Oxidation and diffusion
In regions where the impurity concentration far exceeds the intrinsic carrier density, the diffusion coefficient of arsenic and others varies with concentration. Which statement about this is correct?
- AAt high concentration, the diffusion coefficient grows with concentration, for example because more electrons increase the population of charged point defects
- BAt high concentration, impurities stop diffusing altogether
- CThe diffusion coefficient is always constant regardless of concentration
Show answer and explanation
Answer: A. At high concentration, the diffusion coefficient grows with concentration, for example because more electrons increase the population of charged point defects
With high impurity concentrations, the Fermi level shifts and the concentrations of charged vacancies and interstitials increase, and internal electric fields also arise, so the diffusion coefficient becomes concentration-dependent and larger. High-concentration profiles therefore take a box-like shape with a steeper front than simple erfc or Gaussian profiles.
- A. Correct. More charged point defects and similar factors increase the diffusion coefficient.
- B. Diffusion still occurs at high concentration.
- C. At high concentration, the diffusion coefficient depends on concentration.