Name: p-n junction depletion region
Uploaded: 2024-07-08T13:20:08.000Z
Duration: 1 min 11 s
Channel: JoVE

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.

p-n junction

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-side and electrons from the n-side begin to diffuse into the opposite sides due to the concentration gradient. This diffusion of carriers leads to a region around the junction where there are no free charge carriers, known as the depletion region. The charge density within the depletion region for the n-side and p-side can be described by the equations:
<img alt="Static equilibrium formulas, semiconductor charge equations, Q+=+qND, Q-=&#8722;qNA, diagram." src="/files/ftp_upload/15156/15156_Equation_1.svg" style="text-align: center; height: 20px;" data-altupdatedat="1762173717000" data-alt="Equation 1" />
Where q is the elementary charge, ND and NA are the donor and acceptor doping concentrations, respectively.
The fixed charges in the depletion region create an electric field (E), pointing from the n-side to the p-side, opposing further diffusion of carriers. This electric field gives rise to a potential difference across the junction, known as the built-in voltage (V0), which can be calculated by:
<img alt="Semiconductor voltage equation: \( V_0 = V_T \ln\left(\frac{N_A N_D}{n_i^2}\right) \), static equilibrium." src="/files/ftp_upload/15156/15156_Equation_2v2.svg" style="text-align: center; height: 40px;" data-altupdatedat="1762173717000" data-alt="Equation 2" />
 Where, VT is the thermal voltage, and ni is the intrinsic carrier concentration.
There are two types of currents that exist in a p-n junction: diffusion current due to carrier diffusion and drift current due to the electric field. At equilibrium, the magnitude of the diffusion current equals the magnitude of the drift current, leading to no net current flow across the junction. Under open-circuit conditions, there is no external current, and the depletion layer's built-in voltage balances the contact potential at the metal-semiconductor junctions, resulting in zero net voltage across the terminals.
The built-in barrier voltage and the width of the depletion region play critical roles in the junction's behavior. The width of the depletion region determines the junction's capacitance and affects how the junction will respond to external voltages.

Solar streetlights function independently without the requirement of an external power supply, as they contain p-n junctions within their solar cells.
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Basics of Semiconductors

KEY TERMS AND CONCEPTS

p-n junction depletion region

A p-n junction forms when p-type and n-type semiconductor materials are joined together. At the boundary, holes from the p-side and electrons from the n-side diffuse into the opposite side because of the concentration gradient. This movement leaves a region with no free charge carriers near the junction, called the depletion region.

The charge density in the depletion region can be described on the n-side and p-side using the donor and acceptor doping concentrations, ND and NA. The fixed charges in this region create an electric field that points from the n-side to the p-side. That field opposes further carrier diffusion and helps set up the junction’s built-in voltage, V0.

The built-in voltage across the junction depends on the thermal voltage, VT, and the intrinsic carrier concentration, ni. A p-n junction also has two kinds of current: diffusion current from carrier movement and drift current from the electric field. At equilibrium, these currents are equal in magnitude, so there is no net current flow across the junction.

Under open-circuit conditions, no external current flows. The built-in voltage in the depletion layer balances the contact potential at the metal-semiconductor junctions, so the terminals show zero net voltage. The barrier voltage and the width of the depletion region are important because they control junction behavior and affect the capacitance and response to external voltages.

P-N Junction Depletion Region

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A p-n junction forms when p-type and n-type semiconductor materials are joined together. At the boundary, holes from the p-side and electrons from the n-side diffuse into the opposite side because of the concentration gradient. This movement leaves a region with no free charge carriers near the junction, called the depletion region. The charge density in the depletion region can be described on the n-side and p-side using the donor and acceptor doping concentrations, ND and NA. The fixed...

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Learn how a p-n junction creates a depletion region, built-in voltage, and balance between diffusion and drift currents in semiconductor devices.

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JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

electrical engineering

P-N Junction Depletion Region

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