What is an engineered substrate?

The term "engineered substrate" has different meanings depending on the context. For semiconductor device manufacturing it usually refers to a special type of substrate upon which epitaxy is carried out to create a device epiwafer. The special type in this case is that it is neither a bulk or free-standing substrate such as GaN or SiC or sapphire, nor is it a simple template substrate such as that of a GaN on sapphire template. One example of an engineered substrate is that of a thin crystalline layer of GaAs, InP, or GaN that is bonded on top of a silicon, sapphire, ceramic, or metal substrate. Considerations into engineered substrate design includes that the base material may be chosen for its physical properties (diameter, thermal expansion coefficient, thermal conductivity) and low cost while the upper region may be chosen primarily for its ability to support high quality epitaxy.