Influence of sapphire substrate thickness on the characteristics of AlGaN/AlN/GaN heterostructure field-effect transistors

• The sapphire substrate influences the characteristics of AlGaN/AlN/GaN device.
• With more than 250 μm substrate grinding, the strain in the GaN layer enhances.
• The shrinkage of the GaN layer affects the carrier scatterings.

A standard AlGaN/AlN/GaN heterostructure field-effect transistor (HFET) on a 420 μm thick sapphire substrate was fabricated, and then the sapphire substrate was thinned to several different thicknesses by grinding. When the remaining substrate thickness is more than 170 μm, the characteristics of the substrate thinned AlGaN/AlN/GaN HFET are almost the same as the original device. However, when the substrate thickness is less than 170 μm, the drain-to-source current, the threshold voltage, and the electron mobility of the two-dimensional electron gas (2DEG) in the channel are varied compared to the original one. It is attributed to the shrinkage of the GaN layer with the reduction of substrate thickness, which induces the decrease of both the 2DEG sheet density and the polarization charge sheet density, and a possible increase of dislocation scattering and the reduction of polarization Coulomb field scattering.

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