Charge Transport in GaN High Electron Mobility Transistor With Positive Substrate Bias

Charge transport in 650-V-rated GaN high electron mobility transistors (HEMTs) was investigated using positive substrate bias up to +600 V. Positive substrate bias resulted in a reduction in channel current, attributed to negative charge storage in the buffer, resulting in up to a >50% reduction in the 2-D electron gas (2DEG) channel density. The dynamics of the accumulated charge was investigated using recovery transients after substrate bias stress, with recovery times >1000 s for substrate stress bias <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;+$</tex-math></inline-formula>200 V. The recovery time was reduced significantly with the application of a negative substrate bias of short duration, immediately following the positive substrate bias stress. A comprehensive explanation is presented, which requires a detailed understanding of the transport (both ohmic and non-ohmic) through each of the layers in the epitaxial stack.