Do we have to use heatsink above the GaN FET?
You can use a heatsink to take advantage of the top side cooling features of EPC GaN FETs. We have almost same thermal resistance of junction to board as MOSFET, and we have very low thermal resistance of junction to top side, so top side cooling is an advantage of EPC GaN FETs. This link: https://epc-co.com/epc/DesignSupport.aspx provides a resource for many design topics and there is a section on thermal management. The top side of our eGaN is Si structure and will be same voltage level as source terminal, so you need TIM to provide electrical insulation. There is information on the thermal interface material (TIM) between GaN FET and heatsink in How to Get More Power Out of a High-Density eGaN-Based Converter with a Heatsink (How2AppNote012). For additional information on heatsinking, i recommend the webinar Thermal Management of GaN FETs