  Filamentous cable bacteria exhibit unprecedented long-range biological
electron transport, which takes place in a parallel fibre structure that shows
an extraordinary electrical conductivity for a biological material. Still, the
underlying electron transport mechanism remains undisclosed. Here we determine
the intrinsic electrical properties of individual cable bacterium filaments. We
retrieve an equivalent electrical circuit model, characterising cable bacteria
as resistive biological wires. Temperature dependent experiments reveal that
the charge transport is thermally activated, and can be described with an
Arrhenius-type relation over a broad temperature range (-196{\deg}C to
+50{\deg}C), thus excluding metal-like electron transport. Furthermore, when
cable bacterium filaments are utilized as the channel in a field-effect
transistor, they show n-type transport, indicating that electrons rather than
holes are the charge carriers. Electron mobilities are in the order of
10$^{-1}$ cm$^2$/Vs, comparable to many organic semiconductors. This new type
of biological centimetre-range semiconductor with low resistivity offers new
perspectives for both fundamental studies and applications in (bio)electronics.
