A high mobility (3.6 x 10(5) cm(2)/V a) n-type Si/Si1-xGex heterostructure has been Schottky gated using An metal to cover the whole of the device. Over the front-gate voltage (V-g) range for which there was negligible gate leakage (-0.1 V < V-g < 0.1 V), the carrier density could be varied from 1.7 x 10(11)to 4.4 x 10(11) cm(-2). Measurements of the quantum lifetime suggested that the quality of the device varied in a manner not indicated by the transport mobility. This may arise from inhomogeneities caused by energy-level broadening. The results can be explained by a surface-roughened Au gate whose influence is negated at zero front-gate voltage due to a frozen-in screening charge in the doping layer.