biased mode as shown in Figure 2

below.

Figure 2

5. The DMM is used to measure the potential across the diode as you vary the power

supply.

6. The reverse current, IR is recorded through the diode at respective 4 values of VR,

that are 1V, 3V, 8V and 15V.

(B) Zener Diodes

Figure 3


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1. The breakdown characteristics (VR vs IR) for TWO different diodes using the

circuit shown in Figure 3.

2. The identification information is noted on the diodes.

(C) Light Emitting Diodes

Figure 4

1. The circuit is wired as shown in the schematic diagram in Figure 4. Loads are

connected to 200 ohms control. The LED short lead is connected to ground during the

installation process. Rs is adjusted to midrange.

2. The test requirements are set so that the millimeter in 100mV DC range and

voltmeter in 3V DC range.

3. Power is applied to Trainer and Rs is adjusted for an indication 40mA on

Inillian1meter. The brightness of LED is noted.

4. The forward voltage of the LED is recorded with 40mA of forward current.

5. The data points obtained in Steps 3 and 4 are plotted.

6. To prove the results of your calculations, use ohmmeter to set the Rs calculated by

applying the formulae:

VRS = VS - VF

RS =

7. To prove the results of calculations in Step 6, the 200 control is set to the RS

value calculated to produce 40 mA of forward current using the ohmmeter. The

control is inserted into the circuit; power, applied to the trainer; and milliammeter

indication, observed. If the curve and calculations are correct, the milliammeter

should indicate very close to 40 mA.

Power is removed from the trainer.


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This procedure is repeated for 30 mA, 20 mA and 10 mA (depending upon the LED

used, it may be necessary to use the 1 k control to obtain 10 mA IF).


8. The test circuit is modified as shown below. The 1 k control is used for RS and a68 resistor is added in series with the LED.

Power is applied to the trainer and RS is adjusted for minimum resistance (maximum

IF). The LED should emit a bright red light. Power is removed from the trainer and the

colour of the LED lens is noted.

The red LED is replaced with the green LED and power is applied. The relationship

between the LED's lens colour and the colour of its emission is again noted.

This procedure is repeated using the amber LED.


Results:

(A)The Junction Diodei) Forward-bias Characteristic

IF (mA) VF(V)1.0 0.554

2.0 0.594

3.0 0.614

4.0 0.628

5.0 0.640

6.0 0.649

7.0 0.656

8.0 0.662

9.0 0.667

10.0 0.670

A

V

VOLTMETER

OR

SCOPE

MILLIAMMETER

1

2

3

+ 5 V

Rs

1k

CONTROL

ADD THIS RESISTOR TO

EXISTING CIRCUIT

LED

68


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The graph of VF versus IF is then plotted on a graph on page 13.

ii) Reverse-Biased Characteristic

VR (V) IR (mA)

1.000 0.03.000 0.0

8.000 0.0

15.000 0.0

(B)Zener DiodeBreakdown characteristics for 2 different breakdown diodes:

I: 3.2V breakdown voltage

II: 3.9V breakdown voltage

(mA)(V)

I II

0.05 1.813 2.338

0.10 1.982 2.794

0.20 2.492 3.158

0.50 3.042 3.878

5.00 3.195 4.043

(C)Light Emitting DiodesSTEPS 1-5

IF VF/V

RED GREEN YELLOW AMBER

40mA 2.021 2.587 2.288 1.989

35mA 1.986 2.528 2.217 1.976

30mA 1.963 2.452 2.186 1.964

25mA 1.889 2.345 2.129 1.942

20mA 1.870 2.263 2.086 1.931

15mA 1.834 2.165 2.032 1.915

10mA 1.792 2.051 1.975 1.895Table showing the forward current versus forward voltage of different types of LEDs.

For Red LED (graph on page 14),

From the graph, gradient =

=27.5m

0.210

= 0.131


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Forward resistance =

= 7.636

For Green LED (graph on page 15),


=24.5m

0.390

= 0.0628


= 15.918

For Yellow LED (graph on page 16),


=22.5m

0.235

= 0.0957


= 10.444

For Amber LED (graph on page 17),


=26.5m

0.0775

= 0.342


= 2.925


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STEPS 6-7

Calculation of the values of RS needed to obtain the following values of IF using the

equation:

VRS = VS - VF

but VS = 5 V and IRS = IF

RS =

IF RS/

Red Green Yellow Amber

40 mA 74.475 60.325 67.800 75.275

30 mA 101.23 84.933 93.800 101.20

20 mA 156.50 136.85 145.70 153.4510 mA 320.80 294.90 302.50 310.50

By setting the control to the value RS calculated in step 6, the forward current reading

is obtained.

For Red LED,

RS / IF / mA

74.475 40.6

101.23 32.5156.50 21.7

320.80 10.8

For Green LED,

RS / IF / mA

60.325 40.1

84.933 30.9

136.85 21.2

294.90 10.4

For Yellow LED,

RS / IF / mA

67.800 40.7

93.800 31.2

145.70 20.9

302.50 10.6

For Amber LED,


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RS / IF / mA

75.275 40.8

101.20 31.5

153.45 20.7

310.50 10.3

STEP 8

Type of LED Colour of emissions at minimum

RS resistance (IF maximum) and

power on

Emission colour when power off

Red LED Bright red light The colour became dimmer slowly

and then went off.

Green LED Greenish yellow light The colour became dimmer slowly

and then went off.

Yellow LED Bright yellow light The colour became dimmer slowly

and then went off.

Amber LED Yellow brownish light The colour became dimmer slowly

and then went off.


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DISCUSSION:

(A)The Junction Diode

Being semiconductor device, the junction diode is formed by fusing a p-typematerial and an n-type material together. It allows the current to flow through it in one

direction and hence, it acts like a switch. When the positive voltage is applied to the

p-region relative to the n-region, the junction diode is said to be in forward biased

condition. In forward biased conditions, the depletion region has already been reduced

and hence, majority electrons from n-junction can diffuse to the p-junction and

majority holes from p-junction can diffuse to the n-junction. The injection of minority

carries in both p and n junctions cause the current to produce.

As seen in the graph on page 13 , the diode conducts current when it is in forward

biased condition. The higher the voltage applied, the higher the diode current. In fact,

the current (I) is an exponential function of the applied voltage (V), I = IS(eV/Vt-1),

where Vt is given as kT/e. From the VF versus IF graph, we can also see that the slope

decreases when the current increases. This means that as the at higher applied voltage,

a slight increase of voltage will cause a significant increase in diode current.

Ideally, when the diode is in forward bias, the diode can be treated as short

circuit and hence zero voltage drop across it. Nevertheless, we found out that the

voltage drop is approximately 0.6V across the diode when it is in forward biased (you

can see from the graph too). It is due to the junction voltage across the diode that

needs to be achieved in order to obtain a forward biased current.

From the table when reversed biased voltage is applied, there is essentially no

diode current in the diode. You can verify this from the table of reversed biased

shown in the result section. This is due to the widen of space charge region that

prevents the diffusion of carries from n-junction to the p-junction and the diffusion of

carries from p-junction to the n-junction Besides, the graph obtained is non-linear

because the diode is dependent on the temperature change.

(B)Zener DiodeZener diode is a diode which allows current to flow in the forward direction in

the same manner as an ideal diode during forward biased condition. However, it also

permits the current to flow in the reverse direction when the voltage is above a certain

value known as the breakdown voltage, zener knee voltage or zener voltage.


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In this experiment under reversed biased condition, when the current supplied

increases from 0.50mA to 5mA, the voltage across the 3.2V breakdown-voltage zener

diode is maintained in the range of 3.042V to 3.195V. For 3.9V breakdown-voltage

zener diode, the voltage across it is maintained in the range of 3.878V to 4.034V

when the current supplied increases from 0.50mA to 5mA. As the zener diode canmaintain the voltage across it, it can be used as a voltage regulator.

(C)Light Emitting DiodesThe forward-current-versus-forward-voltage characteristic of LEDs (Step 1 Step 5)

The curves formed provide us with workable Forward Current Versus-

Forward Voltage characteristic curve for this LED. This curve is usually available in

manufacturings data sheet. After the experiment was repeated for red, light green,yellow and amber LEDs, we obtained three similar curves, showing the linear

relationship between the forward current, IF and forward voltage, VF.

These outcomes obey the general behavior of a diode which shows linear

forward property after the threshold voltage was exceeded. The threshold voltage of a

typical LED is around 2V.

The selection of current limiting resistance in an LED circuit (Step 6 Step 7)

The result of IF is similar to the value we get at step 6. By using the forward

characteristic curve obtained, it allows us to design an LED circuit, limiting the LED

forward current to any predetermined value. This is done by choosing the appropriate

control resistance RS.

The independence of the colour of light emitted by an LED and its lens colour (Step 8)

Based upon the observation in Step 8, we might conclude that the light emitted by

an LED is strictly a function of its lens color. However, as we will recall from the

text, this is not true. The LED produces only a narrow band of wavelengths.

Therefore, the lens color can modify its appearance only slightly. The primary

purpose of using pigmentation in lens construction is to improve its visual contrast

between the on and off condition.


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Conclusion:

1) Diode is a device that only allow the current to flow in one direction.

2) As obtained in this experiment, the threshold voltage of p-n silicon diode is

approximately 0.6V

3) Zener diodes allow the current to flow in reverse direction with reverse bias

configuration provided the breakdown voltage is reached.

4) The forward voltage current characteristic show straight line relationship if the

forward voltage is greater than the turn-on threshold voltage of an LED.

5) The color of the emission of an LED is independent of its lens color and is

solely a function of its chip material.

6) Intensity of the LED emission is directly proportional to its forward current forcurrent levels far below its maximum swing.

References:

1. Neamen, Donald A. (2001). Electronic circuit analysis and design. (4th ed.).

McGraw-Hill.

2. Boylestad R. (2012). Electronic Device and Circuit Theory. (11th ed.). Prentice

Hall International.

diode (lab report)

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