  Characteristic electrical curves of GaN HEMT devices from Infineon and
Transphorm are compared at different X-ray radiation dose. It is shown that the
device with pGaN gate is more robust having a stable threshold voltage (Vth).
The Vth of device with MIS gate shifts towards negative direction firstly and
shifts to forward direction then. A qualitative analysis is performed in the
paper. Such dynamic phenomenon is caused by releasing and trapping effects of
radiation induced charges both in the dielectric layer and the interface of the
device. It is summarized that pGaN gate based GaN HEMT is a promising solution
for further space use of electric source
