Neither R_th(ch-a) nor R_th(j-a) is specified for MOSFETs, IGBTs and bipolar transistors. Why is that?

Since it is common to install heat sinks in self-standing power devices, we are offering R_th(ch-c) and R_th(j-c). (There are exceptions in some products.)
Channel-to-case (or junction-to-case) thermal resistance values should be used for more accurate thermal calculations instead of channel-to-ambient (or junction-to-ambient) thermal resistance values. Be sure to use R_th(ch-c) or R_th(j-c) if it is specified.

Thermal Design and Attachment of a Thermal Fin: Power MOSFET Application Notes (PDF:1,061KB)

• https://toshiba.semicon-storage.com/info/docget.jsp?did=13417