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Effects of Si Deposition on AlGaN Barrier Surfaces in GaN Heterostructure Field-Effect Transistors

We observed that Si deposition on AlGaN/GaN heterostructure field-effect transistors (HFETs) can increase the two-dimensional electron gas (2DEG) density up to a level comparable to that achieved as a result of SiN passivation. Silicon atoms located at the AlGaN barrier surface might act as positively ionized donors, which can partially neutralize the negative polarization charges of the AlGaN surface and thereby increase the 2DEG density through polarization effects. This phenomenon is not only interesting from the scientific point of view, but also promising from the practical point of view. By using a 2-nm-thick Si insert layer in the ohmic contact, we obtained a contact resistance value 2.5 times lower than that obtained without the Si insert layer (conventional ohmic contact). GaN HFETs with Si deposited on the AlGaN barrier surface exhibited excellent device characteristics owing to a reduction in the source and drain sheet resistances and the ohmic contact resistances.