Pn Junction 2

CONTENT PAGE1. Introduction ______________________________________________________________ 2

1.1 Background

1.2 Objective

1.3 Scope

2. Theory __________________________________________________________________ 3

2.1 Theoretical Formulaes

3. Light-Current Characteristics and Emission Spectrum of an LED ________________ 5

3.1 Experimental Apparatus and Procedure for L-I experiment

3.2 Data and Comments

3.3 Experimental Apparatus and Procedure for Emission Spectrum experiment


4. Light-Voltage Characteristics and Emission Spectrum of an LD _________________ 11

4.1 Experimental Apparatus and Procedure for L-V experiment




5. Current-Voltage Characteristics of a P-N Junction Solar Cell ___________________ 25

5.1 Experimental Apparatus and Procedure


6. Conclusion ______________________________________________________________ 25

7. References _____________________________________________________________ 12

1. Introduction

1.1 Background

In the continual evolution of electronic circuits and devices, optoelectronics is a branch of electronics engineering whose devices are widely used in many complex systems, in fact, they form an integral part of a system and lead to many unique and highly specialised products. As the name optoelectronics implies, such circuits utilize an optimum combination of electronics and optics for signal processing, storage and transmission. Some examples of optoelectronic devices are the light emitting diode, the photodiode, the laser diode and the solar cell.

In this experiment, Characteristics of PN Junction-Based Optoelectronic Devices, tests were done on three different optoelectronic devices, the light emitting diode (from this point on referred to as the LED), the laser diode (from this point on referred to as the LD) and the solar cell, to study their characteristics with respect to various parameters like light, current and voltage.

1.2 Objectives

The objectives of this experiment were two-fold:

(i) To study the characteristics of two common optoelectronic components: the LED and

the LD.

(ii) To study the I-V characteristics of a P-N junction solar cell

1.3 Scope

This experiment included a study of the Light-Current characteristics and emission spectrum of an LED and the Light-Voltage characteristics and emission spectrum of an LD and also the comparison of the I-V characteristics of a P-N junction solar cell under different illumination conditions. Detailed theory can be found in Section 2. Data, calculations and discussions are grouped together under Sections 3, 4 and 5 for the LED, LD and solar cell respectively.

2. Theory

2.1 Theoretical Formulaes

The basic introduction of the Light Emitting Diode, the Laser Diode and the P-N Junction Solar Cell can be found in the L217 Lab Manual.

The current-voltage characteristic of a p-n junction diode is given by the equation

I = Io[exp()-1] __eqn (1)

Where Io is the reverse saturation current, k the Boltzmanns constant and T the absolute temperature. The equivalent circuit of a solar cell is shown in Figure 1, where Rsh and Rs are the shunt and series resistances of the solar cell while IG is a current source representing the optically generated current given by

IG = qG(Ln + Lp)A __eqn (2)

Here, Ln and Lp are the minority carrier diffusion lengths for electrons and holes respectively, and are effectively the distances that a minority carrier can diffuse before recombining with a majority carrier. G is the generation rate of electron-hole pairs and A is the cross sectional area of the cell. If the effect of Rsh and Rs is assumed to be negligible then the load current is

IL = IG I __eqn (3)

Figure 1If the solar cell is short circuited, V=0 and IL=ISC=IG (As indicated in Figures 1 and 2).

The maximum voltage that can be generated across the junction occurs when the cell is open circuit. It is readily shown from equation (1) that I=IGVOC = ln(1+) __eqn (4)

Figure 2 shows the I-V characteristic of a p-n junction with and without illumination. It is a usual practise to ignore all but the fourth quadrant and to redraw this quadrant into the first as shown in Figure 3. The product VOCISC represents the maximum electrical power that can be delivered by an ideal solar cell. Finite values of RL, Rsh and RS reduce the maximum power that can be generated by a real cell to VmpImp, the quantities Imp and Vmp being the length of the sides of the rectangle of largest area that can be drawn within the I-V characteristic, the so-called maximum power rectangle(MPR). A measure of the squareness of the characteristic is given by the fill factor (FF):

FF= __eqn (5)

Figure 2

Figure 33. Light-Current Characteristics and Emission Spectrum of an LED3.1 Experimental Apparatus and Procedure for L-I experiment

The experimental apparatus given to us are:

1 Keithley Source Management Unit(SMU)

1 RS577-780 ultrabright red light emitting diode

1 RS 111-346 1.5mW cw laser diode module

1 black box

1 luxmeter

1 Oriel spectrometer

1 DC power supply

2 Digital Multimeters

1 P-N junction solar cell

1 halogen lamp

1 solar simulator

Figure 4

Figure 4 shows the experimental setup for the Light-Current characteristic experiment. An LED mounted onto a mounting was secured to the left side of the black box and it was connected to the SMU. At the right hand side of the black box, a luxmeter was secured and the lid of the black box was closed. There were no objects inserted into the slots in the black box. The SMU was switched on, set as a constant current source and the readings were taken at intervals of 5mA from 0mA to 30mA and the corresponding luxmeter readings were also tabulated. These results are shown in Section 3.2.


The Luxmeter Offset when there was no light = 6 lux.

I/mARecorded Luxmeter Reading (Lux)Actual Luxmeter Reading (Lux)

060

160

5104

102014

153226

204438

255549

306660

Table of current and luxmeter readings for the LED i) Plot the L-I characteristic curve for the LED on a graph paper.

The L-I characteristic curve for the LED was plotted based on the readings above and is included in this section.

ii) Comment on the results.Judging from the specifications in Appendix 2 in the lab manual, the maximum current that the LED can handle is 30mA. The L-I graph shows a linear relationship between light intensity and current input. This means that the LED will grow steadily brighter in predictable amounts as the input current is increased.


Figure 5

This experimental setup is similar to the L-I characteristic experiment (see Figure 5) except that the grating spectrometer has taken the place of the black box. On the left side of the grating spectrometer the LED mounting (still connected to the SMU) was attached and on the right side, a 10 VDC supply together with the signal output to the DMM were attached. The output current of the SMU was set at I=30mA (the maximum light intensity of the LED at the maximum rated current) and the frequency was set initially at 600 nm. The subsequent voltage output of the photodetector module was recorded on the DMM. A series of readings were taken from 600nm to 680nm by turning the knob on the side of the grating spectrometer. These results are shown in Section 3.4.


Luxmeter Offset in total darkness = 6 lux.

SMU Current fixed at I=30mA

V/mVPower/mWWavelength/nm

86.752.603600

86.762.603610

86.762.603620

86.782.603632

98.502.955635

163.204.896638

236.607.098640

332.109.963642

502.0015.060645

707.4021.220648

847.5025.425650

959.0028.770652

1027.7030.831655

940.1028.203658

791.0023.730660

639.8019.194662

404.6012.138665

227.506.825668

136.504.095670

86.762.603680

Table of Voltage, Power and Wavelength for the LEDi) Plot the emission spectrum of the LED.The power was calculated using the formula P=VI and subsequently a graph of the emission spectrum (Power/mW against Wavelength/nm) of the LED was plotted. The graph is included in this section.

ii) Comment on the results.The output power is maximum at the wavelength of 655nm. This means that the LED gives off the most red light of wavelength 655nm. The LED itself gives off red light of different wavelengths from 640nm to 670nm but with different intensities as shown by the graph. This can be explained by the properties of the LED itself. The LED gives off light by spontaneous emission and as the emitted photons have random phases, the LED is an incoherent light source and hence its output light spectra consists of red light of varying wavelengths. The light emitted has a narrow band of wavelengths, about 30nm at room temperature and the spectral position of this band is directly related to the bandgap energy of the semiconductor.

iii) Calculate the bandgap energies of for the LED emission.The peak wavelength, , of the LED is at 655nm.

Eg=

Eg=

Eg= 3.0328 x 10-19 J

Eg= 1.893 eV

Hence the bandgap energy for the LED is 1.893eV.

iv) Describe the operation of a 3-colour LED.

Figure 6

Figure 6 shows the schematic diagram of a 3-colour LED. The LED emits green light the diode on the left is forward biased (Pin 3 should be at least 0.7V higher than Pin 2 and Pin 1 should be at the same potential or less than Pin 2) and the diode on the right is not biased or is reverse-biased. The converse is also true. The LED emits red light when the diode on the left is not biased or is reverse-biased and the diode on the right is forward biased (Pin 1 should be at least 0.7V higher than Pin 2 and Pin 3 should be at the same potential or less than Pin 2). For the LED to show a third colour, both diodes must be on, in other words both diodes must be forward biased (Pin3 and Pin 1 must be at least 0.7V higher than Pin 2) and the spectra of light produced from the 2 diodes will mix and produce a new colour.

v) Describe the operating principles of the grating spectrometer.

A spectrometer is an apparatus designed to measure the distribution of radiation of a source in a particular wavelength region. Its principle components are a monochromator (diffraction grating), an instrument designed for isolating a narrow portion of the spectrum, and a radiant power detector such as a photoemissive cell or a photomultiplier tube. Radiant power enters the entrance slit of the monochromator. The monochromator selects a narrow spectral band of radiant power and transmits it through the exit slit to the photosensitive surface of the detector.

Hence a grating spectrometer consists of the following elements:

1. An entrance slit or aperture stop.

2. A collimating element to make all the rays parallel which pass through one point of the entrance slit or field-stop. This collimator may be a lens, a mirror or an integral part of the dispersing element, as in a concave grating spectrometer.

3. A dispersing element, usually a grating, which spreads the light intensity in space as a function of wavelength.

4. A focusing element to form an image of the entrance slit or field-stop at some convenient focal place.

5. An exit at the focal plane which transmits the light from the image that the focusing system has formed.

4. Light-Voltage Characteristics and Emission Spectrum of an LD4.1 Experimental Apparatus and Procedure for L-V experimentThe experiment is set up as shown in Figure 1, except that an LD should be used in the mounting on the left side of the black box. The SMU was set a constant voltage source and the voltage was varied and the subsequent luxmeter readings were tabulated. The results are shown in Section 4.2.

4.2 Data and CommentsLuxmeter offset in total darkness = 6 lux.

I/mAV/VRecorded Luxmeter Reading (Lux)Actual Luxmeter Reading (Lux)

12.4360

52.6771

102.8382

152.95104

203.06137

253.161812

303.253327

313.274236

323.299084

333.31235229

343.32405399

353.34580574

365.51817811

375.51817811

Table of current, voltage and luxmeter readings for the LDi) Plot the L-V characteristic curve.The L-V graph was plotted and included in this section. A L-I graph was also plotted and included.

ii) Comment on the results.As the L-V graph shows, increasing from 2.43V to 3.29V the luxmeter readings stays relatively low and constant. This indicates that the LD is not switched on yet. But from 3.30V to 3.36V, the luxmeter readings suddenly jumps to very high levels. In the space of 0.06V the LD is switched on and gives off a very high intensity of light. At 3.6V, the maximum reading of the luxmeter was reached, at 811 lux. Any increase of the voltage does not increase the output power of the LD. The L-I graph also shows that at about 32.5mA the light intensity suddenly increased sharply and leveled off at 37mA. Again, in a short span of 4.5mA the LD switched on and produced a high intensity of light. This can be explained by understanding the way that a laser works.

Before a laser can produce any useful output 2 conditions must be fulfilled, the first one being optical gain within the medium and the second being population inversion. Population inversion is the promotion of electrons from the valence band to the conduction band (by means of energy bombardment to promote optical absorption) so that more reside in the conduction band than the valence band in the semi-conductor. So part of the light emitted within the active region is reflected back and forth through the active medium and stimulates further electron-hole recombination and this results in stimulated emission (the emission of more photons in the same direction and with the same phase and frequency as those already present in the optical medium) and thus a coherent beam is produced.

As the device current is increased the intensity of the beam builds up (optical gain) and at the onset of lasing the net gain inside the resonator exactly balances the light output through the partially reflective front facet of the laser. Hence at a particularly narrow range of currents the laser is said to be lasing where it produces a maximum power output when population inversion and optical gain reaches the maximum values. In the LD that was tested the input current at which maximum power output was produced was 37mA.

4.3 Experimental Apparatus and Procedure for Emission Spectrum Experiment

The experiment was set up according to Figure 2 except the optoelectronic device coupled to the grating spectrometer should be the LD. The SMU was set at a constant current of 36mA (maximum output power at input current of 36mA) and the wavelength of the grating spectrometer was varied from 600nm to 650nm and the subsequent voltage readings recorded on the DMM were tabulated. This table is shown in Section 4.4.

4.4 Data and CommentsSMU current fixed at =36mA.

V/mVPower/mWWavelength/nm

86.143.10600

86.143.10610

86.143.10620

86.143.10622

86.143.10625

86.143.10628

240.208.65629

484.0017.42630

983.2035.40632

1400.0050.40635

605.2021.79638

276.009.94639

86.143.10640

86.143.10650

Table of Voltage, Power and Wavelength for the LDThe power was calculated by using the formula P=VI.

i) Superimpose the emission spectrum of the LD on the emission spectrum of the LED plotted.

The emission spectrum of the LD was plotted and superimposed on the emission spectrum of the LED. The graph is shown in Section 3.4.

ii) The linewidth of an emission spectrum is defined by its full width at half-maximum(FWHM). Determine the linewidths of the red LED and the LD emissions.

For the LED:

Half-maximum value of the Emission Spectrum (on the graph) is

= {[(30.831-2.603)/2] + 2.603} mW

= 16.717mW

Linewidth of the red LED emission = 665nm-646nm

= 19nm

For the LD:

Half-maximum value of the Emission Spectrum (on the graph) is

= {[(50.40-3.10)/2]+3.10} mW

= 26.75mW

Line width of the red LD emission = 637.5nm-631.5nm

= 6nm

iii) Comment on the results obtained.The line width of the LD emission at 6nm is much smaller than the line width of the LED emission at 19nm. The LED is an incoherent light source, working on the principle of spontaneous emission whereas the LD is a coherent light source which works on the principle of stimulated emission. The LD coherent light output consists only of a narrow bandwidth of red light, due to stimulated emission where light waves are produced in the same direction, with the same frequency and phase. The LED incoherent light output consists of a wide bandwidth of red light, due to spontaneous emission where electron hole pairs combine at any values greater than the bandgap energy thus producing a wide range of red light frequencies. Hence the linewidth of the LED emission spectrum is greater than that of the LD emission spectrum.

5. Current-Voltage Characteristics of a P-N Junction Solar Cell5.1 Experimental Apparatus and Procedure

Figure 7The solar cell was inserted into the black box into Slot One as shown in Figure 7. The solar cells position can be varied between Slot One and Two as desired. The Halogen lamp was calibrated by placing it together with the face of the luxmeter so that any light incident on the luxmeter will be those from the halogen lamp. The voltage into the halogen lamp was increased until the desired brightness was obtained on the luxmeter. The halogen lamp was then affixed to the right side of the black box and the SMU was switched on and set to constant voltage mode. The voltage of the SMU was varied from 0V to 13V and the subsequent current was also recorded. This table is shown in Section 5.2.


V/VCurrent Reading (mA) DarknessCurrent Reading (mA) 20000 luxCurrent Reading (mA) 30000 luxCurrent Reading (mA) 40000 lux (Slot 1)Current Reading (mA) 40000 lux (Slot 2)Current Reading (mA) 50000 lux

00-0.32-0.41-0.49-0.65-0.58

10-0.26-0.35-0.41-0.58-0.49

20-0.20-0.26-0.30-0.48-0.37

30.01-0.13-0.18-0.21-0.38-0.26

40.02-0.07-0.10-0.12-0.30-0.16

50.03-0.02-0.05-0.06-0.21-0.09

60.040.02-0.01-0.03-0.12-0.04

70.080.080.050.02-0.040.00

80.130.220.130.110.110.08

90.320.440.390.370.380.33

100.961.401.341.331.401.29

112.454.374.424.484.614.56

124.759.729.8410.0110.1410.24

137.7916.7716.9717.1917.3717.52

Table of Current and Voltage for the Solar CellThe various graphs of I-V were plotted and included in this section. The expanded fourth quadrant of the main graphs were also included on the graphs and the subsequent VOC, ISC, maximum power generated and fill factor(FF) were calculated and are shown below.

In total darkness:There is no power generated by the solar cell in darkness.

20000 lux:ISC=0.32mA

VOC=5.3V

Imp=0.16mA

Vmp=2.65V

Maximum power generated = Imp x Vmp

= 0.16mA x 2.65V

= 0.424mW

FF= 0.25

30000 lux:ISC=0.4mA

VOC=6.5V

Imp=0.21mA

Vmp=2.6V


= 0.21mA x 2.6V

= 0.546mW

FF= 0.21

40000 lux(Slot 1):

ISC=0.49mA

VOC=7V

Imp=0.25mA

Vmp=2.5V


= 0.25mA x 2.5V

= 0.625mW

FF= 0.182

40000 lux (Slot 2):ISC=0.65mA

VOC=7.5V

Imp=0.33mA

Vmp=3.7V


= 0.33mA x 3.7V

= 1.221mW

FF= 0.25

50000 lux:ISC=0.58mA

VOC=7V

Imp=0.29mA

Vmp=2.7V


= 0.29mA x 2.7V

= 0.783mW

FF= 0.20

i) Under AM1 (1 sun) condition, the solar cell will give VOC=12V, and ISC=54mA. Explain why the maximum power generated and, hence, FF are relatively low in this experiment.

If a large energy gap material is used for the solar cell, only a fraction of the solar power will be absorbed and while allowing a large value of VOC to be generated, ISC would be small. To get maximum power the product VOCISC should be a maximum so both VOC and ISC cannot be small. In this case VOC is large and ISC is small and the Maximum Power Rectangle will be taking up only a small portion of the area enclosed by VOC and ISC. Thus the FF will be relatively low as FF is given by the formula FF=. The product VmpImp will be small compared to VOCISC.

ii) Suggest how the p-n junction solar cell can be used as a photodetector. Sketch the appropriate circuit set-up for this experiment.When the p-n junction is forward biased, the photodiode acts as a solar cell. But if the p-n junction is reverse biased, the photodiode acts as a photodetector. When the reverse biased p-n junction is illuminated, the photons impacting the junction cause covalent bonds to break, thus electron-hole pairs are generated in the depletion layer. The electric field in the depletion region then sweeps the liberated electrons to the n side and the holes to the p side, give rise to a reverse current across the junction. This current, known as photocurrent, is proportional to the intensity of the incident light. Hence the photodetector can convert incident light into electrical signals.

Figure 8As shown in Figure 8, the photodetector should be connected in such a way that the SMU will reverse-biase the photodiode and the DMM will measure the current that flows in the circuit when there is light incident on the photodetector.

iii) Set up a circuit to assess the performance of the solar cell as a power source in driving different loads. Determine its output power versus junction voltage under the same lamp-on illumination condition.

R/OhmsV/mVI/mAPower/W

45.0627.600.5316.9 u

25061.800.5215.28 u

12k2.81 V0.230.658 m

50k4.812 V0.090.463 m

100k5.628 V0.050.317 m

150k5.948 V0.040.236 m

200k6.172 V0.030.190 m

250k6.27 V0.020.157 m

Table of Load Resistance, Voltage, Current and Output Power for the solar cellThe solar cell was illuminated using a lamp source of 50000 lux. The output power was calculated using the formula P=V2/R.

iv) Comment on the viability of using solar power as an energy source.

Solar cells can only (like the one used in the experiment) of small area can only produce a small amount of power (0.783mW) under illumination of 50000 lux. And from the table above the larger the load used, the smaller the output power is going to be. From the FF calculations above, the solar cells conversion efficiency (from solar energy to electrical power) is about 20% to 25%. The solar cells power efficiency is low and it cannot drive large loads. To make it commercially viable to use solar power as an energy source, solar panels with a large area should be used so that more power can be produced and large loads can be supported.

6. Conclusion

From the experiments conducted in this report, it can be concluded that the Light Emitting Diode, the Laser Diode and the Solar Cell have specific characteristics that allow them to be used in complex systems that perform specific functions. The LED is used in place of light bulbs in traffic lights, for instance. The laser diode is an ubiquitous device in disc players, be it CD-ROM drives, DVD players or just normal CD players. Solar cells are used in creative building design where solar panels are incorporated into the buildings design so that solar power can complement the electrical power from the grid which is used for powering the daily energy needs of the building. Optoelectronic devices have many advantages over conventional devices that are used in many electrical systems today and they can, in fact, better our lifes.

7. References(1) Horowitz, P. & Hill, W. (1980) The Art of Electronics. 1st Edition. Cambridge:

Cambridge University Press.

(2) Sedra, A. & Smith, K. (1998) Micrelectronic Circuits. 4th Edition. New York: Oxford

University Press.

Luxmeter

SMU

(I-V mode)

Black Box

Slot 1

Slot 3

Slot 2

LED/LD

3

Green

2

1

Red

Halogen Lamp

SMU

(I-V mode)

Slot 1

Slot 3

Slot 2

Black Box

Solar Cell

DMM

Light Source

Black Box

Photodetector

To SMU

_1011477852.unknown

_1011477879.unknown

_1011478312.unknown

_1011275627.unknown

_1011477372.unknown

_1011469910.unknown

_1011275613.unknown

_1011274789.unknown

Pn Junction 2

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