Effect of GaN Cap Thickness on the DC Performance of AlGaN/GaN HEMTs

We prepared AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap thicknesses of 0, 1, 3, and 5 nm and compared the material characteristics and device performances. It was found that the surface morphology of the epitaxial layer was effectively improved after the introduction of the GaN cap layer. With the increase of the GaN cap thickness, the carrier concentration (n_s) decreased and the carrier mobility (μ_H) increased. Although the drain saturation current (I_dSat) of the device decreased with the increasing GaN cap thickness, the excessively thin GaN layer was not suitable for the cap layer. The thicker GaN layer not only improved the surface topography of the epitaxial layer but also effectively improved the off-state characteristics of the device. The optimal cap thickness was determined to be 3 nm. With the introduction of the 3 nm GaN cap, the I_dSat was not significantly reduced. However, both the off-state gate leakage current (I_gLeak) and the off-state leakage current (I_dLeak) decreased by about two orders of magnitude, and the breakdown voltage (BV) increased by about 70 V.