Analysis and Design of Fourth Harmonic Boosting Technique for THz Signal Generation

In this paper, an analysis method is presented to mathematically characterize the harmonic current generated by the voltage at fundamental or harmonic frequency in the transistor of a high-frequency harmonic oscillator. Using this analysis method, the relationship between the second harmonic voltages and the fourth harmonic drain current in the transistor is investigated, and the requirement on the second harmonic voltages for fourth harmonic boosting is found. Moreover, an approach of modeling the voltage-current relationship in the transistor of a high-frequency harmonic oscillator at harmonic frequency as an equivalent linear relationship is proposed. This modeling method facilitates the design of the second harmonic embedding network around the transistor, thus helping to propose a harmonic oscillator topology in which the requirement on the second harmonic voltages for fourth harmonic boosting is fulfilled. Using the proposed fourth harmonic boosting technique, a 0.6-THz radiator array is designed in 40-nm bulk CMOS. In measurement, the 0.68-
$\text {mm}^{2}$
radiator array achieves a radiated power of 0 dBm and a DC-to-THz efficiency of 0.08% at 586.7 GHz under a 0.9-V supply voltage.