Actually, it is wrong to call it a raw 'filesystem'. The moment a filesystem is created on a device, it ceases to be raw. A raw device is basically a character/block disk device on which no filesystem has been created.

The process and commands involved in creating a raw device will differ for each OS. On Solaris, for instance, you just have to create a new slice on a disk and then label the disk. In HP-UX, you have to create a new volume in LVM.