Effects of Multigate-Feeding Structure on the Gate Resistance and RF Characteristics of 0.1- $\mu{\hbox {m}}$ 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-mum depletion-mode metamorphic HEMTs. From these structures, we observe great reduction in R_g with the increase of N_gf, and their relationship is given by R_gprop 1/[2middot(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 from the 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 times 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.