Forward Biasing: The “On” State

switching-diodes  When a positive voltage (forward bias) is applied across the diode, with the anode connected to the positive terminal of the power source and the cathode to the negative terminal, the external electric field opposes the built-in electric field of the depletion region. As the forward voltage exceeds the barrier potential (approximately 0.7V for silicon diodes and 0.3V for germanium diodes), the depletion region narrows significantly. This allows electrons from the N-region and holes from the P-region to recombine across the junction, creating a continuous flow of current. In this state, the diode exhibits low resistance, typically in the order of a few ohms, and is considered “on,” enabling current to pass through with minimal voltage drop.

Reverse Biasing: The “Off” State

Conversely, when a negative voltage (reverse bias) is applied, with the anode connected to the negative terminal and the cathode to the positive terminal, the external electric field strengthens the built-in electric field of the depletion region. As a result, the depletion region widens, preventing the flow of majority carriers across the junction. Only a very small reverse leakage current, typically in the microampere range for silicon diodes, can flow due to the thermally generated minority carriers (electrons in P-region and holes in N-region). In this reverse-biased condition, the diode presents a high resistance, often in the megohm range, effectively acting as an open circuit or being “off.”switching-diodes