PN junction diodes work by exploiting the properties of the junction formed between p-type and n-type semiconductor materials. When these materials are brought together, electrons from the n-type region (which has an excess of electrons) diffuse into the p-type region (which has an excess of holes). This movement creates a depletion region around the junction where no free charge carriers exist. The electric field formed in this region opposes further diffusion of charge carriers, creating a balance. When forward-biased (positive voltage on the p-side), the barrier is reduced, allowing current to flow. When reverse-biased (positive voltage on the n-side), the barrier increases, preventing current flow.
The working principle of a diode is based on the directional flow of current. A diode allows current to pass in one direction (forward direction) while blocking it in the opposite direction (reverse direction). This is due to the formation of the depletion region at the PN junction, which acts as a barrier to charge carrier movement. In forward bias, the external voltage reduces the barrier, enabling current flow. In reverse bias, the barrier is enhanced, and current is blocked, except for a small leakage current due to minority carriers.
A PN junction rectifier works by converting alternating current (AC) into direct current (DC). During the positive half-cycle of the AC input, the PN junction is forward-biased, allowing current to flow through the diode. During the negative half-cycle, the diode is reverse-biased, blocking current flow. This process results in the rectification of AC into pulsating DC. In a full-wave rectifier, multiple diodes are used to rectify both halves of the AC cycle, providing a more constant DC output.
The PN junction enables a diode to function by creating a depletion region that controls the flow of charge carriers. In forward bias, the external voltage applied reduces the width of the depletion region, allowing electrons and holes to recombine and current to flow. In reverse bias, the depletion region widens, preventing charge carriers from crossing the junction, thus blocking current flow. This directional control of current is the fundamental operating principle of the diode.
In an unbiased condition, a PN junction diode is characterized by the presence of a depletion region at the junction. Without any external voltage applied, the electric field within the depletion region prevents the free movement of charge carriers across the junction. Electrons in the n-type region and holes in the p-type region remain in their respective areas, maintaining equilibrium. The diode, in this state, does not conduct significant current because the internal electric field of the depletion region balances the diffusion of charge carriers.