  High-$T_{\rm C}$ superconductors show anomalous transport properties in their
normal states, such as the bad-metal and pseudogap behaviors. To discuss their
origins, it is important to speculate whether these behaviors are
material-dependent or universal phenomena in the proximity of the Mott
transition, by investigating similar but different material systems. An organic
Mott transistor is suitable for this purpose owing to the adjacency between the
two-dimensional Mott insulating and superconducting states, simple electronic
properties, and high doping/bandwidth tunability in the same sample. Here we
report the temperature dependence of the transport properties under electron
and hole doping in an organic Mott electric-double-layer transistor. At high
temperatures, the bad-metal behavior widely appears except at half filling
regardless of the doping polarity. At lower temperatures, the pseudogap
behavior is observed only under hole doping, while the Fermi-liquid-like
behavior is observed under electron doping. The bad-metal behavior seems a
universal high-energy scale phenomenon, while the pseudogap behavior is based
on lower energy scale physics that can be influenced by details of the band
structure.
