A silicon-on-insulator (SOI) substrate includes a silicon substrate including an active region defined by a field region that surrounds the active region for device isolation. The field region includes a first oxygen-ion-injected isolation region and a second oxygen-ion-injected isolation region. The first oxygen-ion-injected isolation region has a first thickness and is disposed under the active region, a center of the first oxygen-ion-injected isolation region being at a first depth from a top surface of the silicon substrate. The second oxygen-ion-injected isolation region has a second thickness that is greater than the first thickness, the second oxygen-ion-injected isolation region disposed at sides of the active region and formed from a ton surface of the silicon substrate, a center of the second oxygen-ion-injected region disposed at a second depth from the top surface of the silicon substrate.