Why only odd harmonics in an HBV (Heterojunction Barrier Varactor)?

Quick question that the seasoned EEs will be able to answer pretty easily. Why does an HBV only generate odd harmonics? It is essentially several capacitors in series. I cannot rationalize in my head why this setup would only generate odd harmonics.

Any ideas?

(I’mg guessing this thread sinks like a rock, but it’s worth a shot)

Clipping generates odd harmonics?