Microwave Measurement of the Velocity—Electric-Field Characteristic of Inhomogeneous Epitaxial GaAs with Conducting Substrate

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Abstract
The velocity—electric‐field (v‐E) characteristic of n‐type GaAs measured by microwave techniques corresponds to the intrinsic material characteristic only when the sample is uniform. This paper presents a theoretical and experimental investigation of the measured v‐E characteristic for epitaxial samples with nonuniform doping concentration. The theory predicts substantial degradation of apparent peak‐to‐valley ratio and negative differential mobility from intrinsic values, as well as a decrease in apparent critical field for nonuniform material. The measurements are in good agreement with theory, and show a measured peak‐to‐valley ratio of 2.0 for uniform GaAs, decreasing to 1.5 for material with a linear doping gradient (1.5≤n≤2.5)×1015 cm−3 across the 11‐μ‐thick layer. Values as low as 1.2 were observed for samples with a localized high‐resistance region. The measurements were obtained by a 35‐GHz microwave heating method which permitted convenient characterization of practical device material with conducting substrate. The v‐E data were corrected for high‐frequency effects arising from energy relaxation. The results indicate that much of the divergence in reported v‐E characteristics can be explained on the basis of sample inhomogeneity.