We studied the mechanism of low-temperature-annealed Ohmic contacts to AlGaN/GaN heterostructures via formation and removal of Ta/Al/Ta Ohmic-metals. Multi-probe Hall device measurements show one order increase in the sheet electron concentration after Ohmic-metal formation compared with that before formation, indicating that high-density doping takes place in the AlGaN/GaN heterostructure under the Ohmic-metal. However, after Ohmic-metal removal, the increased sheet electron concentration returns to the value before formation. Moreover, we formed Ni/Au Schottky contacts on the AlGaN/GaN heterostructures before Ohmic-metal formation and after Ohmic-metal removal, and confirmed that the characteristics are almost the same. These results indicate that donors do not exist after Ohmic-metal removal, suggesting that, although high-density doping takes place, high-density donors are not formed under the Ohmic-metal. The high-density doping without high-density donors could be attributed to polarization doping, playing a significant role in Ohmic contact formation.

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.