I. Introduction
As the demand of higher operation frequency and bandwidth range, the key component for gain setting and function controlling digital attenuator with perfect performances is desperately needed in Radar, Communication, Electronic Warfare and other electronic equipments [1]–[2]. Due to the fast speed and high operation frequency, GaAs PHEMT has already been the dominant device. However, because of the low isolation, high substrate loss and the prominent parasitic parameter, the attenuator with high accuracy and low insertion loss is difficult to realize in high operation frequency. Nowadays, high accuracy and low insertion loss are still crucial index for digital attenuator and radio-frequency system.