Electrical characterization of AlGaN/GaN heterostructures under Ohmic metals by using multi-probe Hall devices

By using multi-probe Hall devices, we characterized electrical properties of AlGaN/GaN heterostructures under Ohmic metals. The characterization makes it possible to evaluate the sheet resistance, the sheet electron concentration, and the electron mobility of AlGaN/GaN heterostructures after Ohmic contact formation, by analyzing the voltage and current distribution based on a transmission line model. As a result, we find a decrease in the sheet resistance under an Ohmic metal with a decrease in the specific Ohmic contact resistivity, attributed to significant increase in the sheet electron concentration. The high sheet electron concentration indicates a parallel conduction in the AlGaN and GaN layers, caused by a high doping concentration of the near-surface AlGaN ≳2×1019 cm−3, which leads to an Ohmic contact dominated by field-emission. Moreover, it is suggested that polarization doping induced by a strain in the AlGaN layer has a contribution to the high doping concentration. Multi-probe Hall devices provide a useful method to characterize electrical properties of semiconductors under Ohmic metals.