my onion, radish, carrot bed is what I call semi raised. I used OSB scraps 4 feet long and 12 inches wide for the sides, and 6 feet long 12 inches wide for one end. The other end is uphill enough to be "ground level". With this arrangement I have noticed that the higher raised end (down hill end) does indeed warm up quicker, but also dries out quicker. I am putting more mulch (straw) on that end than the up hill or ground level end, for moisture control. Just something to consider.
Keith