Intel R 82801er Sata Raid Controller Driver
You need a driver for that controller, as the driver provides the RAID functionality. I don't think your going to find a driver for 2012 or 2012 R2. The controller is a software based raid controller. Your best option is to just keep the contrioller in SATA mode and use the RAID function of your OS
Intel r 82801er sata raid controller driver
The controller has options for reparing/replacing failed mirror, but this isn't an option without a driver. I'm not familiar with how Windows handles a disk failure when Windows is providing the RAID services.
I'll start with why you cannot use AHCI mode without modification. When Windows is installed, it only installs (enables) AHCI/RAID drivers if you have a storage controller it recognises as AHCI/RAID. Windows Vista and 7 would (usually) have the drivers anyway, but typically disable them. Vista and 7 come with a generic AHCI driver, with a more hardware-specific driver potentially available from the controller manufacturer. A manufacturer provided driver is necessary for 'FakeRAID'. The generic AHCI driver can be enabled with two quick, easy registry modifications.
Motherboard manufacturers often include an AHCI driver on a CD or on their websites. This driver is provided by the manufacturer of the storage controller, typically Intel on systems with an Intel CPU. Some other candidates are nVidia (nForce) and Marvell (on some AMD boards). They are generally not interchangeable.