The ever-increasing demands for high-speed communication to connect people to vehicles, sensors, data, computing resources, cloud storage, and even robotic and artificial intelligence agents have inspired and motivated the development of sixth-generation (6G) communications operating at sub-THz frequencies. Additionally, thanks to their short wavelength, high penetration, and non-ionizing radiation properties, sub-THz waves have vital roles in security screening, medical and biomedical imaging, and DNA sequencing instruments. Moreover, the THz spectrum covers the spectral signatures of many molecules, making them advantageous in chemical identification, material characterization, gas sensing, and Earth and planetary sciences. These THz systems comprise sub-THz amplifiers, signal generators, and frequency multipliers with key performance in terms of bandwidth, gain, and output power. In addition to the developments of solid-state technologies and integrated circuit processes, novel system architectures, design techniques, and circuit configurations are always imperative to utilize and unlock the full capability of these advanced processes.
This Ph.D. research introduces a new Darlington cell configuration to improve the bandwidth, gain, and output power of sub-THz broadband amplifiers. The new cell implements two diode-connected transistors to balance the voltages and currents of two main transistors inside the conventional Darlington pairs. As a result, the pairs’ output voltage swing and output power are both improved. When used in the gain stages of distributed amplifiers (DAs), the high input impedance and high current gain characteristics of the new cells reduce the input line loss and increase the operating bandwidth of the DAs. This new Darlington cell is suitable for high-frequency designs and wideband amplifiers. Various indium phosphide (InP) Darlington DAs were designed and characterized to verify the concept. Among them, a Darlington DA using triple-stacked HBTs exhibits an average gain of 13 dB and an average output power of 18.3 dBm across an extended 3-dB bandwidth of 3 to 230 GHz. Meanwhile, a quadruple-stacked HBT version of this DA provides a flat gain of 12.5 dB with a gain ripple of less than 1.5 dB up to 230 GHz. The quadrupled stacks are designed to widen the 3-dB power bandwidth to 180 GHz with a maximum saturated power of 17.4 dBm.
In addition to power and bandwidth improvements, gain enhancement is addressed by combining matrix topology and tapered input capacitors, building a matrix amplifier with more than 20 dB of power gain across the entire bandwidth from 5 to 230 GHz. Moreover, a continuous gain control mechanism is introduced using several diode-connected HBTs distributed along the input line of a variable gain DA. By adjusting the bias of these HBTs, a continuously variable gain range of 8 dB was demonstrated for a DA operating up to 240 GHz. Furthermore, transistor stacking, high-isolation cell shielding, and low-crosstalk transmission lines are investigated to enhance the reverse isolation of an active isolator. The fabricated prototype provides isolation of better than 50, 37, and 30 dB up to 40, 190, and 220 GHz, respectively, with a flat gain of 10 dB and an output third-order intercept point of 18.2 dBm at 140 GHz. Finally, a new wideband low-loss, low-imbalanced Marchand balun is introduced and integrated into several differential DAs, frequency doublers, and distributed doublers operating up to 220 GHz. By employing multiconductor coupled lines in co-planar waveguide configurations, the designed balun achieves an insertion loss, amplitude, and phase imbalances of less than 1.5 dB, 0.3 dB, and 1.5 degrees across a 42-126-GHz bandwidth. The balun’s enhanced performance enables a frequency doubler to exhibit a high FRR of up to 42 dBc, a peak Pout of 6.5 dBm, and a peak conversion gain of 0 dB across frequencies from 140 to 210 GHz.
