Search

In this study, negative gate bias stress (NGBS) is applied to the Si3N4 metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs). The gate electron injected into the Si3N4 dielectric layer and the holes generated by the impact ionization trapped into the AlGaN layer are the two main degradation mechanisms, which cause the unstable threshold voltage. Moreover, it is discovered that an abnormal ${C}$ – ${V}$ hump is generated after NGBS because of the uneven hole trapping at the AlGaN near the channel under the gate. The unusual direction of the electric field, which is related to the mesa isolation technology, is observed under NGBS condition in the Silvaco TCAD simulation. The location of the hole trapping is verified by the positive gate bias after NGBS during the recovery time. Finally, the device without the mesa isolation is used to verify the abnormal degradation results.

In this study, the reliability issues are discussed under dc and ac negative gate bias stress (ac-NGBS) in Al2O3/Si3N4 metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs). Converse degradation between these two stress conditions is discovered. The holes generated by trap-assisted thermionic field emission (TA-TFE) are trapped into the Al2O3 layer, so that the threshold voltage ( ${V}_{\text {th}}$ ) decreases under dc negative gate bias stress (dc-NGBS). ${V}_{\text {th}}$ increases because of the hot electrons injected into the GaN layer, while the device is turned on quickly under ac-NGBS. In addition, the degradation mechanisms under dc-NGBS and ac-NGBS are confirmed by the OFF-state gate and drain leakages, respectively. Silvaco TCAD is used to validate the degradation mechanism under ac-NGBS. Finally, the characteristics of the ${V}_{\text {th}}$ shift transition from dc to ac NGBS are discussed.

In this study, X-ray irradiation of metal–insulator–semiconductor (MIS) AlGaN/GaN high-electron-mobility transistors (HEMTs) is performed. After the X-ray irradiation, the threshold voltage ( ${V}_{\text {th}}{)}$ shift and ON-state current ( ${I}_{\text {on}}{)}$ variation are observed. However, after a recovery period, the degradation trend of ${V}_{\text {th}}$ and that of ${I}_{\text {on}}$ are in opposite directions. Such opposite degradations are demonstrated and explained in this study. As X-rays irradiate the devices, holes, and defects are generated in the GaN layer and Si3N4 layer, respectively. To prove the degradation mechanism induced by X-rays in MIS HEMT, the following characteristics are offered. The drain current ( ${I}_{d}{)}$ and the source current ( ${I}_{s}{)}$ under the X-ray irradiation are introduced to prove the hole generation. The two-step degradation of the gate current ( ${I}_{g}{)}$ after X-ray irradiation provides evidence of the formation of defect states. Moreover, the different degradation behaviors between Schottky-gate HEMT and MIS HEMT are compared and verification of the position of generation of defect states in the Si3N4 layer is given accordingly.