Towards efficient high power mm-wave and terahertz sources in silicon: One decade of progress

All promising applications of terahertz (THz) and millimeter-wave (mm-wave) systems, from imaging and spectroscopy to high data-rate communication, necessitate the design of high efficiency signal generators. In addition to the high propagation loss of the signals in these frequency ranges, the lack of activity of the current CMOS/SiGe devices (since the desired frequencies are close to their ƒ
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or beyond it) emphasizes on the importance of coming up with new design methods in order to generate high output power signal sources. At UNIC group of Cornell university, we have a long history of designing mm-wave and terahertz signal generators. It started from designing oscillators close to the ƒ
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with output power much higher than the state of the art oscillators in CMOS and SiGe technologies. It embarked on a 121 GHz fundamental oscillator with −3.5 dBm output power in a 130 nm CMOS process. To generate higher frequencies, harmonic generators and frequency multipliers have led to oscillators with oscillation frequencies beyond the ƒ
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. For instance, a triple-push oscillator is fabricated in 65 nm CMOS process with −7.9 dBm output power at 480 GHz. In the next phase, to cope with the challenges of using varactors for frequency tuning, a novel injection-locked loop of oscillators is designed using these advanced oscillators as the core, which shows a 4.5% of tuning range at 290 GHz and has increased the output power to −2.1 dBm by combining the power of the fourth harmonic of four push-push oscillators. In another attempt, the injection-locked tuning loop is built upon eight voltage controlled oscillators (VCO's). Combining the output power of the second harmonic of the eight core VCO's leads to a maximum output power of 4.1 dBm at 256 GHz and the resulting VCO has a tuning range of 4.3% by employing two varactors, one inside the oscillator block and the other one in the phase shifter. At this point, the maximum DC-to-RF efficiency of all these oscillators were below 1.1% which compels the next challenging step to improve the DC-to-RF efficiency to a reasonable value. In this vital step, using a completely novel idea of shaping and maximizing the unilateral power gain of a two-port network (the measure of its activity), a fundamental oscillator is designed in a 130 nm SiGe process which has improved the DC-to-RF efficiency by a factor of 10 and has increased the output power to more than 4.8 dBm, utilizing only one transistor. This new approach enables the future THz and mm-wave systems to become both efficient and also capable of producing high output power. High tunability and frequencies higher than the ƒ
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in addition to much higher output power can be attained by employing previous steps of combining the output powers and tuning through injection-locked loop of oscillators.