  A semiclassical formalism is used to investigate the transistor-like behavior
of ultracold atoms in a triple-well potential. Atom current flows from the
source well, held at fixed chemical potential and temperature, into an empty
drain well. In steady-state, the gate well located between the source and drain
is shown to acquire a well-defined chemical potential and temperature, which
are controlled by the relative height of the barriers separating the three
wells. It is shown that the gate chemical potential can exceed that of the
source and have a lower temperature. In electronics terminology, the
source-gate junction can be reverse-biased. As a result, the device exhibits
regimes of negative resistance and transresistance, indicating the presence of
gain. Given an external current input to the gate, transistor-like behavior is
characterized both in terms of the current gain, which can be greater than
unity, and the power output of the device.
