High resistivity silicon with d.c. conductivity levels (σdc) of 10-4 (Ωcm)-1 at 300 K has been identified as a low dielectric loss material at mm-wavelengths [1] and was also proposed as an alternative window material for high power gyrotrons because the dielectric loss tangent (tanδ) falls well below that of sapphire, the presently used material, in the 100 GHz range [2]. A major increase, however, is expected for the power levels that can be transmitted through cryogenically cooled sapphire windows because with decreasing temperature T, a strong reduction of tanδ is observed by different authors [3,4] apart from other beneficial trends in operation relevant thermophysical parameters. For Si, tanδ at 77 K was found to be at similar level or even above that at 300 K [5]. A more detailed data set is presented for which the features of extrinsic conduction in semiconductors yield a adequate description.
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