V-I Characteristics of Forward and Reverse Bias PN Junction Diode

1. Aim

To study the V-I characteristics of a forward and reverse biased PN Junction Diode and to draw the load line.

2. Apparatus Used

• PN Junction Diode (1N4007 or similar)
• DC Power Supply (0-30V)
• Digital Multimeter (2 units)
• Rheostat/Potentiometer (10kΩ)
• Resistor (1kΩ)
• Breadboard
• Connecting wires
• Graph paper

3. Circuit Diagram

Fig.1: Circuit Diagram for V-I Characteristics Measurement

4. Theory

A PN junction diode is a semiconductor device formed by joining P-type and N-type semiconductor materials. It allows current to flow easily in one direction (forward bias) but restricts flow in the opposite direction (reverse bias).

Forward Bias:

When the positive terminal of the voltage source is connected to the P-type material and the negative terminal to the N-type material, the diode is said to be forward biased. In this configuration:

• The holes in the P-region and electrons in the N-region are pushed toward the junction
• The width of the depletion region decreases
• Once the applied voltage overcomes the barrier potential (approximately 0.7V for silicon diodes), current begins to flow
• The current increases exponentially with the applied voltage

Reverse Bias:

When the positive terminal of the voltage source is connected to the N-type material and the negative terminal to the P-type material, the diode is reverse biased. In this configuration:

• The holes in the P-region and electrons in the N-region are pulled away from the junction
• The depletion region widens
• Only a very small leakage current flows
• If the reverse voltage exceeds the breakdown voltage, the diode will conduct in the reverse direction

Load Line:

A load line is a graphical representation of all possible operating points of a circuit. For a diode circuit with a series resistor:

• The load line represents the relationship between the diode voltage and current based on Kirchhoff's voltage law
• It helps determine the operating point (Q-point) of the diode
• The intersection of the load line with the diode's I-V curve gives the operating point

5. Formula

6. Procedure

For Forward Bias Characteristics:

1. Set up the circuit as per the diagram with the PN junction diode in forward bias configuration (P-side connected to the positive terminal through a resistor and N-side to the negative terminal).
2. Keep the power supply voltage initially at zero.
3. Gradually increase the supply voltage in small steps (0.1V) and record the corresponding Junction diode voltage and current readings.
4. Continue until sufficient readings are obtained (typically up to 1V for the diode voltage).
5. Plot a graph of PN Junction diode current vs. diode voltage.

For Reverse Bias Characteristics:

1. Reverse the diode connections in the circuit (N-side connected to the positive terminal and P-side to the negative terminal).
2. Keep the power supply voltage initially at zero.
3. Gradually increase the supply voltage in steps (0.5V) and record the corresponding junction diode voltage and current readings.
4. Continue until a sufficient range is covered (typically up to 10V, but not exceeding the diode's maximum reverse voltage rating).
5. Plot a graph of diode current vs. diode voltage.

For Drawing the Load Line:

1. For a given circuit with resistor R and supply voltage $V_{CC}$, mark two points on the graph:
   • Point 1: When $V_D = 0$, $I_D = \frac{V_{CC}}{R}$
   • Point 2: When $I_D = 0$, $V_D = V_{CC}$
2. Draw a straight line connecting these two points. This is the load line.
3. The intersection of this load line with the forward characteristic curve gives the operating point (Q-point) of the diode.

7. Observation Table

Forward Bias:

Reverse Bias:

8. Calculations

Static Resistance Calculation:

The static resistance of the diode at a particular operating point is given by:

Example calculation for forward bias (values to be filled after performing the experiment):

Dynamic Resistance Calculation:

The dynamic or AC resistance of the diode at a particular operating point is given by:

Example calculation using forward bias data points (values to be filled after performing the experiment):

Load Line Calculation:

For a circuit with a 1kΩ series resistor and a supply voltage of 5V:

The operating point can be determined by finding the intersection of this load line with the forward characteristic curve from your experiment data.

9. Result

1. The V-I characteristics of the PN junction diode in forward bias and reverse bias were successfully studied.
2. From the forward bias characteristics, it was observed that:
   • The PN junction diode starts conducting significantly once the voltage across it exceeds approximately 0.7V (silicon diode).
   • Beyond the threshold voltage, the current increases exponentially with a small increase in voltage.
3. From the reverse bias characteristics, it was observed that:
   • The PN junction diode allows only a very small leakage current (in μA range) to flow in reverse bias.
   • This current remains almost constant with an increase in reverse voltage until breakdown occurs.
4. The load line was successfully drawn, and the operating point (Q-point) was determined for a circuit with $V_{CC} = 5V$ and $R = 1k\Omega$.
5. The static resistance at the operating point was calculated.
6. The dynamic resistance at the operating point was calculated.

10. Precautions

1. Handle the diode carefully to avoid damage due to static electricity.
2. Connect the diode in the circuit with proper polarity. The band or mark on the diode indicates the cathode (N-side).
3. Do not exceed the maximum ratings of the diode, especially in reverse bias.
4. Ensure all connections are tight and secure before turning on the power supply.
5. Start with zero voltage and gradually increase it to avoid sudden spikes.
6. Use proper range settings on the multimeters to get accurate readings.
7. For forward bias, keep the voltage across the diode below 1V to avoid overheating.
8. Allow the diode to cool down between successive readings if necessary.
9. Record the readings quickly, especially at higher currents, to prevent heating effects from influencing the measurements.
10. Draw the graphs carefully, labeling all axes and marking important points like threshold voltage.

11. Viva Voice Questions

1. What is a PN junction diode?

   A PN junction diode is a semiconductor device formed by joining P-type and N-type semiconductor materials. It allows current to flow easily in one direction (forward bias) but restricts flow in the opposite direction (reverse bias).

2. What is meant by forward and reverse bias?

   Forward bias means connecting the positive terminal of the voltage source to the P-type material and the negative terminal to the N-type material, which allows current to flow easily. Reverse bias means connecting the positive terminal to the N-type material and the negative terminal to the P-type material, which restricts current flow.

3. What is the knee voltage or threshold voltage for silicon and germanium diodes?

   For silicon diodes, the threshold voltage is approximately 0.7V. For germanium diodes, it is approximately 0.3V.

4. What is the depletion region in a diode?

   The depletion region is a zone at the PN junction where mobile charge carriers (electrons and holes) have diffused across the junction, leaving behind exposed charged ions. This region is depleted of mobile charge carriers and creates a potential barrier to further current flow.

5. What is meant by reverse saturation current?

   Reverse saturation current (Is) is the small current that flows when the diode is reverse biased. It is due to the thermally generated minority carriers in the semiconductor material.

6. What is the difference between static and dynamic resistance of a diode?

   Static resistance is the ratio of voltage across the diode to the current flowing through it at a specific operating point (V/I). Dynamic resistance is the ratio of change in voltage to change in current (dV/dI) at a specific operating point, representing the AC resistance of the diode.

7. What is a load line and what is its significance?

   A load line is a graphical representation of all possible operating points of a circuit. It shows the relationship between diode voltage and current based on Kirchhoff's voltage law. Its significance lies in determining the operating point (Q-point) of the diode, which is the intersection of the load line with the diode's I-V curve.

8. What happens if the reverse voltage exceeds the breakdown voltage?

   If the reverse voltage exceeds the breakdown voltage, a phenomenon called avalanche breakdown occurs. The reverse current increases rapidly and can damage the diode if not limited by an external circuit component.

9. Why is the forward characteristic curve exponential?

   The forward characteristic curve is exponential because the current through the diode is related to the voltage by the Shockley equation: $I_D = I_S(e^{\frac{V_D}{nV_T}} - 1)$, which has an exponential term.

10. How does temperature affect the diode characteristics?

    With an increase in temperature: (1) The forward voltage at a given current decreases, (2) The reverse saturation current increases, and (3) The breakdown voltage increases. These effects are due to the increased thermal energy of charge carriers at higher temperatures.