M1 p-n junction

Topic

The contact between a p-region and an n-region of this semiconductor material creates a depletion region between them, driven by the diffusion of majority charge carriers triggered by the contact itself as it moves toward equilibrium: majority free electrons from the n-region diffuse across the contact into the p-region, while majority holes from the p-region diffuse in the opposite direction into the n-region. This mutual crossing of majority charge carriers depletes the region on both sides of the contact—forming the depletion region—a phenomenon modeled elsewhere in this set to determine the junction's capacitance based on applied voltage. If this diffusion of majority charge carriers continues for a longer period before equilibrium is reached, the depletion region consequently extends further on both sides of the contact.

As majority charge carriers diffuse across the contact and leave their regions of origin, they expose the fixed charges of ionized donor and acceptor impurities—charges described in greater detail by another model in this set. The internal electric field generated by these exposed fixed charges eventually halts the diffusion of majority charge carriers, establishing the p-n junction's contact potential at equilibrium; this contact potential is precisely the potential barrier that the internal electric field opposes against the further diffusion of majority charge carriers across the contact. Consequently, a higher concentration of these ionized donor and acceptor impurities on either side of the contact results in a higher contact potential. Consequently, in this model—which began by defining the depletion region and the contact potential—that contact potential, combined with any externally applied bias across the p-n junction, determines the junction's diode current (calculated using the Shockley model, whose parameters are derived from a measured current-voltage curve). A forward bias reduces the potential barrier imposed by the contact potential, allowing the diode current to increase exponentially; conversely, a reverse bias increases the barrier and suppresses the diode current, until a sufficiently strong reverse electric field triggers a breakdown—whether through band-to-band tunneling or avalanche multiplication—thereby overcoming that suppression. Since this p-n junction—characterized by the depletion region and contact potential introduced at the start—serves as the common physical foundation for other models in this set (each describing a semiconductor device built from that same junction), any variation in the depletion region or contact potential inevitably propagates to affect the behavior of all those other semiconductor devices.

ID:4501