PN Junction Diode (Forward-Bias Condition)

PN Junction Diode: Forward Bias Condition

Key Concepts:

• Forward Bias: Applying a positive voltage to the P-side and a negative voltage to the N-side of a PN junction diode.
• Depletion Layer Width (WD): The region devoid of mobile charge carriers at the PN junction.
• Barrier Potential (VB): The potential difference across the depletion region that opposes the flow of majority carriers.
• Diffusion Current (I majority): Current due to the flow of majority carriers across the junction.
• Reverse Saturation Current (IS): Current due to the flow of minority carriers across the junction.
• Diode Current (ID): The net current flowing through the diode.

Forward Bias Setup and its Effects

In forward bias, the positive terminal of a voltage source (VD) is connected to the P-side, and the negative terminal to the N-side of the PN junction. This configuration has several key effects:

• Repulsion of Charge Carriers: The positive terminal repels holes (majority carriers in the P-side) towards the depletion region. Similarly, the negative terminal repels electrons (majority carriers in the N-side) towards the depletion region.
• Reduction of Depletion Layer Width: The injected holes recombine with negative ions in the depletion region on the P-side, and the injected electrons recombine with positive ions in the depletion region on the N-side. This recombination reduces the width of the depletion layer from WD to WD'.
• Mathematical Relationship: When VD = 0, the depletion width is WD. When VD > 0, the depletion width becomes WD', and WD > WD'. Therefore, increasing the forward bias potential reduces the width of the depletion layer.

Diode Current and its Components

The diode current (ID) in forward bias is the difference between the diffusion current (I majority) and the reverse saturation current (IS):

• Equation: ID = I majority - IS
• Reverse Saturation Current (IS): IS remains relatively constant because it depends primarily on temperature and the concentration of minority charge carriers. Since the temperature is assumed to be constant, IS does not change significantly with the applied forward bias.
• Diffusion Current (I majority): The diffusion current increases significantly due to the reduction in the depletion layer width and the barrier potential.

Barrier Potential Reduction

The forward bias voltage (VD) directly opposes the built-in barrier potential (VB):

• New Barrier Potential: The new barrier potential becomes VB - VD.
• Silicon Example: For silicon, VB is approximately 0.7 volts. If VD is increased to 0.7 volts, the new barrier potential becomes 0 volts (0.7 - 0.7 = 0).
• Electron Behavior: As the barrier potential decreases, electrons on the N-side experience a reduced barrier at the junction and are strongly attracted to the positive potential on the P-side.

Exponential Rise in Diode Current

As the magnitude of the applied forward bias voltage increases, the width of the depletion layer continues to decrease. Once the barrier potential is effectively overcome (approaches zero), a "flood" of electrons can pass through the junction, leading to an exponential rise in the diode current (ID).

Summary of Key Points

1. Connection: Positive terminal to P-side, negative terminal to N-side.
2. Charge Carrier Repulsion: Holes and electrons are repelled towards the depletion region.
3. Depletion Layer Reduction: Recombination of mobile charge carriers and immobile ions reduces the depletion layer width.
4. Barrier Potential Reduction: The forward bias voltage reduces the barrier potential.
5. Exponential Current Rise: Once the barrier potential is overcome, the diode current increases exponentially.
6. Reverse Saturation Current: IS remains relatively constant as it primarily depends on temperature.

Conclusion

The forward bias condition of a PN junction diode results in a reduction of the depletion layer width and the barrier potential. As the forward bias voltage increases, the diode current increases exponentially once the barrier potential is overcome. The understanding of these principles is crucial for plotting the volt-ampere (VI) characteristics of the diode in forward bias, which will be discussed in future lectures. The reverse saturation current remains relatively constant during forward bias.