10.6
P-N junction
A p-n junction is formed when p-type and n-type semiconductor materials are joined together. At the interface of the p-n junction, holes from the p-si…
Solar streetlights function independently without the requirement of an external power supply, as they contain p-n junctions within their solar cells.
A p-n junction is formed by adding pentavalent impurities to a thin p-type silicon sheet, converting part of it into n-type silicon.
The concentration gradient of charge carriers on both sides of the silicon sheet leads to the diffusion of holes from the p-side to the n-side and electrons from the n-side to the p-side, generating a diffusion current.
The region near the junction becomes devoid of free charge carriers, forming the depletion region. Also, net bound positive and negative charges remain in this region, creating an electric field and potential difference across the junction.
The electric field drifts the minority carriers in the p-n junction, generating a drift current that counteracts the diffusion current.
This electric field also opposes any further diffusion of majority carriers across the junction.
At equilibrium, the total current across the junction is the same in magnitude in both directions and is indicated by constant Fermi energy.
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Q1: How is a p-n junction formed in a semiconductor?
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.
Q2: What is the depletion region in a p-n junction?
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.
Q3: What causes diffusion current in a p-n junction?
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.
Q4: How does drift current oppose diffusion current at equilibrium?
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.
Q5: What is built-in voltage and why does it matter?
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.
Q6: Why do solar streetlights work without external power?
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.
Q7: What happens to the p-n junction under open-circuit conditions?
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.