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 g prop 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.