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The aim of the experiment was to examine the characteristics of semiconductor materials, operation and to design simple circuits. Circuits involving semiconductor materials such as LED (Light emitting diode) and a normal semiconductor diode. The IV characteristics of the normal diode and the LED obtained concurred with the available theory. The diode started to conduct when the forward biasing voltage was more than 0.6 volts. However, the reverse current remained zero as the external voltage was increased from 0 to -0.7 volts. According to theory, it was expected that some leakage current could flow but it was not measured because its value was less than microampere. The multimeter used could not measure such small values of current. This explains the shape of the IV characteristic graphs plotted during the experiment. For the LED, conduction was a witness only on forward biasing condition. The knee voltage for the LED was obtained to be 1.8 volts.
Semiconductors are type materials whose electrical properties are depended on temperature of the material. The commonly used semiconductor materials are silicon and germanium. They have found applicability in different area such as manufacture of electronic devices such diodes, solar cells and transistors. Semiconductor materials exhibit some unique characteristics in their operations as investigated and explained in this report.
A p-n junction semiconductor material can operate in two states, either in forward bias or reverse bias. In forward, it allows flow of current when the external voltage exceed the value of the potential barrier created by its depletion layer. In reverse bias, it only allows small reverse leakage current to flow due minority charge carriers. If the reverse voltage exceeds the break down voltage, avalanche breakdown occurs leading to sudden increase the amount of current that flow across the p-n junction (Bird, 2014).
The LED is a special type of a diode that emits light. They are made of different semiconductor compounds which are determined by the wavelength of light required. It consists of a small layer of heavily doped region. Recombination of electrons and holes from the conduction and valence bands respectively occurs releasing energy in form of light (Electronic Tutorials, 2019).
Transistors are three terminal semiconductor devices used in either switching or amplification of signals. they are classified as either BJT or FETs. BJTs can either be n-p-n or p-n-p transistors. BJTs can be used in different configuration which includes common emitter, common base and common collector configuration. A common emitter configuration has the emitter terminal grounded with the output being obtained from the collector terminal (Theraja et al., 2010). To turn ON the transistor, a small current is applied at the base terminal.
- To describe the basis of semiconductor action and its application to simple electronic circuit
- Light emitting diode
- Semiconductor Diode
- Power source
The experimental set up shown in figure 1 was constructed. A 100Ω ohms resistor was connected in series with the LED (light emitting diode) to limit the flow of current. The supply voltage was increased from zero volt to determine the point at which both the diode and the light emitting diode stated conducting. Measurement were taken at various interval and recorded in table 1,2 and 3.
Figure 1: The circuit constructed during the experiment
Lab Report Help : Normal semiconductor Diode
Table 1: Results Obtained for a Zener diode in both forward and Reverse bias condition
|Forward Bias Test||Reverse Bias Test|
|Voltmeter (V)||Ammeter (mA)||Voltmeter (V)||Ammeter (mA)|
LED (Light Emitting Diode)
Table 2: Results Obtained for LED test for both forward and Reverse bias Condition
|Forward Bias Test||Reverse Bias Test|
|Voltmeter (V)||Ammeter (mA)||Voltmeter (V)||Ammeter (mA)|
Table 3: Results for Transistor n-p-n
|Table 3: n-p-n Transistor|
|Voltmeter reading mV-||Ammeter Reading IBE|
Figure 2: IV characteristics of semiconductor diode
This section covers task six and seven given in the laboratory manual. It explains the shapes of the plotted graphs, compares the performance of the semiconductor device used with the available semiconductor theory and describes their application.
Figure 2 is a graph that show the I-V characteristics of a diode in both forward and reverse biased. When forward biased, zero current was measured at the output until the supply voltage was 0.5 volts when a current of 0.002 mA was measured. After exceeding 0.6 volts, the current at the output was observed to rise suddenly and thus making 0.6 volts the Knee voltage or forward threshold voltage. The knee voltage is equivalent to the potential barrier of the diode. It was observed that supply voltage must exceed the value of the potential barrier for diode to start conducting while in forward basing condition.
In reverse bias, a zero current was measured at the output for values of the input voltage. This implies that the diode blocked the flow of current. However, according to semiconductor theory, silicon diodes allows the flow of very small reverse leakage currents which are less than a microampere. The multimeter used could only measure currents up to microampere levels and thus, zero reverse leakage currents recorded. In addition, the reverse voltage was not enough to break down the diode. Therefore, the reverse break down voltage could not be determined. The IV characteristics observed for the forward biasing conditions concurred with semiconductor theory.
A light emitting diode allows current to flow in forward bias while blocking it when reversed biased. LEDs emit light due to the presence of thin layer of heavily doped layer where recombination occurs releasing photons of energy. The forward threshold voltage of LEDs varies depending on their colors. A Red LED was used during the experiment. The forward threshold voltage was 1.8 volts above which the flow of current increased with increase in supply voltage. According to semiconductor theory, red LEDs have forward threshold voltage of 1.8 volts which was similar to the value obtained during the experiment. From the graphs in figure 3, it is evident that the LED did not conduct when reverse biased.
The transistor conducted zero current when the base-emitter voltage was zero. Increase in VBE turned ON of the transistor and at 2 Volts a base current of 0.03 mA was measured. Further increase in the VBE led to rise in base current to 0.58 mA at supply voltage reached 24 volts. The graph in figure 4 shows the variation of VBE with base current.
A normal diode can be used in rectification, that is conversion of alternating current to Direct current. It suits this application since a diode conducts only in one direction and thus clipping the negative cycle of the alternating current. Filters are used together with the diode(s) to smoothen the direct current obtained.
LED emit visible light and thus, they can be used in advertisements, aviation lighting, lighted wall papers and traffic lighting.
NPN transistors have amplification characteristics and thus can be used in electronics where a large output is required such as phones.
The aim of the experiment was to investigate the characteristics of several semiconductor devices such as diodes, LEDs and n-p-n transistors. The diode and LED tested were observed to have a forward threshold voltage of 0.6 volts and 1.8 volts respectively. These values were similar to the values given in semiconductor theory. It was also observed that a n-p-n transistor in common emitter configuration could only conduct when a small current or voltage was supplied to the base. Moreover, the IV characteristics obtained were similar to ones given in semiconductor theory.
Task 1: Description of Terms
- Conduction in intrinsic semiconductors
Intrinsic semiconductors are undoped semiconductor materials such as germanium and silicon. Free electrons in these materials are generated at room temperature through a process called thermal generation of electron hole pairs leading to existence of holes which are positively charged. When voltage is applied across the materials holes and electrons move towards the negative and positive terminals respectively and thus leading to a flow of current.
- Majority and Minority carriers
There are two types of doped semiconductors materials that is n-type and p-type semiconductors. For n-type semiconductor materials, the number of free electrons is greater than the number of holes. Therefore, electrons are considered as majority carriers while holes the minority carriers. In p-type semiconductor materials, the number of holes surpasses the number of free electrons and thus holes form the majority carriers while electrons the minority carriers.
It’s the process by which mobile electrons and holes move to the p-type and n-type materials respectively due to free random motion within the structures of the semiconductor materials.
Task 2: Action of a p-n Junction Diode
- On Open Circuit
The is no current that flows within the circuit since the charge carries lack enough energy to overcome the potential barrier that exits at the junction of the two semiconductor materials.
- Forward bias
Application of a forward bias volts acts against the potential barrier that exist due to depletion layer at the semiconductor junction. When the value of the external voltage exceeds 0.2V and 0.6 V for germanium and silicon respectively, free charge carriers flow across the junction and thus, flow of current. The amount of current in the circuit increases with increase in the amount of voltage applied.
- Reverse Bias
Application of reverse bias voltage increase or widens the depletion layer thus allowing no current to flow within the circuit. However, due electron-hole generation by thermal excitation at room temperature, minority charge carriers are attracted by the applied voltage resulting to flow of small currents. This current is called the reverse leakage current which is in terms of microamperes for germanium and less than microamperes for silicon.
- Sketch of forward and reverse bias Characteristics
- Semiconductor diode in forward and reverse tested
Semiconductor diodes only conducts in one direction. In forward bias condition, it allows current to flow though when the external voltage exceeds forward threshold voltage which can be about 0.6 volts for normal diode. A diode does not conduct in reverse bias condition
However, it conducts small leakage currents until the external voltage exceeds the reverse breakdown voltage. Above the reverse breakdown voltage, the diode undergoes irreversible damage and the current across it suddenly increases. For a normal diode, the reverse breakdown voltage is about 200V.
- Function of 100Ω
The resistor is used to limit current the current that flows around the circuit to prevent burning of the diode.
Results obtained during the experiment were recorded in table 1 and 2 in the results section.
Knee voltage for the light emitting was obtained to be 1.8 V
- Circuit of n-p-n transistor in Common emitter configuration
- The current flowing through the circuit is always directly proportional to the supply voltage but inversely proportional to the resistance according to Ohms law. If the resistor value is fixed and DC source is variable, input voltage can be varied allowing current to flow which is limited by the value of the resistor. For instance, when 2 volts is supplied, a current of 20mA will flow within the circuit.
- Results obtained upon construction of the circuit were recorded in table 3 in the results section.
Bird, J. (2014). Electrical circuit theory and technology. 5th ed. NewYork: Routledge, pp.130-149.
Electronic Tutorials. (2019). The Light emitting diode. [online] Available at: https://www.electronics-tutorials.ws/diode/diode_8.html [Accessed 29 Apr. 2019].
Theraja, B., Theraja, A., Khedkar, M. and Pandey, V. (2010). A textbook of electrical technology. New Delhi: S. Chand & Co., pp.2238-2246.
Lab Report Help More sample questions
Diodes 1 Pre-lab Assignment
- Write the diode equation, ID(VD), identifying the key parameters: saturation current and ideality factor. Extract the values of these parameters from the data obtained in the experiments.
- Draw an equivalent circuit of a real diode that includes the symbol of a diode, series resistance, and shunt resistance.
Using PSPICE or MULTISIM simulate ID(VD) curves for a real diode in the forward and reverse bias. Use the diode equivalent circuit from with an ideal diode, and two resistors, one in 5Ω – 15Ω range, and one in 10 GΩ – 20 GΩ range. Plot ID(VD) forward bias curves on a linear and semi-log graphs in 0 – 1 V voltage range and the reverse bias curve on a linear graph in 1 – 20 V range.
- Diodes in switching circuits are rapidly turned on and off. Draw a schematic of a simple circuit consisting of a resistor and a diode connected in series with one terminal of the diode at the ground. If a square wave ± 2 V is applied between the end of the resistor (the one not connected to the diode) and the ground, the diode will be switched on and off. Draw this waveform and the waveform of the voltage across the diode. Indicate when the dioce is on and off. Do you expect a delay in the diode switching as the waveform frequency increases? Will the delay mainly occur when the diode is switched on or off? Why?
Reference: Jasprit Singh Semiconductor Devices, John Wiley & Sons 2001. pp. 174 – 207.
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Experiments with Semiconductor Diodes
- Measure the forward bias I(V) characteristic of a silicon diode (1N4148) with digital meters.In the forward bias, the diode current increases rapidly with small increase of the voltage across the diode so a safer and more convenient measuring method is using the power supply in the current source mode. Increase the current in small intervals while measuring the voltage with the benchtop DVM; do not relay on the less accurate voltage meter on the power supply. The rated power of N4148 diode is 500 mW, the rated current is 300 mA (400 mA for a short time). While recording the diode voltage VD and the corresponding current ID, plot these values on ID (VD) graph using Excel or other graphing program. This will help you to decide how many data points (and where) are needed to obtain a smooth curve of the diode characteristic.To measure reverse bias current you will need a very sensitive ammeter. Connect it in series with the diode. Connecting to the power supply make sure that the diode is reverse bias! Putting a 10 k to 100k resistor in series may be a good idea. Increase the reverse bias voltage form 0 to 20 V and observe the current. You may not have zero current at zero V as the sensitive meter may be difficult to balance but note the difference between 0 V and 20 V and calculate the resistance based on these two points. Note: If you use a voltmeter to measure the voltage, be careful how you connect it so that you do not disturb the measurement of ID.The goal of these measurements is determining the saturation current, the ideality factor (using a semi-log graph) and possibly estimating the series resistance from the forward bias I-V curve and the shunt resistance from the reverse bias curve (on a linear graph).
Hint:The diode equation on the semi-log graph is a straight line (except near the origin). Its slope and the intercept are defined by the diode parameters. If all data points do not follow a straight line on a semi-log plot select those that do and fit to them an exponential trend line, displaying its equation on the graph. If points corresponding to high current divert from the exponential trandline, they may be plotted on a linear graph to estimate the diode series resistance from a linear trend-line equation. You may not reach high enough forward current to find this resistance precisely but at least you may estimate its lower limit.
- Measurements with Agilent U2722A source measure unit.This instrument is a computer controlled voltage or current power supply which also measures both current and voltage supplied to the circuit. To measure a diode characteristic you can program the instrument to step up the voltage in increments. You can then obtain an I-V curve after exporting the data to a graphing program like Excel. In setting up the instrument, you should select the proper range of voltage and current to obtain precise measurements. Just like using a digital multimeter, you do not select a 10 A range to measure a current of 1.5 mA because you will not get precise results. When your measurements cover a wide range of values you may have to switch ranges for low and high values.
- Measure the forward bias ID (VD) characteristics of the same diode used in (1). Use the maximum current range (120 mA). Compare graphically with the results in (1)
- Using Agilent source measure unit obtain forward bias characteristics of another diode, for example 1N4001.
- Use Agilent U2722A source measure unit for testing a Zener diode (such as 1N4733A). Obtain both forward and reverse bias curve but set up the instrument to obtain enough points in the “interesting” voltage range, which will be different for the two biasing directions. Plot data on a linear I-V graph and fit the linear trend line only to the points on the straight part of the reverse bias curve. Its slope will give you Zener resistance Rz and its intercept with the voltage axis Zener voltage Vzfor this particular diode.
- Diode switching. Supply a square wave signal to a 1N4001 diode through a series resistor (1k to 10k). Use two properly adjusted oscilloscope probes, one across the input terminals (waveform generator), and the other across the diode. The oscilloscope inputs should be set to DC. Save the oscilloscope screen images on a computer, preferably showing the measurements with cursors. Include the images in the lab report.
- Adjust the square wave amplitude (± 2V, measured on the oscilloscope) and the frequency of 1 kHz to 5 kHz. Record signals from the two oscilloscope channels simultaneously (on one screen). Identify parts of the signal that correspond to the diode forward and revers bias and measure their amplitude.
- Without changing the generator signal amplitude, increase the frequency to between 50 kHz and 100 kHz. Measure the time relationship between the input and output signals. Do you see a delay in the diode switching ON or OFF?
Lab Report Help : Report and Analysis
- Present clearly all schematics of the experimental circuits. Show all relevant clearly labeled graphs. Include data tables for part 1 (they can be put in an appendix). The data tables must have titles indicating to which measurements they refer. Do not include large data spreadsheets for part 2. Your instructor may ask you to upload them, in which case make sure that you label them indicating which measurements they represent.
- Extract parameters of the diodes: saturation current, ideality factor, and when possible equivalent series and shunt resistances from measurements 1 and 2. Determine parameters Rz and Vz from measurements 2 b. The values of the parameters must be clearly related to the graphs of the data and trend lines form which they were derived.
- For part 3 include schematic of the diode and the resistor showing which terminals were connected to the oscilloscope, the waveform generator, and ground.
- Comment if the values of the derived parameters are reasonable, in agreement with your understanding of the measured devices.
- Explain the observed amplitude difference between the input and output waveforms in 3a and between the input and output waveforms timing for the two frequencies in 3b.
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