Millimeter-wave and terahertz subharmonic CMOS transmitters

The terahertz frequency spectrum has become a focal point for next-generation communication, sensing, and imaging systems. There is a pressing need for compact, power-efficient, and integrable transmitter architectures. Conventional transistor-based solutions face fundamental challenges in achieving high gain and power efficiency at terahertz frequencies due to inherent limitations in carrier transit time in advanced CMOS technologies. Conversely, compound semiconductor technologies, while offering superior high-frequency performance, are often constrained by low fabrication yield and significantly higher cost. These limitations motivate the exploration of reactive frequency-conversion techniques that can overcome the constraints of traditional active circuits. This dissertation presents a comprehensive study of reactive, multi-port asymmetric varactor-based up-conversion architectures implemented in CMOS process. The proposed designs leverage the nonlinear capacitance characteristics of accumulation-mode MOS varactors to achieve efficient third- and fourth-order subharmonic up-conversion without relying on active devices. By repurposing the varactor’s n-well as an additional signal port, the architecture enables simultaneous injection of local oscillator and intermediate frequency signals without external combiners. This multi-port configuration reduces circuit complexity, minimizes signal loss, and enhances signal-to-signal isolation. A key contribution of this work is the implementation of a Hartley-inspired quadrature signal-combining network, realized through transmission lines designed to introduce precise phase shifts at both the fundamental and third-harmonic frequencies. The resulting 90º phase relationship between the two asymmetric varactor branches allows intrinsic image and harmonic rejection, improving spectral purity and enabling efficient power combination at the desired output frequency. The dissertation also investigates the impact of bias-dependent harmonic generation and self-biasing effects arising from even-order nonlinearities within the devices. These effects dynamically influence the operating point and impedance environment. Controlled asymmetry in biasing conditions is shown to enhance rejection of undesired harmonics while maintaining constructive combination at the target up-converted frequency. Beyond its circuit-level innovations, this research establishes a foundational framework for passive terahertz up-conversion in CMOS technology. It addresses key challenges in matching, harmonic control, and bias optimization, offering a pathway toward highly integrated, energy-efficient terahertz transmitters. The proposed approach demonstrates that passive, varactor-based architectures can serve as viable alternatives to power-intensive active circuits.