Review of GaN-based devices for terahertz operation

What is it about?

GaN provides the highest electron saturation velocity, breakdown voltage, operation temperature, and thus the highest combined frequency-power performance among commonly used semiconductors. The industrial need for compact, economical, high-resolution, and high-power terahertz (THz) imaging and spectroscopy systems are promoting the utilization of GaN for implementing the next generation of THz systems. As it is reviewed, the mentioned characteristics of GaN together with its capabilities of providing high two-dimensional election densities and large longitudinal optical phonon of ∼90  meV make it one of the most promising semiconductor materials for the future of the THz emitters, detectors, mixers, and frequency multiplicators. GaN-based devices have shown capabilities of operation in the upper THz frequency band of 5 to 12 THz with relatively high photon densities in room temperature. As a result, THz imaging and spectroscopy systems with high resolution and deep depth of penetration can be realized through utilizing GaN-based devices. A comprehensive review of the history and the state of the art of GaN-based electronic devices, including plasma heterostructure field-effect transistors, negative differential resistances, hetero-dimensional Schottky diodes, impact avalanche transit times, quantum-cascade lasers, high electron mobility transistors, Gunn diodes, and tera field-effect transistors together with their impact on the future of THz imaging and spectroscopy systems is provided.

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Why is it important?

Among commonly used wide bandgap semiconductor materials, GaN provides the highest bandgap energy, electron saturation velocity, and thermal conductivity. GaN has been a well-known semiconductor material for electroluminescent visible light diodes since their first introduction in 1971.1,2 The urge for renewable energy systems and electric cars has promoted the application of GaN in power electronic converters.3 GaN-based devices provide high switching frequency and high power,4 which are needed for realization of the next generation of the electric grid, namely, the smart grid,5 and electric vehicles.6 High-frequency converters need less passive storage elements and are attractive for vehicle applications in terms of size, weight, reliability, and cost.7 Among the emerging systems in photonics, with developing the first terahertz (THz) imaging system in no more than two decades ago,8 THz systems including THz-time-domain spectroscopy (TDS) and THz-continuous wave (CW) imaging systems are developed with a fast pace. Thanks to low photon energy in the THz regime, the THz beam can traverse through most of the nonmetallic materials. x-ray photons can also traverse through nonmetallic materials. However, high-energy x-ray photons ionize the molecules of the object. Ionization is harmful to wide varieties of objects as it causes cancer for live tissues9 and degrades semiconductor devices.10 For avoiding degrading effects, for investigation of valuable historical artifacts,11 x-ray and chemicals might not be recommended. In addition, detaching the historical structures, such as wall paintings, to place them in a protected x-ray imaging chamber is not feasible. Although the degrading effect of intense THz radiation on DNA is reported,1213.–14 no other degrading effect is observed, and THz is still considered to be safer than x-ray in many areas. As a result, THz spectroscopy and imaging systems provide promising substitutes for ionizing x-ray and invasive chemical characterization tools in wide varieties of applications.15 THz-TDS systems are powerful tools for material spectroscopy, layer inspection, and transmission imaging of packaged objects.16 These capabilities of THz-TDS systems are utilized in authentication,1718.–19 nondestructive inspection of composite materials,2021.22.23.24.25.26.27.–28 three-dimensional imaging,2930.31.–32 metrology and quality control of industrial products,3334.35.36.–37 detection of concealed weapons,3839.40.41.42.43.44.–45 art investigations,46,47 tomography,4849.50.51.–52 biomedical diagnosis,5354.55.–56 material characterization,5758.59.60.61.–62 thickness measurement,63,64 and holography.6566.67.–68 Despite this variety of applications, THz systems are suffering from two major drawbacks, namely, (1) low resolution and (2) low photon intensity. Low resolution is the intrinsic feature of THz systems. According to the Gaussian beam and diffraction theories, the focused beam diameter, divergence, and bending of the beam are directly related to the wavelength.69 Common THz imaging systems are not capable of generating THz beams of higher than 5 THz. Moreover, the attenuation of the beam drops exponentially with respect to the wavelength, transmission imaging with beams with frequencies of higher than 1.5 THz has not been possible yet. This deficiency is shown in Fig. 1. As this figure indicates, the signal-to-noise ratio (SNR) of the generated beam via conventional GaAs-based photoconductive antennas (PCA) drops to zero by 4 THz and the intensity of the beam after passing through the sample, which in this case was a 2.3-mm packaged integrated circuits (IC), drops to zero for frequencies above 2 THz.70 Fig. 1 The SNR of the generated beam by conventional GaAs-based PCA drops to zero by 4 THz. When a 2.3 packaged IC is placed as the sample in the system, the SNR drops to zero at frequencies below 2 THz. OE_56_9_090901_f001.png A great deal of research is dedicated to the enhancement of the resolution of the THz imaging systems. In this regard, different groups are working on the enhancement of the resolution by approaching it from different aspects. Stantchev et al.71 have proposed a near-field THz imaging of hidden objects using a single-pixel detector. However, the drawback of near-field imaging is the fact that objects thicker than a few hundred micrometers cannot be imaged. Trofimov et al.72,73 have realized conventional image processing techniques for increasing the quality of THz imaging systems. Kulya et al.74 have proposed taking material dispersion into account for enhancing the quality of THz images. For suppressing the absorption in the physical lenses, diffraction lenses with low absorptions are proposed.7576.77.–78 Ahi and Anwar,79 Ahi et al.80 have proposed a mathematical algorithm to incorporate the THz imaging features into Gaussian beam theory in order to model the THz point spread function and demerge the merged feature through deconvolution. Chernomyrdin et al.81 have achieved promising resolution enhancement by utilizing solid immersion imaging and wide-aperture spherical lens.82 In another trend, for the enhancement of the imaging systems, subwavelength focusing using hyperbolic meta materials is proposed by Kannegulla et al.8384.–85 However, as it will be discussed in this paper, GaN-based devices can fundamentally address the resolution by enabling THz imaging systems with frequencies higher than 5 THz and enhancing the photon intensity. For instance, GaN-based quantum-cascade lasers (QCL) can operate in 5 to 12 THz,86 whereas the operation of conventional naturally cooled GaAs-based QCLs in the upper THz frequency band is limited by longitudinal-optical (LO) phonon of 36 meV.87 For overcoming the large beam diameter, diffraction, and absorption issue, THz imaging and spectroscopy systems that can operate in the upper THz frequency band and with higher photon densities are needed. Wide bandgap semiconductor devices provide promising features for implementing such devices. Figure 2 shows the numerical values of characteristics of GaN and GaAs. The bandgap energy, saturation velocity, and thermal conductivity of GaN are all more than twice of those of GaAs. As a result, GaN devices offer higher output power and operation frequency compared with other conventional III to V devices.8889.90.–91 The mentioned characteristics of GaN together with its capabilities of providing high two-dimensional (2-D) election densities and high LO phonon of ∼ 90 meV ∼90  meV make it one of the most promising semiconductors for the future of the generation, detection, mixing, and frequency multiplication of the electromagnetic waves in THz frequency regime. As Fig. 3 shows, plasmonic GaN-based heterostructure field-effect transistors (HFETs) and QCL have capabilities of operating in the upper THz frequency band of 5 to 12 THz, in room temperature and with relatively high emission powers.9293.–94 Fig. 2 Comparison between GaN and GaAs. OE_56_9_090901_f002.png Fig. 3 Superiority of GaN-based devices over other devices in THz regime. Updated and reprinted from Ref. 92, with permission of Nature Publishing Group. OE_56_9_090901_f003.png In this paper, a comprehensive review of the history and state-of-the-art of the GaN-based electronic devices and their impact on the future of THz imaging and spectroscopy systems is provided. Plasma HFETs, negative differential resistances (NDRs), hetero-dimensional Schottky diodes (HDSDs), impact avalanche transit times (IMPATTs), QCLs, high electron mobility transistors (HEMTs), Gunn diodes, and tera field-effect transistors (TeraFETs) together with their impact on the future of THz imaging and spectroscopy systems are reviewed. Challenges that are in front of scientific groups for implementing GaN devices are discussed, and a detailed report on the timeline and state of the art of achievements of different research groups in developing models, theories, and implementing practical GaN THz devices is given. Characterization techniques of GaN in THz frequency range are reviewed as well. This paper is organized as follows: Sec. 2 provides a review of the history and state-of-the-art of GaN for THz applications. Section 3 reviews characterization techniques of GaN in the THz frequency range. Section 4 concludes this paper and proposes a roadmap for utilization of GaN in order to address the growing demands of THz imaging and spectroscopy systems.

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