A PN junction diode is a fundamental semiconductor device with two regions: the P-region, which is positively doped with holes as majority carriers, and the N-region, which is negatively doped with electrons as majority carriers. When the P and N regions are joined together, a PN junction is formed.
Forward biasing a PN junction diode means applying an external voltage across the diode in a way that allows current to flow through it easily. Specifically, for a standard silicon PN diode, the positive terminal of the voltage source is connected to the P-region, and the negative terminal is connected to the N-region. This arrangement decreases the potential barrier at the junction, which is formed due to the accumulation of positive and negative charges at the interface.
As a result of forward biasing, the following happens:
Reduced Barrier: The external voltage reduces the electric field across the PN junction, allowing electrons from the N-region and holes from the P-region to move toward the junction.
Current Flow: When the external voltage is sufficient to overcome the remaining potential barrier, majority carriers (electrons from the N-region and holes from the P-region) start to flow across the junction. This flow of carriers constitutes the forward current, and the diode is said to be conducting.
Low Resistance: The PN junction behaves like a low-resistance path for current flow, and the voltage drop across the diode remains relatively small.
It's important to note that even in forward bias, there is some small voltage drop across the diode due to the internal resistance, known as the forward voltage drop (typically around 0.6 to 0.7 volts for silicon diodes).
Reverse biasing a PN junction diode means applying an external voltage in a way that opposes the natural flow of majority carriers. In this case, the positive terminal of the voltage source is connected to the N-region, and the negative terminal is connected to the P-region.
As a result of reverse biasing, the following happens:
Increased Barrier: The external voltage increases the potential barrier at the junction, preventing majority carriers from crossing the junction easily.
Minimal Current Flow: Due to the increased barrier, only a very small reverse current, known as the leakage current, flows through the diode. The magnitude of this current is usually negligible for standard diodes.
High Resistance: The PN junction behaves like a high-resistance path for current flow, and the voltage drop across the diode remains relatively high.
It's important to highlight that reverse biasing is an essential mode of operation for certain semiconductor devices, like diodes in rectifier circuits, Zener diodes (used for voltage regulation), and avalanche diodes (used in some types of voltage protection circuits).
Overall, forward and reverse biasing play critical roles in the operation of PN junction diodes and other semiconductor devices, allowing them to function as rectifiers, switches, and voltage regulators in electronic circuits.