BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing a general configuration of a power transistor; FIGS. 2A and 2B are a plan view and a partial sectional view, respectively, of a CBC substrate disclosed in Japanese Unexamined Patent Publication No. Hei. 3-145748; FIGS. 3A and 3B are graphs respectively showing a dependence on the amount of added zirconia of the heat conductivity