A monolithically integrated CMOS and MEMS device. The device includes a first semiconductor substrate having a first surface region and one or more CMOS IC devices on a CMOS IC device region overlying the first surface region. The CMOS IC device region can also have a CMOS surface region. A bonding material can be provided overlying the CMOS surface region to form an interface by which a second semiconductor substrate can be joined to the CMOS surface region. The second semiconductor substrate has a second surface region coupled to the CMOS surface region by bonding the second surface region to the bonding material. The second semiconductor substrate includes one or more first air dielectric regions. One or more free standing MEMS structures can be formed within one or more portions of the processed first substrate.