Design and fabrication of THz frequency-range RTD oscillators with enhanced output power and frequency tunability

Abstract

In recent years, the demand for various THz application systems such as spectroscopy, imaging, and wireless communication has greatly increased. For these applications, THz signal sources are one of the most important components. In both electronic and optical devices, various THz sources have been reported such as III-V transistor-based oscillators (HBTs and HEMTs), Si CMOS transistor-based oscillator, diode-based oscillators (Gunn diodes, IMPATT diodes and RTDs), multiplier chains and quantum cascade lasers.

The RTD-based oscillator is one of the most promising candidates for solid-state compact and coherent THz signal sources due to the high fmax (maximum oscillation frequency) of intrinsic NDC (negative differential conductance) from the resonant tunneling. However, the relatively small output power (0.1~$10 \mu W$) and the narrow frequency tuning range are limitations of RTD THz oscillators to be implemented in practical THz applications. In this thesis, the THz frequency range RTD oscillators with enhanced output power and frequency tunability have been developed.

The device fabrication and characterization of the terahertz RTD/SBD IC technology has been conducted. The InP-based RTD, InP-based SBD, CPW transmission line inductor and on-chip dipole antenna were fabricated and integrated by using an InP terahertz IC technology.

The terahertz RTD oscillators with enhanced output power have been fabricated and characterized. To improve the output power of the RTD oscillator without modifying the RTD device structure for high output power, the power combining techniques such as coherent power combining and multi-device power cell have been proposed and implemented.

In order to enhance the output power of the RTD oscillator, a differential-mode RTD oscillator has been suggested to generate differential output signals and increase the output power from the single-mode RTD oscillator. To combine and radiate the differential output signals of the differential-mode RTD oscillator effectively, an RTD-based differential-mode oscillator using an integrated on-chip dipole antenna has been demonstrated based on the InP RTD terahertz monolithic IC technology for the first time. The fabricated oscillator showed the oscillation frequency of 675 GHz and the output power of $47 \mu W$. The total DC power consumption and the DC-to-RF efficiency were 14.1 mW and 0.33 %, respectively.

As a two-element RTD oscillator, the RTD-pair oscillator has been confirmed by the experimental result. From the on-wafer measurement, the fabricated RTD-pair differential oscillator shows the oscillation frequency of 219 GHz with the maximum output power of $65 \mu W$ at each output port. For radiation measurement, the RTD-pair differential-mode oscillator integrated with an on-chip dipole antenna shows approximately 2 times higher output power than that of the single-RTD differential oscillator in the frequency range of 250 GHz.

The tunable-frequency oscillators by using the RTD and the SBD were proposed and demonstrated for the first time in the sub-THz frequency range. The single-RTD tunable-frequency oscillator shows the oscillation frequency from 159 to 179 GHz with varying a bias voltage of the SBD. The frequency tuning range was 12 % and the output power was 71 μW by the on-wafer measurement setup.

To enhance the output power of the tunable-frequency RTD oscillator, the RTD-pair configuration was applied. The RTD-pair tunable-frequency oscillator shows the frequency tuning range of 18% with the center frequency of 151 GHz. The oscillation frequency was tuned from 137 GHz to 165 GHz. the output power of 221 μW was measured by the on-wafer measurement setup. The measured performances show that the frequency tunable RTD oscillator with the SBD varactor is very promising for THz-range applications.