Forward and Reverse Bias

Definition:

A PN junction diode works in two modes depending on how the external voltage is applied:

• Forward Bias: Current flows easily.
• Reverse Bias: Current is almost blocked.

Forward Bias (GIF Reference):

In the GIF:

• The P-side (pink) is connected to the positive terminal of the battery, and the N-side (blue) is connected to the negative terminal.
• This reduces the depletion region width, making it easier for electrons (from N-side) and holes (from P-side) to cross the junction.
• Once the applied voltage reaches around 0.6V (for silicon diode), a large current begins to flow, as shown in the I–V curve on the right.
• This is called forward biasing.

Reverse Bias (concept explanation):

• If the battery is reversed (P-side negative, N-side positive):
  • The depletion region widens, blocking the movement of carriers.
  • Only a tiny leakage current flows due to minority carriers.
  • The diode effectively blocks current flow.

Quick Recap:

• Forward Bias: P-side positive, N-side negative → Current flows once voltage > threshold (~0.6V for silicon).
• Reverse Bias: P-side negative, N-side positive → Almost no current flows.
• Graph Insight: The I–V curve shows very small current in reverse, and a rapid increase in current once forward voltage passes the threshold.