Oxidation and diffusion

Why can SiO2 serve as a diffusion mask against boron and phosphorus but works poorly as a mask against gallium and aluminum?

  1. ABecause SiO2 reacts with gallium and aluminum to become a conductor
  2. BBecause gallium and aluminum diffuse very fast in SiO2
  3. CBecause gallium and aluminum are electrically inactive in silicon
Show answer and explanation

Answer: B. Because gallium and aluminum diffuse very fast in SiO2

Boron and phosphorus diffuse much more slowly in SiO2 than in silicon, so an oxide can block them. Gallium and aluminum, however, diffuse quickly through SiO2, making them unsuitable for selective diffusion using an oxide mask. This is one reason boron is the main p-type dopant in silicon.

  • A. They do not react to form a conductor.
  • B. Correct. They diffuse fast in SiO2, so it cannot block them.
  • C. Gallium and aluminum also act as acceptors in silicon.
Practice this topic Back to topic

More Oxidation and diffusion questions

  1. 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?
  2. Which correctly describes the thermal oxidation of heavily doped polycrystalline silicon (polysilicon)?
  3. Which correctly compares dry oxidation (O2) and wet oxidation (H2O) in the thermal oxidation of silicon?
  4. When an SiO2 film of thickness x is formed by thermally oxidizing silicon, approximately how much silicon is consumed?
  5. How does the diffusion coefficient of dopants in silicon depend on temperature?