Evaluation of contact properties of AlGaN/GaN high-electron-mobility transistor using the bridge-contact resistance method

The formation of good ohmic contact is crucial for achieving a high DC and RF performance in a high electron mobility transistor (HEMT), such as AlGaN/GaN HEMTs. The transmission line model (TLM) is still widely used to evaluate the ohmic contact properties of AlGaN/GaN HEMTs. However, this conventional method can lead to inaccuracies during analysis of their contacts because the TLM framework inherently assumes that the source/drain sheet resistance is identical to the channel sheet resistance, and HEMT contacts are typically formed as complicated metal-semiconductor alloys by rapid thermal annealing (RTA). In this study, we report that the contact properties of AlGaN HEMT can be evaluated more accurately using the bridge-contact resistance (BCR) method. We fabricated AlGaN HEMTs with Ti/Al/Ni/Au contacts at different RTA temperatures. Their contact properties were then evaluated using the TLM and BCR methods, followed by a detailed comparison of the results. The result revealed that the specific contact resistance and transfer length obtained from the BCR method were ∼60% lower than those from the TLM method at an annealing temperature of 850 °C. In addition, the BCR method also enabled the analysis of the sheet resistance changes within the AlGaN/GaN region under the electrodes. Thus, it is shown that the BCR method offers an effective contact evaluation approach for HEMTs.