Question

To study the V-I characteristic of silicon diode

Answer 1

Apparatus required
Theory
Procedure
Observation table
Precautions
Or diagram

Answer 2

Experiment: To Study the V-I Characteristic of a Silicon Diode

1. Aim / Objective

To plot the voltage-current (V-I) characteristic curve for a silicon p-n junction diode in both forward and reverse bias conditions and to determine its key parameters like knee voltage and reverse saturation current.

2. Apparatus Required

1. Silicon P-N Junction Diode: 1N4007 (a very common and suitable choice).
2. Variable DC Power Supply: 0-30V.
3. DC Voltmeter: 0-1V (for forward bias) and 0-30V (for reverse bias). A digital multimeter is ideal.
4. DC Ammeter: 0-10mA (milliammeter for forward bias) and 0-100µA (microammeter for reverse bias). A digital multimeter is ideal.
5. Current Limiting Resistor: 1 kΩ.
6. Breadboard.
7. Connecting Wires.

3. Theory

A p-n junction diode is a two-terminal semiconductor device that allows electric current to flow predominantly in one direction. The V-I characteristic of a diode is a graph that shows the relationship between the voltage applied across its terminals and the current flowing through it.

a) Forward Bias:
When the positive terminal of the DC supply is connected to the p-side (anode) and the negative terminal to the n-side (cathode) of the diode, it is said to be forward-biased.
The applied voltage opposes the built-in potential barrier of the junction.
Initially, for small applied voltages (less than the barrier potential), the current is very small.
As the voltage approaches the barrier potential, the depletion region narrows, and a significant current starts to flow.
The voltage at which the current begins to increase rapidly is called the knee voltage or cut-in voltage. For a silicon diode, this is approximately 0.6V - 0.7V.
* The forward current is in the order of milliamperes (mA).

b) Reverse Bias:
When the negative terminal of the DC supply is connected to the p-side (anode) and the positive terminal to the n-side (cathode), the diode is reverse-biased.
The applied voltage aids the built-in potential barrier, increasing the width of the depletion region.
Ideally, no current should flow. However, a very small current, called the reverse saturation current (I₀), flows due to the movement of minority charge carriers.
This current is in the order of microamperes (µA) or nanoamperes (nA) for a silicon diode and is almost independent of the applied reverse voltage.
If the reverse voltage is increased to a very high value, a large current flows suddenly, and the junction breaks down. This is called the breakdown voltage. This region is usually avoided in this experiment to prevent damage to the diode.

4. Circuit Diagrams

a) Forward Bias Configuration

b) Reverse Bias Configuration

Note the reversal of the diode and the use of a microammeter (µA) in the reverse bias circuit.

5. Procedure

Part A: Forward Bias Characteristic
1. Construct the circuit as shown in the forward bias circuit diagram on the breadboard. Ensure the polarity of the diode, ammeter, and voltmeter are correct.
2. Set the variable DC power supply to its minimum value (0V).
3. Switch on the power supply.
4. Slowly increase the applied voltage in small steps.

*   From 0V to 0.5V, increase in steps of 0.1V.
*   From 0.5V to 0.8V, increase in smaller steps of 0.05V or 0.02V to accurately capture the "knee" of the curve.

1. At each step, record the reading of the forward voltage (Vf) across the diode from the voltmeter and the corresponding forward current (If) from the milliammeter.
2. Continue taking readings until the forward current is about 10-15 mA. Do not exceed the maximum current rating of the diode.
3. Tabulate the readings in the observation table.

Part B: Reverse Bias Characteristic
1. Switch off the power supply and disconnect the circuit.
2. Reconstruct the circuit as shown in the reverse bias diagram. Note that the diode terminals have been reversed.
3. Crucially, replace the milliammeter with a microammeter.
4. Set the variable DC power supply to its minimum value (0V).
5. Switch on the power supply.
6. Increase the reverse voltage (Vr) in larger steps (e.g., 1V or 2V).
7. At each step, record the reverse voltage (Vr) and the corresponding reverse current (Ir).
8. Continue taking readings up to about 20V or 30V, but do not approach the breakdown voltage of the diode (for 1N4007, it is very high, >1000V, but it's good practice to be cautious).
9. Tabulate the readings in the observation table.

6. Observation Tables

a) For Forward Bias

| S. No. | Forward Voltage, Vf (Volts) | Forward Current, If (mA) |
| :----: | :-------------------------: | :----------------------: |
| 1 | 0.0 | 0.0 |
| 2 | 0.1 | 0.0 |
| 3 | 0.2 | 0.0 |
| 4 | 0.3 | 0.0 |
| 5 | 0.4 | 0.0 |
| 6 | 0.5 | 0.1 |
| 7 | 0.6 | 0.8 |
| 8 | 0.65 | 2.5 |
| 9 | 0.7 | 7.0 |
| 10 | 0.75 | 12.5 |

(Sample readings)

b) For Reverse Bias

| S. No. | Reverse Voltage, Vr (Volts) | Reverse Current, Ir (µA) |
| :----: | :-------------------------: | :----------------------: |
| 1 | 0.0 | 0.0 |
| 2 | 2.0 | 0.1 |
| 3 | 4.0 | 0.1 |
| 4 | 6.0 | 0.2 |
| 5 | 8.0 | 0.2 |
| 6 | 10.0 | 0.2 |

(Sample readings)

7. Graph

Plot a graph of Current (I) vs. Voltage (V).
Use the first quadrant for the forward bias data (Vf on the positive X-axis, If on the positive Y-axis).
Use the third quadrant for the reverse bias data (Vr on the negative X-axis, Ir on the negative Y-axis).
* Important: The scales for the axes will be different. The forward voltage scale will be in fractions of a volt (0.1, 0.2,...), while the reverse voltage scale will be in volts (1, 2,...). Similarly, the forward current scale will be in milliamperes (mA), while the reverse current scale will be in microamperes (µA).

Sample V-I Characteristic Curve for a Silicon Diode:

8. Results and Inference

1. The V-I characteristic curve for the silicon p-n junction diode was plotted.
2. From the graph, the knee voltage is found to be approximately 0.7 V. This is the point where the forward current starts to increase significantly.
3. The reverse saturation current is observed to be very small (in the order of µA) and remains almost constant with increasing reverse voltage.
4. The results show that the diode has low resistance in forward bias (after the knee voltage) and very high resistance in reverse bias, confirming its unidirectional current-conducting property.

9. Precautions

1. All connections should be neat and tight.
2. Ensure correct polarity for the diode, voltmeter, and ammeter before switching on the power.
3. Always use a current-limiting resistor in series with the diode in the forward bias setup to prevent excessive current from damaging it.
4. The voltage should be increased slowly and gently.
5. Do not exceed the maximum forward current and reverse voltage ratings of the diode.
6. Ensure you use the correct range on the multimeter (mA for forward current, µA for reverse current).