P-N junction

A p-n junction forms when pentavalent impurities are added to a thin p-type silicon sheet, converting part of it into n-type silicon. This creates a junction where p-type and n-type materials meet. The concentration gradient of charge carriers at this interface drives holes from the p-side and electrons from the n-side to diffuse across the boundary, initiating the junction's electrical properties.

The depletion region is the area near the p-n junction that becomes devoid of free charge carriers due to diffusion. Net bound positive and negative charges remain in this region, creating an electric field and potential difference. The width of the depletion region determines the junction's capacitance and affects how it responds to external voltages.

Diffusion current results from the concentration gradient of charge carriers across the junction. Holes diffuse from the p-side to the n-side while electrons diffuse from the n-side to the p-side. This movement of carriers generates a diffusion current that flows across the junction until an opposing electric field develops to counteract it.

The electric field in the depletion region drifts minority carriers, generating a drift current that counteracts the diffusion current. At equilibrium, the magnitude of the diffusion current equals the magnitude of the drift current, resulting in zero net current flow. This balance is indicated by constant Fermi energy across the junction.

Built-in voltage is the potential difference created by the electric field across the depletion region. It arises from fixed charges in the depletion region and can be calculated using thermal voltage and intrinsic carrier concentration. The built-in barrier voltage plays a critical role in determining the junction's behavior and response to external conditions.

Solar streetlights contain p-n junctions within their solar cells that convert light energy into electrical current. The p-n junction's ability to generate current through the photovoltaic effect allows the streetlight to operate independently. This self-powered functionality demonstrates the practical application of p-n junction physics in renewable energy systems.

Under open-circuit conditions, no external current flows through the junction. The depletion layer's built-in voltage balances the contact potential at metal-semiconductor junctions, resulting in zero net voltage across the terminals. The junction maintains equilibrium with equal diffusion and drift currents despite the absence of external current flow.