Question

What is the working principle of forward and reverse biased PN junction diode?

Answer 1

A PN junction diode is a semiconductor device made by joining P-type (positively doped) and N-type (negatively doped) materials. The behavior of the diode changes depending on how it's connected in a circuit. Let's break down the two main conditions:

1. Forward Biased PN Junction Diode:

1. Definition: In a forward bias, the positive terminal of the battery (or power supply) is connected to the P-type material, and the negative terminal is connected to the N-type material.
   

1. Working:
   

    - When forward bias is applied, the positive voltage pushes holes (which are positive charge carriers) in the P-type material towards the junction, and the negative voltage pushes electrons (negative charge carriers) in the N-type material towards the junction.
    - This reduces the depletion region (the area where no free charge carriers are present) at the junction, allowing current to flow easily through the diode.
    - If the voltage is higher than a certain threshold (typically 0.7V for silicon diodes), current flows through the diode in the direction of the P-type to N-type material.

1. Current Flow: The current flows easily, and the diode conducts. It acts like a short circuit (low resistance path).
   

2. Reverse Biased PN Junction Diode:

1. Definition: In reverse bias, the positive terminal of the battery is connected to the N-type material, and the negative terminal is connected to the P-type material.
   

1. Working:
   

    - In reverse bias, the applied voltage increases the depletion region at the junction. This means the holes in the P-type material are pulled away from the junction, and the electrons in the N-type material are also pulled away.
    - This increases the width of the depletion region, preventing current from flowing through the diode under normal conditions.
    - Very little current (called reverse saturation current) may still flow, but it's extremely small and typically ignored in most cases.

1. Current Flow: The diode does not conduct current, acting as an open circuit (high resistance). However, if the reverse voltage becomes too high, it can cause breakdown (like in Zener diodes), leading to a large current.
   

Summary:

1. Forward Bias: Current flows easily through the diode.
   

1. Reverse Bias: No current flows, and the diode is essentially "off."
   

This behavior of diodes makes them very useful for controlling the direction of current in circuits!