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Equivalent circuit models have been developed to study the performance of the devices with two-dimensional electron gas (2DEG) system for terahertz (THz) applications (single- and grid-grating-gated high-electron-mobility transistor (HEMT)) and for sensing (micromachined HEMT with the array of resonant floating gates). The components of the equivalent circuits were related to physical and geometrical parameters of the devices under consideration. The developed equivalent circuits were used to simulate frequency performance of the devices under consideration invoking IsSpice circuit simulator. The results of IsSpice simulation reveal that the highly doped cap layer can cause the fundamental resonant frequency reduction. It is shown that the increase in the number of fingers in gridgrating-gated HEMT-structures is accompanied by the decrease in mode separation between adjacent harmonics which results in spectrum broadening. Possible approach to realize single-mode operation of such plasma wave grid-grating-gated HEMT, i.e., to narrow its spectrum at around a certain frequency, is proposed. IsSpice simulation of the performance of recently proposed micromachined HEMT with the array of floating gates (FGs) over 2DEG channel demonstrates its capability to operate as a multi-target sensor.