  We investigate the transport properties of a new class of ratchets. The
device is constructed by applying an ac voltage to the metallic single electron
tunneling transistor, and a net transport current is induced by the
time-dependent bias-voltage, although the voltage value is on the average zero.
The mechanism underlying this phenomenon is the Coulomb blockade of the single
electron tunneling. The directions and the values of the induced net currents
can be well controlled by the gate-voltage. A net transport current has also
been observed even in the absence of the external bias-voltages, which is
attributed to the noise in the circuit.
