Digitally controlled C-band microwave Variable Gain Amplifiers(VGA's) which use the Segmented Dual-Gate GaAs MESFET's (SDGMESFET's) are designed, fabricated, and characterized. The circuits are fabricated using the Monolithic Microwave Integrated Circuit(MMIC) approach based on GaAs MESFET technology developed at KAIST. Key fabrication techniques are ion implanted MESFET, and MIM(Metal-Insulator-Metal) capacitor with SiN dielectric film deposited by Electron Cyclotron Resonance(ECR) plasm and gold-electroplating. With this technology, fully functional 2 and 6 bit VGA's are successfully fabricated, and RF performance of these circuits is measured using on-wafer probing system. The dynamic range of the 6bit VGA with the second gate termination of constant resistor and capacitor is 17 dB at 4 GHz with only 4 bits control. The insertion phase loss between states is less than 5 degrees throughout C-band. The input matching is almost independent of the gain setting, while the output matching is very much dependent on it. These results show that they have potential application capability not only for the precise gain control of Transmit/Receive(T/R) module in active phased array radar but also for mobile communication.
