Epitaxial growth

In bipolar transistors and similar devices, what is the main purpose of a heavily doped layer (buried layer) beneath the epitaxial layer?

  1. ATo lower the collector series resistance
  2. BTo make the base thicker
  3. CTo make the epi layer highly resistive
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Answer: A. To lower the collector series resistance

In a vertical bipolar transistor, the collector current flows laterally under the epi layer toward its contact. Forming a heavily doped n⁺ layer in the substrate before epitaxy, which then becomes buried, lowers the resistance of this path and improves the saturation voltage and high-frequency performance.

  • A. Correct. It is a heavily doped layer that lowers collector resistance.
  • B. It is unrelated to base thickness.
  • C. The epi layer's resistivity is set by doping during growth.
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More Epitaxial growth questions

  1. Which is closest to the typical growth temperature for SiC epitaxy (CVD)?
  2. Which type of reactor is mainstream for silicon epitaxial growth on 300 mm wafers?
  3. In heteroepitaxy of materials with very different lattice constants, what is the purpose of inserting a "graded buffer layer" whose composition changes gradually?
  4. In the X-ray diffraction rocking curve used to evaluate the crystal quality of an epitaxial layer, what does a narrow peak width (FWHM) indicate?
  5. In MOCVD growth of GaN, what does the controlled condition "V/III ratio" mean?