An MOCVD reactor is a chamber made of a material that does not react with the chemicals being used. It must also withstand high temperatures. This chamber is composed of reactor walls, a liner, a susceptor, gas injection units, and temperature control units.
In the MOCVD reactor, the substrate is supported by a pedestal, which also acts as a susceptor. The pedestal/susceptor is the primary origin of heat energy in the reaction chamber. Only the susceptor is heated, so gases do not react before they reach the hot wafer surface. The reaction chamber walls in a cold-wall reactor, however, may be indirectly heated by heat radiating from the hot pedestal/susceptor, but will remain cooler than the pedestal/susceptor and the substrate the pedestal/susceptor supports. So Boron Nitride is a good choice because the power density, operating temperature, insulation grade and dielectric strength of heating elements can be greatly improved by using Boron Nitride as an insulator. It is also more economical and reduces thermal inertia. The annular shape reduces the size of the heating system and improves heat transfer.