Thanks To recenT cMos advances, it has become possible to implement voltage-controlled oscillators (VCOs) and a Schottky diode detector operating above 100 GHz. In fact, a millimeter-wave Schottky-diode frequency doubler fabricated in CMOS has been developed by Chuying Mao, Chakravartula Shashank Nallani, Swaminathan Sankaran, Eunyoung Seok, and Kenneth K. O from the University of Florida. The doubler, which is built in 130-nm CMOS, can generate signals to 140 GHz. At 125 GHz, it exhibits ~10 dB conversion loss and delivers 1.5 dBm output power. The doubler offers input matching beyond 10 dB from 61 to 66 GHz. Its rejection of fundamental signals at the output port is greater than 12 dB for inputs from 61 to 66 GHz.
Compared to the Schottky diodes in silicongermanium (SiGe) BiCMOS, the Schottky diode in CMOS does not have an n+ buried layer. Thus, it has a smaller unit diode cell area and larger n+ cathode contact areas to lower series resistance. These aspects raise the cathode-to-substrate capacitance. Together with non-negligible substrate resistance, such issues make it difficult for CMOS to use a series topology for frequency multiplication. As a result, the researchers used a balanced topology with two Schottky shunt-barrier diodes with grounded cathodes, which can increase the output power used for the CMOS implementation. U
sing the Schottky barrier diodes, it should be possible to implement CMOS frequency doublers operating above 300 GHz. However, they will have lower conversion efficiency and output power. See "125-GHz Diode Frequency Doubler in 0.13-m CMOS," IEEE Journal Of Solid-State Circuits, May 2009, p. 1531.
