A semiconductor device comprises a substrate (11) having an insulating
layer (12) formed on a surface thereof, and a silicon layer (13) located
on a surface of the insulating layer. A trench (14) extends from a surface
of the silicon layer (13) through the insulating layer (12) and into the
substrate (11). An insulating liner (14a) is located on the side walls and
the base of the trench (14), and an in-fill (14b) of thermally-conductive
material is formed within the insulating liner. The insulating liner
(14a), the in-fill material (14b) and the distance over which the trench
14) extends into the substrate (11) are such as to promote flow of heat
from the silicon layer (13) to the substrate.
