Capacitance-distance (C-s) and capacitance-voltage (C-V) characteristics have been measured for the tip-sample capacitance on Si(001)2 x 1 and Si(111)7 x 7 surfaces. On both surfaces, the C-s characteristics are found to be little affected by the types of sample doping and also show no appreciable bias dependence. These observations suggest that the space-charge region of the semiconductor gives no contribution to the tip-sample capacitance because of the high density of surface states on clean Si surfaces.