Using the MRFE6VS25GNR1 transistor for driving 11.5 MHz is a suitable choice. However, when it comes to applying thermal paste, it is important to use the appropriate amount. Applying too much thermal paste (DOW 340) can indeed cause issues.
Excessive thermal paste can lead to increased thermal resistance, which may result in overheating and potential failure of the transistor. It is recommended to apply a thin, even layer of thermal paste to ensure optimal heat transfer between the transistor and the heat sink.
For more information on the proper application of thermal paste and other best practices, you can refer to the following application note on the community.nxp.com website: https://www.nxp.com/docs/en/application-note/AN1955.pdf
