  Intrinsic limits to temperature-dependent substrate loss for GaN-on-Si
technology, due to the change in resistivity of the substrate with temperature,
are evaluated using an experimentally validated device simulation framework.
Effect of room temperature substrate resistivity on temperature-dependent CPW
line loss at various operating frequency bands are then presented. CPW lines
for GaN-on-high resistivity Si are shown to have a pronounced
temperature-dependence for temperatures above 150{\deg}C and have lower
substrate losses for frequencies above the X-band. On the other hand,
GaN-on-low resistivity Si is shown to be more temperature-insensitive and have
lower substrate losses than even HR-Si for lower operating frequencies. The
effect of various CPW geometries on substrate loss is also presented to
generalize the discussion. These results are expected to act as a benchmark for
temperature dependent substrate loss in GaN-on-Si RF technology.
