Systematic studies were performed on the influence of different cap layers of i-GaN, n-GaN, p-GaN and InGaN on AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on sapphire by metal organic chemical vapor deposition. The decrease of maximum extrinsic transconductance (gm) and maximum drain current density (IDmax) values agrees with the product values of two-dimensional electron gas (2DEG) mobility (µH) and 2DEG sheet concentration (ns) of AlGaN/GaN HEMT structures. An improved Schottky barrier height with low surface roughness has been observed in AlGaN/GaN HEMT structure with i-GaN and n-GaN cap layers. The HEMTs with i-GaN cap layer exhibited low gate leakage current with high breakdown voltage among the other HEMTs. Though the HEMTs with n-GaN cap layer and without cap layers exhibited good µH, gm, IDmax values, the ac characteristics are not up to the extent of HEMTs with i-GaN cap layer. All the devices except the HEMTs with InGaN cap layers were operational even up to the measurement temperature of 350°C. The HEMTs with i-GaN cap layer exhibited collapse-free IDS–VDS characteristics with small ID hysteresis width variations among the other HEMTs. The observation of small threshold voltage variation, small drain current hysteresis width and small white light illumination effects confirms the existence of small trapping effects in HEMTs with i-GaN cap layers. Only one thermally activated trap level at -0.161 eV was observed on AlGaN/GaN HEMTs with i-GaN cap layer. However, each of the three trap levels has been observed in HEMTs with other cap layers and HEMTs without cap layers. From this, it is concluded that the collapse-related traps are screened/passivated from two-dimensional electron gas by the addition of thin i-GaN cap layer on AlGaN/GaN HEMTs. The cap layer i-GaN (3 nm) is a promising candidate to get collapse-free AlGaN/GaN HEMTs.