Investigations on TID Effects-Induced Parasitic Transistors in GaN Cascode Power Transistors

The effects of Total Ionizing Dose (TID) are studied on two different commercial 600 V GaN cascode transistors from the same manufacturer. The components are irradiated with 1 MeV X-rays under different bias conditions in order to distinguish the contribution of each sub-component of the cascode structure to the static response under TID. The combined influence of the electric field and irradiation in these structures is then analyzed. Specific bias conditions are also applied to maximize the potential difference across the GaN HEMT while minimizing the electric field in the Si-MOSFET. The shifts in threshold voltage (VTH) and maximum transconductance (gm,max) are most significant for bias cases that apply a high potential difference across the Si-MOSFET while maintaining a low electric field in the GaN HEMT. These results indicate that the Si-MOSFET is the main contributor to the static degradations observed in both cascode structures, while the GaN HEMT shows tolerance to TID effects up to 100 krad(SiO2). Finally, two unexpected parasitic transistors are triggered by TID irradiation, only in the Cascode 2 device. These parasitic transistors appear when the Si-MOSFET is biased, indicating that their origin lies within the Si-MOSFET. An analytical approach based on cross-sectional views, oxide thicknesses, and experimental threshold voltage shifts (ΔVTH) is proposed to identify the origin of these parasitic transistors. A TCAD model is also used to validate the assumptions made in the analytical approach.