  We discuss the operation of the superconductor - insulator - normal-metal -
insulator - superconductor (SINIS) turnstile. This voltage-biased hybrid
single-electron transistor (SET) provides current quantization even with only
one radio-frequency (rf) control parameter, namely the gate voltage of the
single island. We give an overview of the main error mechanisms of the
turnstile and consider its feasibility as a quantum current standard. We also
present experimental results of pumping with the SINIS structure which show
decreased leakage current compared to earlier measurements with the opposite
NISIN structure.
