Nitride Semiconductor Nanoelectronic Devices and Its Application for Active THz Devices

Abstract

Terahertz (THz) electromagnetic waves are expected to create new functions that cannot be replaced by other electromagnetic wave technologies in applications such as high-speed wireless communication, medical imaging, and biosensors. For industrial application, various demands for components such as THz sources, modulators, and detectors are increasing. In particular, the interest in effective THz wave control technology by modulators is rapidly increasing. In this doctoral thesis, we have developed the nitride semiconductor nanoelectronics platform technology capable of ultrafast switching reaching upto THz frequency range and an electronically controllable THz modulators through integration of varactor diodes and metamaterials. Firstly, the research on design, fabrication, and characterization of gallium nitride (GaN) electronic devices such as transistors and varactor diodes was carried out for the development of GaN semiconductor nanoelectronics platform technology. In heterostructure design, the quaternary In0.05Al0.75Ga0.2N/GaN heterostructure is designed to maximize the polarization doping and minimize the lattice mismatch and short channel effect, so that the sheet resistance of 216 Ohm/ were achieved. In the device fabrication part, the nanoscale T-shaped gate structure is employed into GaN high electron mobility transistor (HEMT) and varactor diodes to fabricate electronic devices capable of ultrafast switching operations. In the device characterization part, we have measured and modeled the DC and RF characteristics of the fabricated GaN HEMTs and varactor diode to extract the device parameters and understand the device physics and ultimately observe the electron transport in the two-dimensional electron gas (2DEG) channel. As a result, we have developed “GaN HEMTs with 100-nm-scale T-gate based on the quaternary In0.05Al0.75Ga0.2N/GaN heterostructure” to achieve a current gain cutoff frequency of 102 GHz, “balanced metal-semiconductor-metal (MSM) varactor diode” to achieve a cutoff frequency of 1.54 THz, and “self-aligned MSM varactor diode” to achieve a cutoff frequency of 1.96 THz. Secondly, the MSM varactor diodes have been integrated into THz fishnet metamaterials to modulate the amplitude of THz waves. This approach for THz wave modulation can achieve high modulation index and low insertion loss by utilizing the kinetic inductance in the 2DEG channel and have the advantage of simple fabrication process and low voltage operation by MSM varactor diodes. The fabricated Al0.25Ga0.75N/GaN-based THz modulators exhibit the operation frequency of 0.85 THz, insertion loss of 2.90 dB, modulation index of 58.0 %, and operation voltage of 5V. THz modulators investigated in this doctoral thesis have several advantages in terms of the small form factor, high modulation speed and low power consumption, opening up the breakthrough in THz technology such as THz imaging and wireless communications.