One aspect of the present invention is a double sided hybrid crystal structure including a trigonal Sapphire wafer containing a (0001) C-plane and having front and rear sides. The Sapphire wafer is substantially transparent to light in the visible and infrared spectra, and also provides insulation with respect to electromagnetic radio frequency noise. A layer of crystalline Si material having a cubic diamond structure aligned with the cubic <111> direction on the (0001) C-plane and strained as rhombohedron to thereby enable continuous integration of a selected (SiGe) device onto the rear side of the Sapphire wafer. The double sided hybrid crystal structure further includes an integrated III-Nitride crystalline layer on the front side of the Sapphire wafer that enables continuous integration of a selected III-Nitride device on the front side of the Sapphire wafer.

A FET based double balanced mixer (DBM) that exhibits good conversion gain and IIP3 values and provides improved linearity and wide bandwidth. In one embodiment, a first balun is configured to receive a local oscillator (LO) signal and generate two balanced LO signals that are coupled to two corresponding opposing nodes of a four-node FET ring. A second balun is configured to pass an RF signal on the unbalanced side. The FET ring includes at least four FETs connected as branches of a ring, with the source of each FET connected to the drain of a next FET in the ring. Each FET is preferably fabricated as, or configured as, a low threshold voltage device having its gate connected to its drain, which causes the FET to operate as a diode, but with the unique characteristic of having close to a zero turn-on voltage.

Provided is a composite substrate which has a high-performance semiconductor layer. A composite substrate of the present invention comprises: a supporting substrate which is formed of an insulating material; a semiconductor layer which is formed of a single crystal semiconductor that is superposed on and joined to the supporting substrate; and interfacial inclusions which are present in the interface between the supporting substrate and the semiconductor layer at a density of 10.sup.12 atoms/cm.sup.2 or less, and which are formed of a metal element that is different from the constituent elements of the supporting substrate and the semiconductor layer.

A method for producing SOS substrates which can be incorporated into a semiconductor production line, and is capable of producing SOS substrates which have few defects and no variation in defects, and in a highly reproducible manner, or in other words, a method for producing SOS substrates by: forming an ion-injection region (3) by injecting ions from the surface of a silicon substrate (1); adhering the ion-injection surface of the silicon substrate (1) and the surface of a sapphire substrate (4) to one another directly or with an insulating film (2) interposed therebetween; and then obtaining an SOS substrate (8) having a silicon layer (6) on the sapphire substrate (4), by detaching the silicon substrate in the ion-injection region (3). This method is characterized in that the orientation of the sapphire substrate (4) is a C-plane having an off-angle of 1 degree or less.