Effects of multigate-feeding structure on the gate resistance and RF characteristics of 0.1-μm metamorphic high electron-mobility transistors

We investigate the effects of a multigate-feeding structure on the gate resistance (R_g) and RF characteristics of the high electron-mobility transistors (HEMTs). In this structure, the increase of R_g with the gatewidth (W) is minimized; therefore, high maximum frequency of oscillation (f_max) is achieved. Various numbers of gate feedings (N_gf) using the air-bridge interconnections are adopted for fabricating the 0.1-μm depletion-mode metamorphic HEMTs. From these structures, we observe great reduction in with R_g the increase of N_gf , and their relationship is given by R_g ∝ 1/[2·(N_gf-1)] ², where N_gf=2,3,4,⋯; on the other hand, the effects of N_gf on other small-signal parameters are negligible. Calculated cutoff frequency (f_T) and f_max fromthe extracted small-signal parameters all show good agreement with the measurement results.f_T is slightly decreased with the increase of N_gf due to the increase of gate-to-source capacitance. f_max is, however, greatly increased with N_gf , and this effect becomes greater at longer total gatewidth (W × number of gate fingers). This is due to the smaller R_g at greater N_gf in the multigate-feeding structure. We propose that this gate-feeding structure provides a very effective way to suppress R_g and maximize f_max for the applications of the HEMTs with long W.