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Chapter 14: Problem 8

Explain the meaning of intrinsic and extrinsic diffusion.

Short Answer

Expert verified
Intrinsic diffusion refers to the spontaneous movement of atoms, ions, or molecules within a material due to thermal vibrations or random motion, governed by factors such as temperature, material composition, and crystal structure. Extrinsic diffusion, on the other hand, is the movement of impurities, dopants, or defects within a material due to an applied external force or gradient, such as an electric field, a chemical potential gradient, or a stress field. Factors affecting extrinsic diffusion include the external force or gradient, material properties, and the presence of impurities or defects. Examples of these phenomena include self-diffusion in a solid metal (intrinsic diffusion) and the doping process in semiconductor materials (extrinsic diffusion).

Step by step solution

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1. Defining intrinsic diffusion

Intrinsic diffusion refers to the spontaneous movement of atoms, ions, or molecules within a material due to thermal vibrations or random motion. This process is a natural property of the material, meaning it occurs without any external forces or gradients applied. Intrinsic diffusion is influenced by factors such as temperature, material composition, and crystal structure.
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2. Defining extrinsic diffusion

Extrinsic diffusion is the movement of impurities, dopants, or defects within a material due to an applied external force or gradient, such as an electric field, a chemical potential gradient, or a stress field. In this case, the diffusion is not an inherent property of the material; rather, it is driven by the external force or gradient. Extrinsic diffusion can be used in various processes, such as doping semiconductor materials or altering material properties.
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3. Factors affecting intrinsic diffusion

Intrinsic diffusion is governed by the following factors: - Temperature: At higher temperatures, the atoms, ions, or molecules in a material have greater kinetic energy, leading to faster diffusion rates. - Material composition: Some materials naturally have higher diffusion rates due to their composition, molecular structure, or atomic arrangement. - Crystal structure: In crystalline materials, the atomic arrangement (e.g., face-centered cubic, body-centered cubic) influences the available diffusion pathways and, subsequently, the diffusion rate.
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4. Factors affecting extrinsic diffusion

Extrinsic diffusion is influenced by the following factors: - External force or gradient: The strength and direction of the applied force or gradient determine the extent to which diffusion occurs. - Material properties: Some materials are more susceptible to extrinsic diffusion due to their inherent properties, such as electrical conductivity, mechanical properties, or chemical reactivity. - Impurities or defects: The presence, concentration, and distribution of impurities or defects within a material can also influence the extrinsic diffusion rate.
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5. Examples of intrinsic and extrinsic diffusion

Intrinsic diffusion example: In a solid metal, atoms can move within the crystal lattice even at relatively low temperatures, due to the natural thermal vibrations of the lattice. This process is called self-diffusion and occurs without any external forces or gradients applied. Extrinsic diffusion example: In the process of doping a semiconductor material, such as silicon, an external force is applied to introduce impurity atoms (e.g., phosphorus or boron) into the silicon crystal lattice. This results in controlled extrinsic diffusion, which changes the electrical properties of the semiconductor and enables the fabrication of various electronic devices.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Intrinsic Diffusion
Intrinsic diffusion is the natural spreading process of particles within a material. Imagine a dance where atoms move subtly and spontaneously inside a solid, with no help from the outside. This movement is mainly due to thermal vibrations. As the temperature rises, atoms jiggle more vigorously, leading to a faster diffusion rate.
Understanding intrinsic diffusion is crucial in materials science because it helps predict material behaviors, like wear and phase transformations.
Some key factors affecting intrinsic diffusion include:
  • Temperature: Higher temperatures lead to increased kinetic energy, enabling atoms to move more freely.
  • Material Composition: Certain materials have higher diffusion rates due to their unique atomic structures or bonding.
  • Crystal Structure: In metals, for example, the arrangement of atoms allows varied diffusion paths, affecting the overall diffusion rate.
Extrinsic Diffusion
Extrinsic diffusion, unlike its intrinsic counterpart, is heavily influenced by external factors or forces. An example is when electronics manufacturers introduce impurities into semiconductors, known as doping. Here, an external gradient鈥攍ike an electric field鈥攄rives the movement of particles.
It's a crafted process, where manufacturers control diffusion to alter material properties effectively.
Factors influencing extrinsic diffusion include:
  • External Forces or Gradients: These could be electric fields, stress, or concentration gradients applied to the material.
  • Material Properties: Some materials are more susceptible due to their inherent electrical or mechanical properties.
  • Presence of Impurities or Defects: This affects how easily and quickly atoms can move.
Semiconductor Doping
Semiconductor doping is a method used to change the electrical properties of a semiconductor. By introducing impurities, such as phosphorus or boron, into a silicon crystal, manufacturers can control its conductivity.
This adjustment is crucial for creating components like diodes and transistors, which are the backbone of modern electronics.
Without doping, semiconductors wouldn't be able to serve diverse purposes in creating circuits and devices.
  • P-type Doping: Involves adding elements like boron to create more positive charges or "holes."
  • N-type Doping: Involves using elements like phosphorus to add negative charge carriers or electrons.
Thermal Vibrations
Thermal vibrations refer to the natural jiggling movements of atoms in a material due to heat energy. This energy causes vibrations, leading atoms to move and bump into each other.
It's a key player in intrinsic diffusion, as it accounts for the random motion of particles, even when no external forces are applied.
As temperature increases, so do these vibrations, enhancing the material's diffusion capabilities and altering its behavior.
  • Impact on Diffusion: Increased thermal vibrations mean higher diffusion rates.
  • Material Behavior: At elevated temperatures, materials can exhibit changed mechanical and electrical properties due to increased atomic activity.
Crystal Structure
The crystal structure is the specific arrangement of atoms within a solid. It plays a significant role in how diffusion occurs in a material. For example, different lattice types like face-centered cubic (FCC) or body-centered cubic (BCC) offer various pathways for atoms to move.
This arrangement influences properties such as strength, ductility, and diffusion rates in metals.
Understanding crystal structures is essential in engineering materials for specific purposes, ensuring they perform well under desired conditions.
  • Lattice Type: FCC and BCC are common structures in metals, each affecting diffusion differently.
  • Influence on Properties: The arrangement of atoms determines mechanical properties and reactivity of materials.

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Most popular questions from this chapter

We would like to form \(0.1 \mu \mathrm{m}\) deep, heavily doped junctions for the source and drain regions of a submicron MOSFET. Compare the options that are available to introduce and activate dopant for this application. Which option would you recommend and why?

If arsenic is diffused into a thick slice of silicon doped with \(10^{15}\) boron atoms \(/ \mathrm{cm}^{3}\) at a temperature of \(900^{\circ} \mathrm{C}\) for 3 hours, what is the final distribution of arsenic if the surface concentration is held fixed at \(4 \times 10^{18}\) atoms \(/ \mathrm{cm}^{3}\) ? What is the junction depth? Assume \(D \quad D_{0} e^{\frac{E a}{k T}} \times \frac{n}{n_{i}}, D_{0} \quad 45.8 \mathrm{~cm}^{2} / \mathrm{s}, E a \quad 4.05 \mathrm{eV}, x_{j} \quad 1.6 \sqrt{D t}\)

If a \(50 \mathrm{keV}\) boron ion is implanted into the silicon substrate, calculate the damage density. Assume silicon atom density is \(5.02 \times 10^{22}\) atoms/cm \(^{3}\), the silicon displacement energy is \(15 \mathrm{eV}\), the range is \(2.5 \mathrm{~nm}\), and the spacing between silicon lattice planes is \(0.25 \mathrm{~nm}\).

Calculate the junction depth and the total amount of dopant introduced after boron predeposition performed at \(950^{\circ} \mathrm{C}\) for 30 minutes in a neutral ambient. Assume the substrate is \(n\)-type silicon with \(N_{D}=1.8 \times 10^{16} \mathrm{~cm}^{-3}\) and the boron surface concentration is \(C_{S}=1.8 \times 10^{20} \mathrm{~cm}^{-3}\).

Assume that a \(100 \mathrm{~mm}\) diameter GaAs wafer is uniformly implanted with \(100 \mathrm{keV}\) zinc ions for 5 minutes with a constant ion beam current of \(10 \mu \mathrm{A}\). What are the ion dose per unit area and the peak ion concentration?
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