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

太阳能路灯无需外部电源供电即可独立运行，因为其太阳能电池中含有p-n结。
p-n结是通过向一层薄的p型硅片中添加五价杂质，使其部分区域转变为n型硅而形成的。
硅片两侧的载流子浓度梯度导致空穴从p区向n区扩散，同时电子从n区向p区扩散，从而产生扩散电流。
结附近的区域会失去自由载流子，形成耗尽区。同时，该区域内仍保留净的束缚正电荷和负电荷，从而在结两侧产生电场和电势差。
电场使 p-n 结中的少数载流子发生漂移，产生漂移电流，以抵消扩散电流。
该电场还会阻碍多数载流子进一步扩散通过结区。
在平衡状态下，结两侧的总电流在大小上相等且方向相反，并由恒定的费米能级表示。

半导体基础知识

P-N junction

当 p 型和 n 型半导体材料连接在一起时，就会形成 p-n 结。在 p-n 结的结点处， 在浓度梯度的作用下，来自 p 侧的空穴和来自 n 侧的电子开始向相反的两侧进行扩散。载流子的扩散会导致结周围出现一个没有自由电荷载流子的区域，并将这一区域称为耗尽区。n 侧和 p 侧耗尽区内的电荷密度可以用以下公式来进行表示：
<img alt="Equation 1" src="/files/ftp_upload/15156/15156_Equation_1.svg" style="text-align: center; height: 20px;">
其中，q 为基本电荷，N_D 和 N_A 分别为供体和受体的掺杂浓度。
耗尽区中的固定电荷产生一个从 n 侧指向 p 侧的电场（E），以此来阻止载流子进行进一步扩散。该电场会在结点处产生一个电位差，称为内建电压 （V_0），可通过以下公式来进行计算：
<img alt="Equation 2" src="/files/ftp_upload/15156/15156_Equation_2v2.svg" style="text-align: center; height: 40px;">
其中，V_T 为热电压，n_i 为本征载流子浓度。
p-n 结中存在两种类型的电流：由于载流子扩散而产生的扩散电流和由于电场而产生的漂移电流。在平衡状态下，扩散电流的大小等于漂移电流的大小，从而导致结点处没有净电流。在开路条件下则没有外部电流，耗尽层的内置电压则会平衡金属-半导体结点处的接触电位，从而导致两端的净电压为零。
内置势垒电压和耗尽区的宽度对结点的行为发挥着至关重要的作用。耗尽区的宽度决定了结点的电容，并且还影响着结点对外部电压的响应方式。

太阳能路灯依靠其太阳能电池中的 p-n 结，可独立运行，无需外部电源供电。  
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JoVE Core

Electrical Engineering

电气工程

Unit 1

第一单元

Basics of Semiconductors

KEY TERMS AND CONCEPTS

关键术语与概念

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

energy bands in solids

Valence and Conduction Bands in Silicon

Valence and conduction bands explain how electrons behave in solids. In isolated atoms, electrons occupy discrete energy levels that the Bohr model describes well. The hydrogen atom, for example, has specific electron energies labeled by the quantum number n, and higher n values give levels that are less negative and closer together. When atoms move close together in a solid, those discrete levels start to split. This happens because electron orbitals from neighboring atoms overlap. The Pauli...

semiconductors

Energy Bands in Metals and Insulators

Materials conduct electricity in different ways because their energy bands are arranged differently. Metals, semiconductors, and insulators can be compared with energy band diagrams. These diagrams show how easily electrons can move when an electric field is applied. Metals such as copper, zinc, and lead have low resistivity. Their conduction band is not fully occupied, or it overlaps with the valence band, so a band gap is essentially absent. Electrons in the highest valence energy levels can...

types of semiconductors

Intrinsic and Doped Semiconductors

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carrier generation and recombination

Semiconductor Charge Carriers: Creation and Loss

Carrier generation and recombination control how free charge carriers change inside a semiconductor. Carrier generation creates electron-hole pairs, or EHPs, when electrons move from the valence band into the conduction band and leave holes behind. Carrier recombination does the opposite and lowers the number of free carriers. In direct-bandgap semiconductors such as gallium arsenide, generation happens efficiently because energy absorption can move a valence electron straight to the...

carrier transport

Drift and Diffusion in Semiconductors

Drift and diffusion are the two main ways electric current forms in semiconductors. These charge-carrier motions help determine how semiconductor devices work and how well they perform. The carriers involved are electrons and holes. Drift current begins when an external electric field acts on charge carriers. Electrons and holes move between collisions with lattice atoms, and this motion creates drift velocity. For electrons, drift velocity depends on electron mobility and the electric field...

p-n junction

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...

biasing of p-n junction

P-N Junction Diode Under Bias

P-N junction diodes change behavior under equilibrium, forward bias, and reverse bias. In equilibrium, no external voltage is applied across the junction. Carriers diffuse at the boundary and leave behind charged dopants. Acceptors remain on the p-side, and donors remain on the n-side. These fixed charges form a depletion region and an electric field that blocks further diffusion. The band diagram at equilibrium shows the Fermi levels on both sides aligned. That alignment means the junction is...

metal-semiconductor junctions

Schottky and Ohmic Metal Contacts

Metal contacts with semiconductors can form either Schottky or Ohmic junctions. The result depends on the work function of the metal, the work function of the semiconductor, and how electrons move when the two materials touch. At equilibrium, the Fermi levels align, so charge shifts until both sides match in energy. Schottky barriers form when a metal with work function Φm contacts a semiconductor with a different work function Φs. If Φm is greater than Φs, the semiconductor Fermi level starts...

biasing of metal-semiconductor junctions

How Bias Changes Metal-Semiconductor Current

Biasing metal-semiconductor junctions means applying a voltage across the junction. The metal connects to the voltage source, and the semiconductor is grounded. This setup helps control the direction and size of current in devices such as diodes, transistors, and photovoltaic cells. In a Schottky junction with an n-type semiconductor, a positive voltage on the metal lowers its Fermi level. A Fermi level is the energy level that helps describe how electrons are distributed. Lowering this level...

fermi level

Fermi Level in Semiconductors

The Fermi level is a key energy level in semiconductors. It sits between the lower-energy valence band and the higher-energy conduction band. The Fermi-Dirac function, which follows an S-shaped curve, shows the chance that an energy state is filled by an electron at a given temperature. At absolute zero, electrons occupy every energy state up to the Fermi level. States above it stay empty. As temperature rises, electrons gain energy and can move into vacant states above the Fermi level. In an...

fermi level dynamics

Vacuum Level and Schottky Barriers

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key terms and concepts

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

P-N Junction Depletion Region | JoVE K12

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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