To draw the I-V characteristics curve of a p-n junction in forward bias and reverse bias | Class 12 | Physics Practical Copy | Handwritten with observations

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Aim: To draw the I-V characteristics curve of a p-n junction in forward bias and reverse bias.

Important Terms:

  1. Forward Bias: When an external voltage is applied to a p-n junction diode in such a way that the p-side is at a higher potential with respect to the n-side, it is said to be forward biased.
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  2. Reverse Bias: When an external voltage is applied to a p-n junction diode in such a way that the n-side is at a higher potential with respect to the p-side, it is said to be reverse biased.
  3. Reverse Saturation Current: When the applied voltage is increased in the reverse biased condition, starting from zero value, the current increases and soon becomes constant. This current is very small(in μA). It is called reverse saturation current.
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  4. Threshold voltage or “Cut-in” voltage: When the p-side is connected to the positive terminal of the battery and the voltage is increased, initially a negligible current flows till the applied voltage crosses a certain value. After, a characteristic voltage, the diode current increases significantly (exponentially), even for a very small increase in the diode bias voltage. This voltage is called the threshold voltage or cut-in-voltage of the diode.

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Procedure:

  1. Note the range and least count of the given voltmeter (V), milliammeter (mA) and the microammeter (μA).
  2. Remove the insulating layers from the connecting wires and leads of diode using a sand paper.
  3. Connect the variable voltage power supply, p-n junction diode, voltmeter, milliammeter, resistor and a plug key in a circuit.
  4. Initially when the key is open, you would note at this stage, that no current is flowing through the circuit. Now close the key.
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  5. Give a small voltage to the circuit by slight and gentle turning of the power supply knob. Note the voltmeter reading across the diode and the corresponding milliammeter reading to find the current I flowing through the diode.
  6. Gradually, increase the applied voltage (in steps) in the circuit and note the corresponding voltmeter and milliammeter readings.
    The value of current flowing through the diode would be negligibly small till the voltage across the diode exceeds the value of its cut in or threshold voltage. After the cut-in voltage, the variation in current will be rapid.
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  7. Once the threshold voltage is reached, vary the diode voltage very slowly noting the corresponding current I flowing through the diode. Continue increasing voltage till the current reaches the limit of the milliammeter.
  8. Now disconnect the circuit and make the connections for the reverse bias characteristics. Connect p-side of p-n junction diode to the negative terminal of the power supply and replace milliameter with a microammeter. Gradually, increase the applied voltage (in steps) in the circuit and note the corresponding voltmeter and microammeter readings in Table E 16.2. In this part of experiment, with the given supply voltage you will get only flat portion of the reverse bias characteristic curve.



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