Semiconductor devices having a silicon-germanium channel layer and methods of forming the semiconductor devices are provided. The methods may include forming a silicon-germanium channel layer on a substrate in a peripheral circuit region and sequentially forming a first insulating layer and a second insulating layer on the silicon-germanium channel layer. The methods may also include forming a conductive layer on the substrate, which includes a cell array region and the peripheral circuit region, and patterning the conductive layer to form a conductive line in the cell array region and a gate electrode in the peripheral circuit region. The first insulating layer may be formed at a first temperature and the second insulating layer may be formed at a second temperature higher than the first temperature.