To Draw The I-V Characteristic Curve For P-N Junction In Forward And Reverse Bias

P-N diode is an electrical device made of many semiconductor materials like Silicon, germanium, gallium arsenide. In the structure of a PN diode, we can observe an interface or a border between the two semiconductor material types, which are the p-type and the n-type materials, called the PN junction diode. The semiconductor’s P side (positive side) has an excessive hole which acts as a relative positive charge and the N side (negative side) has an excess electron.

The level of doping of the semiconductor device judges the behaviour of the semiconductor as a p side or the n side of the diode. The different levels of doping will create a layer in between the diodes which is known as the PN junction. Biasing is the main scheme that determines the extensive properties of the diode to explore. This practical is included in the Class 12 Practicals where we will draw the I-V characteristic curve for P-N Junction in forward and reverse bias and also derive the current and voltage characteristics for the p-n junction in forward and reverse bias.

Key Terms: Semiconductor Devices, Pn Junction Diode, Depletion Layer, Diode, Charge Carriers, Forward Biasing, Reverse Biasing, Resistance of the Diode

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Definitions of Basic Terms Used in the Experiment 

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• Semiconductor Devices: It is a device that has intermediate conductivity with that of conducting material and insulating material. It is largely employed in the electric circuits in different forms such as Diode, Schottky diode, Zener diode, PIN diode, photodiode, tunnel diode, Laser diode, DIAC etc.
• PN Junction: It is an interface or a boundary that is between p-type and n-type, the two types of semiconductor material, is known as a p-n junction. The width of the region varies with the biasing conditions.
• Depletion layer: It is the region that is formed between the p-type and n-type material in a semiconductor device when a voltage source is connected between the diodes.
• Charge carriers: The electrons which were surplus in the n-type material, generally act as the charge carriers in the semiconductor devices.
  • The electrons are the majority charge carriers and the holes are the minority charge carriers on the n side.
  • The electrons are the minority charge carriers and the holes are the majority charge carriers on the P side.
• Forward biasing: when we connect the P-type material to the anode and the N-type material to the cathode. That is called the forward bias.
• Reverse biasing: when we connect the P-type material to the cathode and the N-type material to the anode. That is called reverse bias.

To Draw I-V Characteristic Curve For P-N Junction In Forward And Reverse Bias

• Avalanche Breakdown: When a high electric field is applied in a reverse-biased condition, the electrons gain kinetic energy and move at high velocity. These electrons collide with each other, breaks covalent bonds and give rise to free electrons. As the number of free electrons increases, the net current or reverse biased current also increases and breakdown occurs. This is an irreversible process where the p-n junction diode completely breaks.
• Zener Breakdown: The width of the depletion region of a p-n junction diode depends on the doping. If the p-n junction diode is heavily doped, the width of the depletion region becomes very thin. When high electric field is applied, the electrons gain more kinetic energy and start jumping from the p-type region to n-type region through the depletion region. This frequent crossing of free electrons across the thin depletion area causes Zener breakdown.

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Aim of the Experiment 

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To draw the I-V characteristics curve of a P-N junction in forwarding bias and reverse bias conditions.

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Apparatus 

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• A PN junction diode
• A typical variable low voltage source
• A typical high voltage source
• Sensitive voltmeters
• Two sensitive ammeters
• Copper wires for connection
• Connecting wires for different instruments.

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Theory Related to P-N Junction 

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When we use various semiconductor materials at different doping (by adding an impurity) levels to form a P-N diode junction. There will form a small border that will prevent the opposite charges from moving from one side to the other by arranging the electron along a border.

Theory Related to PN Junction

• The concentration of the holes and electrons will increase on either side of the junction and the electrons from the n-side will start diffusing to the p side and that causes the flow of a small current (in micro-Amperes) called diffusion current, to flow across the junction.
• Hence an electric field directed from the positive end to the negative end of the junction is generated due to the concentration of the opposite charges on either side.
• This causes the motion of the majority charge carriers and generates a current called Drift current

Forward bias characteristics: During the forward biasing of the PN junction diode, the forward current will start increasing and at the certain voltage called threshold voltage, it reaches the maximum current value.

Reverse bias characteristics: During the reverse biasing of the PN junction diode, then a small amount of reverse current flows which remains low at a constant current. But after the biasing voltage reaches a certain voltage called breakdown voltage the depletion layer disappears causes a large amount of reverse current to flow.

P-N Junction: Forward and Reverse Bias

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Procedure 

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The procedure to draw the i v characteristic curve for p n junction in forward and reverse bias is:

For Forward-Bias

Forward Biasing Voltage

• Firstly, take all the elements by following the diagram and arrange them as shown in the diagram above.
• Ideally, the graph we have to get must be as per the diagram shown above. 
• Connections must be made with the copper wires and proper arrangements.
• Check the error correction (through the least count method) of the voltmeters and the milli-ammeter (mA) in order to get accurate results.
• The rheostat must be brought near the negative end by inserting the key K to get the zero reading from the voltmeter and milli-ammeter.
• For forward bias voltage (VF) of 0.1V, the p side is connected to the Positive terminal of the voltage source (battery).
• The voltage range must be increased slowly by keeping the current constant and increasing up to 0.3V (Since the semiconductor diode is made of germanium semiconductor).
• Increase the VF to 0.4 V to record a small current using a milli-ammeter.
• Increase the VF by 0.2 V endnote down the corresponding current. The current will increase rapidly when the VF becomes 0.7 V. 
• The current increases suddenly when VF = 0.72 V, and this stage is known as the forward breakdown stage.
• Take out the key as VF increased beyond forward breakdown.
• Note down the observations.

For Reverse Bias

Reverse Biasing Voltage

• Arrange the circuit as shown in the diagram given above.
• Connections must be made with the copper wires and proper arrangements.
• Check the error correction (through the least count method) of the voltmeters and the micro-ammeter (μA) in order to get accurate results.
• The rheostat must be brought near the positive end by inserting the key K to get the zero reading from the voltmeter and micro-ammeter μA.
• For reverse bias voltage (VR) of 0.5 V, move the rheostat to the negative end so as to flow the reverse current.
• Increase VR by 0.2 V and note down the corresponding current. The current will increase rapidly when VR becomes 20 V.
• The current increases suddenly when VR = 25 V, and this stage is known as the reverse breakdown stage. 
• Note down the current reading and take off the key and record the observations.

The following are the standard notations given to the different values of the parameters.

• VF = forward Voltage
• VR = Reverse Voltage

Read More: Difference Between Diode and Rectifier

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Observations 

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For Forward-Bias: Take the values for the following in case of forward bias:

• Range of voltmeter = …….V
• Least count of the voltmeter = …….V
• Zero error of voltmeter = ……..V
• Range of milli-ammeter = …….mA
• Least count of milli-ammeter = …….mA
• Zero error of milli-ammeter = ……..mA

For Reverse Bias: Take the values for the following in case of reverse-bias:

• Range of voltmeter = …….V
• Least count of the voltmeter = …….V
• Zero error of voltmeter = ……..V
• Range of micro-ammeter = …….μA
• Least count of micro-ammeter = …….μA
• Zero error of micro-ammeter = ……..μA

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Result 

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During the forward biasing of the diode, there is a reduction in the potential barrier. For germanium diodes, the voltage is 0.3V. and for silicon diodes, the voltage is 0.7V.

During the reverse biasing of the diode, there is an increase in the potential barrier and the reverse resistance also increases. Then the minority carriers are present at the junction that creates the reverse saturation current.

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Things to Remember

• Connect all the elements of the circuit with copper wires and check the connections are neat and tight.
• The source voltage must be increased carefully with the help of resistance by moving the sliding bar over the variable resistance.
• Since, we have an idea of the limit of the breakdown voltage, for the reverse biasing we should not increase the voltage beyond it.
• Similarly, while for the forward biasing condition we have the value of threshold voltage limit, the voltage must not be increased beyond it.
• The measuring instruments must be free from errors so they must be corrected by considering the least count values.
• In reverse bias, resistance increases.
• In forward bias, resistance decreases.

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