Gain improvement of a 130 GHz CMOS amplifier by using radial stub as AC ground

Low-loss matching circuit is an important issue since sufficient gain cannot be achieved with over-100 GHz CMOS amplifiers. In the case of matching circuit with short stub, the short stub should be terminated to the ground. Since the matching circuit is applied DC bias to operate the FET of amplifier, the ground should not be directly connected the ground (DC ground) but should be AC ground. AC ground with MIM capacitor is widely used in microwave frequency range, but 1/4 $\lambda_{g}$ open stub is used for over-100 GHz amplifier to reduce the loss of the AC ground. In this paper, we examine the losses of open stub using low-impedance line and radial stub. Theoretical analysis based on the series resistance of the conductor, EM simulation which take into account conductor loss and dielectric loss, and the measurement of fabricated stubs show the superiority of the radial stub than the open stub. The fabricated 130 GHz CMOS amplifiers having an input and an output matching circuits with open stubs shows higher gain than that with open stubs.