When SiC wafer is used as the substrate of RF devices, it is required that SiC should be semi insulating and its resistivity should be greater than 10 ^ 6 Ω· cm. In fact, the resistivity of silicon carbide should be very high, but due to impurities and other defects, the resistivity of silicon carbide has decreased.
According to the principle of increasing resistivity, there are two types of semi insulating SiC, one is doping semi insulation,which is further introduced impurity V to compensate, another one is high purity semi insulation. However, vanadium doped semi insulating silicon carbide has two disadvantages:
- Back gate effect: when the size of the device is small to a certain extent, the vanadium free area is actually conductive, resulting in non insulation of the device;
- High temperature failure:When the temperature is 1000 ℃, the substrate will become conductive.
Therefore, though V doped semi-insulating SiC substrate is ok, in some application, it still need HPSI substrate.
However, the preparation of high purity semi insulating silicon carbide needs more technology. For example, the crucible structure is controlled. Three holes with a diameter of 1-4mm can be opened on the crucible, so that impurities n, B, Al and V can overflow from the holes during the heating process.