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What are forward biased and reverse biased diodes ?

Diodes are semiconductor devices that allow the flow of electric current in one direction while blocking it in the opposite direction. The terms “forward biased” and “reverse biased” refer to the conditions under which a diode is subjected to an external voltage. Let’s delve into the detailed explanations of forward biased and reverse biased diodes:

Forward Biased Diode:

  1. Voltage Application:
    • In a forward-biased diode, a positive voltage (higher potential) is applied to the P-type semiconductor (anode), and a negative voltage (lower potential) is applied to the N-type semiconductor (cathode). This creates an electric field that encourages the flow of current from the P-type region to the N-type region.
  2. Reduction of Barrier Potential:
    • The applied forward voltage reduces the built-in potential barrier at the P-N junction. The electric field from the applied voltage opposes the built-in electric field of the diode, facilitating the movement of charge carriers across the junction.
  3. Conduction:
    • As the voltage across the diode increases beyond the threshold known as the “threshold voltage” or “cut-in voltage,” the potential barrier is overcome. Electrons from the N-type region move towards the P-type region, and holes from the P-type region move towards the N-type region. This movement of charge carriers constitutes the current flow through the diode.
  4. Low Resistance:
    • In the forward-biased state, the diode exhibits low resistance (low impedance) to the flow of current. The voltage drop across the diode is relatively small, typically around 0.7 volts for silicon diodes. This voltage drop is often referred to as the forward voltage drop.
  5. Characteristics:
    • Forward-biased diodes conduct readily, and their current-voltage (I-V) characteristics show a rapid increase in current with a small increase in voltage once the threshold is reached.

Reverse Biased Diode:

  1. Voltage Application:
    • In a reverse-biased diode, a negative voltage (higher potential) is applied to the P-type semiconductor (anode), and a positive voltage (lower potential) is applied to the N-type semiconductor (cathode). This configuration increases the built-in potential barrier at the P-N junction.
  2. Widening of Barrier Potential:
    • The applied reverse voltage adds to the built-in potential barrier, making it more difficult for charge carriers to cross the junction. The wider potential barrier prevents significant current flow through the diode.
  3. Limited Reverse Leakage Current:
    • Although the ideal diode would block all current in the reverse-biased state, real-world diodes exhibit a small amount of reverse leakage current due to minority carriers. This current is usually very small, and reverse-biased diodes are considered to have high impedance under reverse bias.
  4. Breakdown Voltage:
    • If the reverse voltage exceeds a certain critical value called the “reverse breakdown voltage” or “avalanche voltage,” the diode can experience a phenomenon called “avalanche breakdown” or “Zener breakdown.” This results in a sudden increase in reverse current and can damage the diode if the current is not limited.
  5. Characteristics:
    • Reverse-biased diodes typically have a very high resistance to current flow, and their I-V characteristics show a small reverse leakage current until the breakdown voltage is reached.

In summary, forward biased diodes allow current flow easily when a positive voltage is applied to the anode, reducing the built-in potential barrier. Reverse biased diodes block current flow under normal conditions due to the increased potential barrier. However, they may experience breakdown if the reverse voltage exceeds a critical value. Understanding these biasing conditions is crucial for designing and analyzing electronic circuits involving diodes.

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