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To improve the stability and sensitivity of pH sensors based on AlGaN/GaN high electron mobility transistors (HEMTs), we proposed a modified sensing structure by integrating a reference HEMT device. This structural pH sensor exhibits a typical Nernstian behavior with a sensitivity of 54.38 mV/pH, which is higher than the value of 49.43mV/pH derived from the AlGaN/GaN HEMT-based sensor without integrating the reference HEMTs device. Furthermore, the stability of the new structural sensor is enhanced by approximately 19.2% in comparison with that of its traditional counterpart. The improved performances are analyzed using an electrical double-layer model together with an equivalent circuit model, and we find that the new sensor structure has a larger capacitor and exhibits a better rectification effect, hence decreasing the electrical noise and enhancing the stability of the testing signal. Meanwhile, the new sensor structure displays a smaller equivalent resistance and results in a larger available output current, hence exhibiting a higher sensitivity.

Au/TiO2/GaN junctions with a TiO2 interlayer were deposited at various oxygen pressures from 5 mTorr to 80 mTorr, and their surface, microstructural, and electrical properties were investigated. Compared with a single 5 μm-Si-doped GaN film on an (0001) Al2O3 substrate, the 0.2 μm-Si-doped GaN film with an undoped GaN buffer layer demonstrated improved properties for the application to metal/semiconductor junctions. Atomic force microscope and tunneling electron microscope measurements suggested that the TiO2 interlayer deposited at room temperature exhibited a distinctive change above the oxygen deposition pressure of 40 mTorr. In contrast to the small rectification ratio of 101-102 for Au/GaN junctions, the Au/TiO2/GaN junctions with the TiO2 interlayers displayed a large rectification ratio of 106-107 when the oxygen pressure during the deposition of TiO2 interlayer was maintained at 40 mTorr. These results suggest that the leakage current and the rectification behavior in a metal/oxide/semiconductor junction can be effectively controlled using the oxygen deposition pressure for an oxide interlayer.

AlGaN/GaN heterostructures were successfully grown with the AlIn(Ga)N back barrier at 900 °C. However, the atomic composition of the AlIn(Ga)N layer was strongly dependent on the growth pressure, which resulted in a different lattice constant of the layer. The AlIn(Ga)N back barrier grown at 400 torr was almost lattice-matched to GaN layer. The AlGaN/GaN heterostructures with 10 and 15 nm-thick AlIn(Ga)N back barrier exhibited improved 2-DEG properties, compared to those of the conventional AlGaN/GaN heterostructure without the back barrier. The high electron mobility transistors (HEMTs) fabricated on the AlGaN/GaN heterostructure with a 15 nm-thick AlIn(Ga)N back barrier exhibited a very low off-state leakage current of ∼2 × 10−7 A/mm which is about 1 order lower in magnitude than the value of the device without the back barrier. The AlIn(Ga)N back barrier is a promising candidate as an alternative to conventional AlGaN and InGaN back barrier.

In this work, threshold voltage modulation realized by adjusting fin width and dielectric layer were investigated through MIS-FinFETs. As fin width decreases from 120 to 30 nm, threshold voltage shifts toward positive direction and finally becomes positive value while maintaining SS smaller than 60 mV/dec. The phenomenon of achieving sub-60 mV/dec characteristics were illustrated by simulation and the concept of effective channel length. As for dielectric layer, by 20 nm-thick SiO 2 dielectric layer, device with fin width of 90 nm exhibits a threshold voltage of −0.5 V with SS as small as 50 mV/dec. Even when fin width is 30 nm, drain leakage is still not small enough when VG = 0, which indicates that the gate is not able to totally deplete the fin structure. In order to further increase threshold voltage and enhance the gate controllability, by 10 nm-thick Al 2 O 3 dielectric layer, a threshold voltage of 2 V is achieved when fin width is 40 nm with SS as small as 52 mV/dec due to the higher dielectric constant and thinner thickness of Al 2 O 3 compared with SiO 2 . Therefore, by the modulation of fin width, dielectric layer type, and dielectric thickness, threshold voltage can be carefully designed according to the application requirements while maintaining SS below 60 mV/dec.

In this paper, we investigated characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs) with high resistive buffer structure consisted of periodically carbon-doped (PCD) GaN buffer layer and AlGaN back barrier layer. The PCD structure was proposed for reducing undesirable trapping effects, which resulted in effective suppression of the current collapse compared to that in conventional carbon buffer structure. To further improve the dynamic performances of the device and to increase the electron confinement of the 2-D electron gas (2-DEG) channel, AlGaN back barrier was inserted between the GaN channel and the PCD buffer layer, which results in greatly improved current collapse with slightly improved 2-DEG mobility compared to those of the device without back barrier. The OFF-state leakage current of the device with back-barrier is about 2 orders lower in magnitude than that of device without back barrier, which leads to the breakdown voltage of 2 kV and figure of merit of 2.27 GV2Ω-1cm-2 for the device with LGD of 10 μm, one of the highest values ever reported for the GaN-based HEMTs.

In this work, we fabricated AlInGaN/GaN FinFETs and compare electrical performances with those of AlGaN/GaN FinFETs in different channel structures, such as single and double channel. The double-channel structure is promising to compensate for the degradation of the drain current density caused by the fin structure, as well as the FinFET structure is suitable to control the double-channel structure. The fabricated AlInGaN/GaN double channel FinFETs exhibit considerably higher maximum drain current of 290 mA/mm than those of AlGaN/GaN FinFETs such as 245 mA/mm and 165 mA/mm of double- and single-channel FinFETs, respectively. This is because of the high electron density caused by the strong polarization charge of AlInGaN/GaN heterostructure and double channel structure. Moreover, the double-channel FinFETs effectively suppress the current collapse. In conclusion, the AlInGaN/GaN double-channel fin structure is a very promising candidate for the GaN-based RF power applications.

We investigate the DC and 1/f noise properties in Al0.25Ga0.75N/GaN high-electron mobility transistors (HEMTs) with two types of 2 μm-thick periodically carbon-doped GaN buffer layer (PC-doped GaN buffer) with and without inserting the 30 nm-thick Al0.05Ga0.95N back barrier layer between the GaN channel layer and the PC-doped GaN buffer. The PC-doped GaN buffer layer consists of multiple layers of 12 nm-thick C-doped GaN layer with doping concentration of 1 × 1018 cm−3 and 50 nm-thick undoped GaN layer with unintentional n-typing concentration of 2 × 1016 cm−3. A reference AlGaN/GaN HEMT with 2 μm-thick highly-resistive GaN buffer layer without C-doping is also fabricated for comparison. Similarly to the reference AlGaN/GaN HEMT, the AlGaN/GaN HEMTs with PC-doped GaN buffer show typical 1/f noise characteristics mainly due to the trapping effects at the AlGaN/GaN interface from subthreshold region to strong-accumulation region, which indicates that the deep trapping effects in the PC-doped GaN buffer layer is negligible, and experience the correlated mobility fluctuations (CMF), which is convinced from the drain current power spectral density (PSD) versus drain current. At off-state (deep-subthreshold region), on the other hand, the HEMTs with the PC-doped GaN buffer layer exhibit 1/f2 noise characteristics, which are closely related to the generation-recombination (g-r) noise caused by the spatial trapping/detrapping process between the deep acceptor in the C-doped layer and the shallow donor in the undoped layer in the PC-doped GaN buffer, while the reference HEMT still shows typical 1/f noise characteristics.

We have grown an AlIn(Ga)N/GaN heterostructure which is a promising alternative to the AlGaN/GaN heterostructure. The mobility and the carrier concentration of the two-dimensional electron gas (2DEG) formed at the AlIn(Ga)N/GaN heterointerface were strongly dependent on both the growth temperature and pressure. Two optimized growth conditions for the heterostructure with a low sheet resistance less than 300 Ω/sq were obtained by varying the growth temperature and pressure from 750 to 1070 °C and 100 to 300 torr, respectively: (i) AlIn(Ga)N/GaN heterostructure with high 2DEG carrier concentration of 2.4 × 1013 cm−2 with mobility of 1010 cm2 V−1s−1 (grown at 900 °C and 100 torr); (ii) AlIn(Ga)N/GaN heterostructure with high 2DEG mobility of 1910 cm2 V−1s−1 with carrier concentration of 1.13 × 1013 cm−2 (grown at 900 °C and 300 torr). Both structures would be useful for different purpose of device application.