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Monolithic active pixel detectors fabricated in SOI (silicon on insulator) technology are novel sensors of ionizing radiation which exploit SOI substrates for the integration of readout electronics with a pixel detector. The fully-depleted sensing diode is manufactured under the SOI buried oxide (BOX) in the volume of the `handle wafer', while readout circuitry occupies the upper silicon SOI `device layer'. However, the initial prototypes suffered from significant leakage currents and early breakdown of pixel diodes. These effects limited the production yield. Moreover, the p-wells within the device layer extended to the interface with the BOX, which caused some local potential wells below the BOX at the top of the depleted sensor area. As a result, the effective charge collection efficiency was significantly decreased. This paper describes a solution for these problems employing a thicker 4 Â¿m SOI device layer. The use of this detector structure has increased the effective charge collection efficiency to over 99%.