A novel terahertz plasma-wave photomixer that can improve the conversion gain and terahertz radiation power is proposed and evaluated. The photomixer is based on a high-electron mobility transistor and incorporates doubly interdigitated grating strips for the gate electrodes that periodically localize the 2D plasmons in 100-nm regions with a micron-order interval. A vertical cavity structure is formed in between the top metal grating and a terahertz mirror placed at the backside. The device features electronic tuning of plasmon characteristic frequencies, providing continuously-tunable operation below 1 THz to beyond 10 THz. Frequency-dependent finite-differential time-domain analysis demonstrates that the grating-bicoupled plasmonic structure acts as a broadband terahertz photomixer and antenna and that the vertical cavity structure effectively enhances the conversion gain and radiation power.