Active pixel sensors are defined on double silicon on insulator (SOI) substrates such that a first silicon layer is selected to define radiation detection regions, and a second silicon layer is selected to define readout circuitry. The first and second silicon layers are separated by an insulator layer, typically an oxide layer, and the layers can be independently doped. Doping can be provided in the silicon layers of the SOI substrate during assembly of the SOI substrate, or later during device processing. A semiconductor substrate that supports the first and second layers can be removed for, for example, back side radiation detection, using a second insulator layer (typically an oxide layer) as an etch stop.