Using multiple "by"s in a compound sentence

Question

Which one is correct in the following sentences?

Due to the peculiar device operation, the avalanche can be triggered not only by photogenerated carriers but also by carriers thermally generated or by trapping levels in the semiconductor.

Due to the peculiar device operation, the avalanche can be triggered not only by photogenerated carriers but also carriers thermally generated or trapping levels in the semiconductor.

Do I need to use the by that many times in a compound sentence?

Anton · Answer

To avoid confusion and ambiguity I would keep the "by".
How can the avalanche be triggered? There are three possibilities:
by photogenerated carriers;
by carriers thermally generated;
by trapping levels in the semiconductor.
If "by" is omitted, we might understand a fourth and erroneous possibility:
by carriers thermally generated or trapping levels in the semiconductor. (The non-technical reader may wrongly imagine carriers that are either thermally generated in the semiconductor or are somehow "trapping levels" in it.)

Using multiple "by"s in a compound sentence

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