Aim                                                                                                                                                               To draw the I-V characteristic curve of a p-n junction in forward bias and reverse bias .
Apparatus
A p-n junction (semi-conductor) diode, a 3 volt battery, a 50 volt battery, a high resistance rheostat, one 0-3 volt voltmeter, one 0-50 volt voltmeter, one 0-100 mA ammeter, one 0-100 μA ammeter, one way key, connecting wires and pieces of sand paper.
Theory
Forward bias characteristics. When the p -section of the diode is connected to positive terminal of a battery and n-section is connected to negative terminal of the battery then junction is said to be forward biased. With increase in bias voltage, the forward current increases slowly in the beginning and then rapidly. At about 0.7 V for Si diode (0.2 V for Ge), the current increases suddenly. The value of forward bias voltage, at which the forward current increases rapidly, is called cut in voltage or threshold voltage.
Reverse bias characteristics. When the p -section of the diode is connected to negative terminal of high voltage battery and n-section of the diode is connected to positive terminal of the same battery, then junction is said to be reverse biased.
When reverse bias voltage increases, initially there is a very small reverse current flow, which remains almost constant with bias. But when reverse bias voltage increases to sufficiently high value, the reverse current suddenly increases to a large value. This voltage at which breakdown of junction diode occurs (suddenly large current flow) is called zener breakdown voltage or inverse voltage. The breakdown voltage may^tarts from one volt to several hundred volts, depending upon dopant density and the depletion layer.
Diagram
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-1
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-2
Procedure
For forward-bias
  1. Make circuit diagram as shown in diagram.
  2. Make all connections neat, clean and tight.
  3. Note least count and zero error of voltmeter (V) and milli-ammeter (mA).
  4. Bring moving contact of potential divider (rheostat) near negative end and insert the key K. Voltmeter V and milli-ammeter mA will give zero reading.
  5. Move the contact a little towards positive end to apply a forward-bias voltage (VF) of
    0. 1 V. Current remains zero.
  6. Increase the forward-bias voltage upto 0.3 V for Ge diode. Current remains zero, (It is due to junction potential barrier of 0.3 V).
  7. Increase VF to 0.4 V. Milli-ammeter records a small current.
  8. Increase VF in steps of 0.2 V and note the corresponding current. Current increases first slowly and then rapidly, till VF becomes 0.7 V.
  9. Make VF = 0.72 V. The current increases suddenly. This represents “forward break-down” stage.
  10. If the VF increases beyond “forward breakdown” stage, the forward current does not change much. Now take out the key at once.
  11. Record your observations as given ahead.
    For reverse-bias
  12. Make circuit diagram as shown in diagram.
  13. Make all connections neat, clean and tight.
  14. Note least count and zero error of voltmeter (V) and micro-ammeter (μA).
  15. Bring moving contact of potential divider (rheostat) near positive end and insert the key K Voltmeter V and micro-ammeter μA will give zero reading.
  16. Move the contact towards negative end to apply a reverse-bias voltage (VR) of 0.5 V, a feebly reverse current starts flowing.
  17. Increase VR in steps of 0.2 V. Current increases first slowly and then rapidly till VR becomes 20 V. Note the current.
  18. Make VR = 25 V. The current increases suddenly. This represents “reverse break-down” stage. Note the current and take out the key at once.
  19. Record your observations as given ahead.
Observations
For forward-bias
Range of voltmeter                        = …..V
Least count of voltmeter              = …..V
Zero error of voltmeter                = …..V
Range of milli-ammeter              = …..mA
Least count of milli-ammeter    = …..mA
Zero error of milli-ammeter      = …..mA
1.                         Table for forward-bias voltage and forward current
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-3
Note. The readings are as a sample.
For reverse-bias
Range of voltmeter                     = …..V
Least count of voltmeter           = …..V
Zero error of voltmeter              = …..V
Range of micro-ammeter          = …..μA
Least count of micro-ammeter = …..μA
Zero error of micro-ammeter    = …..
2.                        Table for reverse-bias voltage and reverse current
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-4
Note. The readings are given as a sample.
Calculations
For forward-bias
Plot a graph between forward-bias voltage VF (column 2) and forward current IF (column 3) taking VF along X-axis and IF along Y-axis.
This graph is called forward-bias characteristic curve a junction diode.
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-5
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-6
For reverse-bias
Plot a graph between reverse-bias voltage VR (column 2) and reverse current IR (column 3) taking VR along X-axis and IR along Y-axis.
This graph is called reverse-bias characteristic curve of a junction diode.
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-7
to-draw-the-i-v-characteristic-curve-of-a-p-n-junction-in-forward-bias-and-reverse-bias-8
Result
Junction resistance for forward-bias = 40 ohms
Junction resistance for reverse-bias = 2 x 106 ohms.
Precautions
  1. All connections should be neat, clean and tight.
  2. Key should be used in circuit and opened when the circuit is not being used.
  3. Forward-bias voltage beyond breakdown should not be applied.
  4. Reverse-bias voltage beyond breakdown should not be applied.
Sources of error
The junction diode supplied may be faulty.