On the variation of the 2DEG charge density with the density of the surface donor traps in AiGaN/GaN transistors

Gallium nitride (GaN) has a bright future in high voltage device owing to its remarkable physical properties and the possibility of growing heterostructures on silicon substrates. GaN High Electron Mobility Transistors (HEMTs) are expected to make a strong impact in off line applications and LED drives. However, unlike in silicon-based power devices, the on-state resistance of HEMT devices is hugely influenced by donor and acceptor traps at interfaces and in the bulk. This study focuses on the influence of donor traps located at the top interface between the semiconductor layer and the silicon nitride on the 2DEG density. It is shown through TCAD simulations and analytical study that the 2DEG charge density has an `S´ shape variation with two distinctive `flat´ regions, wherein it is not affected by the donor concentration, and one linear region. wherein the channel density increases proportionally with the donor concentration. We also show that the upper threshold value of the donor concentration within this `S´ shape increases significantly with the AIGaN thickness and the Al mole fraction and is highly affected by the presence of a thin GaN cap layer.