Fundamental Research Of Novel HEMT Thz Active Devices

Terahertz(THz) wireless communication systems have recently become the research focus in the field of communication and information technology. The external modulation THz wireless communication system which could realize long distance data transmission has become an important research direction in this field. Thus it attracts lots of attentions all this time. In this system, the key device THz external modulator is difficult to break through for a long time, which also limits the development of the system. The way to achieve effective and ultrafast THz modulation thus to load the data information on the spatial transmitted THz wave has become an important issue to be solved in external modulation THz wireless communication system. Through designs of structures, analog simulations, theoretical analysises, fabrications and measurements, we systematically design and demonstrate high speed mode conversion THz active devices with the combination of metamaterials(MTMs) and high electron mobility transistors(HEMTs). Moreover, utilizing the external modulation THz wireless communication system, we discuss the performance of this modulator and demonstrate the feasibility of this system. The main contents of this paper are shown below.1. We carry out a detailed research on resonant modes and properties of MTMs. There are significant differences for different resonant modes in the transmission spectra, such as the resonant intensity, the full-width at half maximum(FWHM), and the quality factor(Q-factor). The refining and classification of resonant modes will contribute to the design of device structures with various functions. MTMs are the foundation of THz external modulation active devices, consequently, this paper discusses several THz functional devices based on the MTMs.2. This paper constructs the composite structure system of active devices with the combination of MTMs and semiconductors. Electrical control and light control are the most effective ways to realize the high speed modulation of THz waves. For the design of light control modulators, broadband and multiband THz modulators are realized with the combination of MTMs and doped bulk materials. For the design of electrical control modulators, three kinds of mode conversion modulators are achieved based on the mechanism of mode conversion. They are mainly divided into LC-LC mode conversion, Dipole-LC mode conversion, and Dipole-Dipole mode conversion. Dipole resonance and LC resonance are the most basic resonant modes of MTMs. Consequently, the realization of these three mode conversions will improve the THz active device system.3. We design THz active devices exploiting the composite structure of MTMs and HEMTs. MTMs have a good ability to control the THz wave, and HEMTs have recently been widely used in radio frequency(RF) microwave for its highly dynamic response and well switching properties. This paper applies the HEMT to the core region of mode conversion MTMs. During the simulation, Drude mode is used to represent the two-dimensional electron gases(2DEGs) in the channel of HEMT. The transmission curves depending on different carrier concentrations will help to analyze the properties of this device, which will be benefit to the experiment and optimization.4. We carry out a careful research on device properties of the HEMT and completely fabricate THz active devices based on it. This paper introduces the fabrication of THz active devices in three aspects, including the choice of substrate, the photo-mask, and the micro-fabrication. Moreover, we also design and fabricate the packaged and matching circuit to satisfy the THz modulator chip, thus to realize higher integration and stability.5. We measure the specific parameters of THz active devices in detail. THz time-domain spectroscopy system(THz-TDS) is firstly used to observe the transmission spectra of this device under different voltages. Secondly, we use the THz solid-state source and power meter to observe the amplitude modulation under single-frequency baseband signals. Then we get the static resonance ability under actual application conditions and also study the linearity of this device.6. We set up a THz wireless communication system based on the external modulation. This system is composed of the THz transmit-receive(TR) system and the modulation system. The THz TR system includes a 340 GHz continue wave(CW) solid-state source and a THz detector. The modulation system consists of a THz external modulator and the communication baseband module. This modulator is tested by the sinusoidal signals and eye patters, respectively. The sinusoidal signals show a modulation speed up to 1 GHz and the eye patterns show this modulator has the data transmission ability of 0.1 Gbps, which demonstrates the feasibility of this external modulation THz wireless communication system.This paper has carried out a detailed research and study of THz active device, especially for the design, simulation, fabrication and measurement. We mainly realize the THz external modulator with the ability of real time communication. Besides the amplitude modulation, this modulator could also achieve the phase and frequency modulation. Specifically, we mainly fabricate the Dipole-Dipole mode conversion modulator, which has the simple structure and the good performance. After continuous explorations and optimizations, we finally get the THz modulator with 85% modulation depth, 68o phase shift, and 1 GHz modulation speed. THz active devices may provide advancements in THz communications, medical imaging, security checking, radars, telemetering, the aerospace, and so on.