This paper proposes bandwidth enhancement of a cavity-backed slot antenna using a triangular slot on a half-mode substrate integrated waveguide structure antenna. The bandwidth enhancement was achieved by combining the fixed TE101 and the downward shifting TE102 modes, resulting in hybrid modes. The design evolution of the slot antenna from a half nonresonating rectangular slot to a triangular slot antenna increased the fractional bandwidth. The simulation result showed that fractional bandwidth increased from 6.27% to 9.1%. It was confirmed by measurement that the fractional bandwidth of 9.87% was achieved which reflects a 350 MHz bandwidth with center frequency at 3.84 GHz. The measured gain at center frequency was 4.2 dBi. It is shown that the radiation characteristics obtained from both measurement and simulation results are in very good agreement.