Simple analytical model is developed to evaluate spatial distribution of sheet electron density in the channel of the high-electron mobility transistor (HEMT)-like structure periodically modulated by the bias voltages applied to interdigitated gate. Resonant frequencies of plasma oscillations excited in the two-dimensional electron gas (2DEG) channel of such structures are evaluated in the ideal and realistic situations. The realistic model accounts for the ungated regions which due to nonideality of gate contact—2DEG channel system can be also affected by the gate bias voltages. It is shown that plasma resonances in realistic structures with fringed ungated regions deviate from those predicted by the ideal model. The model developed can be used to interpret the terahertz plasmon spectra measured experimentally.