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I am currently studying the textbook Light-Emitting Diodes (3rd Edition) by E. Fred Schubert. Chapter 1.3 Oleg Lossev's research on SiC LEDs says the following:

The study of a 1933 publication (Lossev, 1933) leaves little doubt that Lossev indeed performed experiments on p-n junctions and not just metal-semiconductor junctions. Tapered grinding of a SiC crystal surface allowed Lossev (1933) to attribute specific voltage drops to the (i) metal-semiconductor contacts, (ii) semiconductor bulk crystal, and (iii) pn-junction region (active region). Lossev (1933) concluded that the voltage drop (i) across the metal-semiconductor contacts is small, (ii) across the semiconductor bulk crystal is small, (iii) across the pn-junction region depends on the polarity of the applied voltage and is large for reverse-biased junctions and small for forward-biased junctions. This undoubtedly is the signature of a p-n junction. In addition, Lossev reported the emission of light from the pn junction region. Lossev calls this region the “active layer”.

What is the difference between the "bulk semiconductor" and the "active layer/region"?

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Active region is the p-n junction (or other type of junction/structure) where the interesting stuff happens. Bulk semiconductor is the part of the semiconductor far away from the junction, where it can be viewed as an effectively infinite semiconductor crystal. In some contexts it may also mean an equivalent semiconductor with such properties.

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  • $\begingroup$ Thanks for the answer. But, ultimately, the material at the active region is the same material in the bulk semiconductor? Or is the material actually different between these two regions? $\endgroup$ Feb 2, 2021 at 15:24
  • $\begingroup$ It is alose to the border, so its properties are modified - e.g., the band is bent, there is the depletion layer, etc. I am not sure on what level you are... much of the semiconductor theory assumes an infinite crystal, which determines most of its electronic properties. While far away from the borders it can still be viewed as infinite, it is a bad approximation near borders/junctions. $\endgroup$
    – Roger V.
    Feb 2, 2021 at 15:27
  • $\begingroup$ Ok, I understand. Thanks for the clarification. $\endgroup$ Feb 2, 2021 at 15:29

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