Impact of Substrate Bias on the Stability of Bidirectional GaN HEMT in Hard-and Soft-Switching

Many power electronics circuits for DC/AC conversion require power semiconductors with bidirectional voltage-blocking capability. Such a bidirectional device is typically achieved by connecting two unidirectional devices in series. The emerging monolithic bidirectional GaN-on-Si HEMTs (MBD-HEMT) offer an attractive alternative by replacing these two devices with a single-device solution. Despite the promise, substrate management in GaN MBD-HEMTs remains a critical challenge. The substrate of GaN MBD-HEMTs is usually floating, resulting in a constantly changing substrate bias that can be either positive or negative during the hard- or soft-switching. This issue can be resolved by adding a substrate management circuit, which, however, increases device cost and complexity. To optimize the substrate management for GaN MBD-HEMT, the impact of dynamic substrate bias on device parametric stability needs to be thoroughly understood. This work addresses this gap by in situ characterizations of dynamic on-resistance (dyR<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</inf>) and threshold voltage (V<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</inf>) in a GaN HEMT with the floating substrate under both hard- and soft-switching conditions. A relatively large dyR<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</inf> shift is found when the substrate is floating or shorted to drain during the hard switching, whilst the dynamic V<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</inf> shift is minimal in these cases. Instead, the back-gating effect, attributed to the changing substrate bias, i.e., negative during hard-switching and positive during soft-switching, is suspected to be the primary cause. These results can provide practical guidance for optimizing the stability of MBD-HEMT.