Question

Draw I - V characteristic curve of p-n junction in forward bias and reverse bias.

Hint:

Use appropriate ranges for the meters. A voltmeter of 0-3V and a milliammeter are suitable for forward bias, while a voltmeter of 0-30V and a microammeter are needed for reverse bias. Be cautious not to apply a reverse voltage beyond the diode's breakdown rating to prevent permanent damage.

Answer 1

Step 1: Understanding the Concept:
The I-V characteristic of a p-n junction diode describes its behavior under different biasing conditions.
Forward Bias: When the p-side is connected to the positive terminal of a DC source and the n-side to the negative, the diode is forward-biased. The potential barrier is reduced, and a significant current (in mA) flows once the applied voltage exceeds the knee voltage (\(\sim\)0.7V for Si, \(\sim\)0.3V for Ge).
Reverse Bias: When the polarity is reversed (p-side to negative, n-side to positive), the diode is reverse-biased. The potential barrier increases, and only a very small leakage current (in \(\mu\)A) due to minority carriers flows, until the breakdown voltage is reached.

Step 2: Apparatus:
Apparatus Required:
A p-n junction diode, a variable DC power supply (0-3V for forward, 0-30V for reverse), a DC voltmeter, a milliammeter (mA), a microammeter (\(\mu\)A), a key, and connecting wires.
Step 3: Detailed Procedure:
Part A: Forward Bias Characteristics
1. Circuit: Connect the power supply, key, milliammeter, and diode in series. Connect the voltmeter in parallel across the diode. Ensure the p-side of the diode is connected to the positive terminal of the supply.
2. Readings: Start with zero voltage. Gradually increase the forward voltage (Vf) in small steps (e.g., 0.1 V). For each Vf, record the corresponding forward current (If) from the milliammeter. Note the sharp increase in current after the knee voltage.
Part B: Reverse Bias Characteristics
1. Circuit: Reverse the connections of the diode and the power supply. Replace the milliammeter with a microammeter to measure the small reverse current.
2. Readings: Increase the reverse voltage (Vr) in larger steps (e.g., 1 V or 2 V). For each Vr, record the corresponding reverse current (Ir) from the microammeter. The current will be very small and almost constant. (Do not exceed the breakdown voltage of the diode).
Step 4: Graph:
1. Plotting: Plot the collected data on a graph paper. Use the first quadrant for forward bias (Vf vs If) and the third quadrant for reverse bias (Vr vs Ir). The voltage is on the X-axis and the current on the Y-axis.
2. The Curve: - The forward bias curve will be almost flat until the knee voltage, after which it will rise steeply.
- The reverse bias curve will be a nearly horizontal line very close to the voltage axis, indicating a very small, constant reverse saturation current.