Draw the I-V characteristic curve for a p-n junction

I-V characteristic curve for a p-n junction

Aim

To draw the I-V characteristic curve for a p-n junction diode in forward and reverse bias.

Apparatus and Material Required

• A p-n junction diode (OA-79 or 1N4007),
• a resistor of value (3W, 1/2W),
• one variable voltage power supply (0-12V),
• voltmeter (0-12V),
• milliammeter (0-200 mA),
• a plug key,
• connecting wires,
• sand paper and
• a microammeter (0-200 μA).

Principle

1. \(\textbf{Forward Bias:}\) A p-n junction diode is forward biased if its p-side is at a higher potential with respect to its n-side when connected to an external voltage.
   • The external voltage reduces the built-in potential barrier across the junction, allowing current to flow.
   • \(\text{Threshold voltage or “Cut-in” voltage:}\) the minimum forward-bias voltage required to overcome the built-in potential barrier of a diode and allow significant current to flow through it is threshold voltage.
   • Current through diode increases exponentially with voltage after the external voltage surpasses threshold voltage.
2. \(\textbf{Reverse Bias:}\) A p-n junction diode is reverse biased if its n -side is at a higher potential with respect to its p-side when connected to an external voltage.
   • The external voltage applied in reverse bias increases the potential barrier through the diode and hence prevents significant current flow.
   • \(\text{Reverse Saturation Current:}\) It is the small, constant current that flows through the diode when it is reverse-biased. The current is maximum for the given diode and temperature when the diode is in reversed bias.
   • Only a small reverse saturation current flows due to minority carriers.

Procedure

1. Note the range and least count of the given voltmeter \((V)\), milliammeter \((mA)\) and the microammeter \((\mu A)\).
2. Connect the variable voltage power supply, p-n junction diode, voltmeter, milliammeter, resistor and a plug key in a circuit as shown in the adjoining diagram of forward biased diode.
3. Give a small voltage to the circuit by slight and gentle turning of the power supply knob. Close the key. Note the voltmeter and milliammeter readings.
4. Increase the voltage supply gradually. Note down the voltmeter and milliammeter readings in the observation table.
5. Look for the threshold voltage, the external voltage supply in forward bias after which there is surge in the current.
6. After the threshold voltage, increase the voltage supply very slowly (preferably in steps of 0.1 V). Continue noting observations till the limit of millimeter is reached.
7. Disconnect the circuit. Reconnect the components in a circuit as per the diagram of reverse biased diode. Important: Replace the millimeter with micrometer.
8. Gradually, increase the applied voltage (in steps) in the circuit and note the corresponding voltmeter and microammeter readings in another table.

Caution

Never increase the value of the voltage too much across the diode. Excessive current flow may damage the diode if it exceeds its limit. Maximum permissible current that can pass through the diode is available from the technical data specified by the manufacturer.

Observations

1. p-n junction diode used (diode no.) = OA-79
2. For forward biasing
   (i) Range of the voltmeter = 0 V to 1 V
   (ii) Least count of the voltmeter scale = 0.01 V
   (iii) Range of the milliammeter = 0 mA to 50 mA
   (iv) Least count of the milliammeter scale = 0.01 mA
3. For reverse biasing
   (i) Range of the voltmeter = 0 V to 50 V
   (ii) Least count of the voltmeter scale = 0.1 V
   (iii) Range of the microammeter = 0 μA to 50 μA
   (iv) Least count the microammeter = 0.01 μA

Plotting Graph

Plot a graph between forward voltage across the diode \((V_f)\) along the \(\textbf{positive}\) x-axis and current flowing through the diode \(( I)\) along the \(\textbf{positive}\) y-axis. Locate the \(\textbf{knee}\) and determine the threshold (cut-in) voltage.

Plot another graph on the same graph paper. Take the reverse voltage \((V_r)\) along the
\(\textbf{negative}\) x-axis and the corresponding current in \((\mu A)\) along the \(\textbf{negative}\) y-axis. Determine the reverse saturation current.

Result

1. The value of cut-in voltage for the given diode is 0.5 \(V\).

Precautions

• .Find out manufacturer’s specification for maximum permissible current through the given diode in forward bias. Do not exceed this limit.
• Find out manufacturers specification for maximum reverse voltage to be applied to the diode. Do not exceed this limit.
• It is important to take care that the potential difference across the diode is increased gradually, in small steps. Keep your eyes on the ammeter and let the current not exceed the specified limit.

Conclusion

The experiment demonstrated the voltage and current relationship for a p-n junction diode. The current increases exponentially in forward bias after the threshold voltage. The current through diode is very small in reverse bias and becomes constant after some reverse voltage applied.

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Reference: NCERT Lab Manual