Recently, THz technology has received considerable attention because of its various applications, for example, spectroscopy, quality control system, and next-generation communication. Especially, the low THz frequency range of 0.1 to 0.3 THz, where oscillation performances for applications are easily achieved, has been widely researched for imaging applications such as medical diagnosis, non-destructive inspection, and security investigation.
The resonant tunneling diode (RTD) is regarded as one of the promising devices for the THz signal sources, due to the high fundamental oscillation frequency and the high output power characteristics. However, in order to utilize RTD-based signal sources for practical imaging applications, a frequency tunability of their oscillation frequency is highly required. The frequency tunable RTD-based signal sources can be used for various systems including frequency modulated continuous wave (FMCW) radar system and 3D mapping.
In this thesis, the RTD-based frequency tunable oscillators integrated with schottky barrier diodes as varactor diodes have been developed for 0.1 ~ 0.3 THz imaging applications. Generally, the schottky barrier diodes have been widely used for THz circuits such as mixer, multiplier, and detector, because the high cutoff frequency characteristic of schottky barrier diodes are highly desired for THz systems.
At first, requirements of a schottky barrier diode for the RTD-based tunable oscillators are discussed based on operation principles and characteristics of the schottky barrier diode. In order to improve the cutoff frequency performance of the schottky barrier diode, developments for reducing the series resistance (Rs) and the junction capacitance (Cj) are essential.
To reduce the series resistance, a Ti/Pd/Au ohmic contact fabrication process with the rapid thermal annealing (RTA) process is developed, which shows a 27% reduction of the specific contact resistivity $\rho$, compared to the conventional Ti/Pt/Au ohmic contact. In addition, to reduce the junction capacitance which is proportional to the device size, the schottky barrier diodes are fabricated with various device sizes of 1 ~ 9 $\mu m^2$. The fabricated schottky barrier diodes show the high cutoff frequencies of 2.3 THz and 0.7 THz with the device size of 1 $\mu m^2$ and 9 $\mu m^2$, respectively. The quality factors of 18.6 and 7.1 are also achieved with 1 $\mu m^2$ and 9 $\mu m^2$ schottky barrier diodes.
Finally, RTD-based frequency tunable oscillators are fabricated with the schottky barrier diodes. The simulated performance of the fabricated oscillators demonstrates the oscillation frequency of 0.12 ~ 0.25 THz. The oscillator with a 1.44 $mu m^2$ RTD and a 9 μm2 schottky barrier diode shows the wide tuning range of 11% with the center frequency of 164 GHz in the simulation. Consequently, the fabricated RTD-based frequency tunable oscillators are suitable for THz imaging applications.
본 연구는 300GHz-600GHz 대역의 THz imaging 응용을 위한 주파수 변조 발진기 개발을 목표로 하였다. 먼저, 주파수 변조를 위해 Schottky barrier varactor diode를 제작하고 이를 모델링하여 분석하였다. 그리고 제작한 varactor diode를 활용하여 주파수 변조 발진기 회로를 설계하고 제작한 다음, 측정 과정을 통해 성능을 검증함으로서 THz 이미징 분야에 응용할 수 있음을 확인한다.