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We developed an AlGaN/GaN heterojunction phototransistor with a recessed detection area to enhance the photoresponsivity. The recessed-AlGaN/GaN phototransistor exhibited a maximum photoresponsivity of 1.6 × 107 A/W at 375 ㎚, which was ~30% higher than that obtained with a conventional AlGaN/GaN phototransistor. A comparable photoresponsivity was also achieved at 260 ㎚ in UV-C range due to the dual absorption process in conjunction with the polarization induced built-in electric field characteristics of AlGaN/GaN heterojunction.

We optimized a silicon nitride (SiNx) passivation process using a catalytic-chemical vapor deposition (Cat-CVD) system to suppress the current collapse phenomenon of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs). The optimized Cat-CVD SiNx film exhibited a high film density of 2.7 g/cm3 with a low wet etch rate (buffered oxide etchant (BOE) 10:1) of 2 nm/min and a breakdown field of 8.2 MV/cm. The AlGaN/GaN-on-Si HEMT fabricated by the optimized Cat-CVD SiNx passivation process, which had a gate length of 1.5 &mu, m and a source-to-drain distance of 6 &mu, m, exhibited the maximum drain current density of 670 mA/mm and the maximum transconductance of 162 mS/mm with negligible hysteresis. We found that the optimized SiNx film had positive charges, which were responsible for suppressing the current collapse phenomenon.

We demonstrated gallium nitride (GaN) based negative capacitance field-effect transistors (NCFETs) that achieve steep switching operation. Ferroelectric phase characteristics were successfully demonstrated using a 10 nm thick undoped-HfO2 ferroelectric thin film fabricated through atomic layer deposition. The HfO2 metal-ferroelectric-metal (MFM) capacitor was connected in series with the gate electrode of a recessed normally-off AlGaN/GaN MOS-HFET. The forward/reverse subthreshold slopes were dramatically reduced from 104/105 to 22/23 mV/dec.

This paper reports the first-time evaluation of the time-dependent dielectric breakdown of recessed AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with plasma enhanced chemical vapor deposition (PECVD) SiO2 gate oxide. The interface fixed charge density and oxide bulk charge density extracted from the flat-band voltage characteristics were 2.7 × 1011 ± 6.54 × 1010 cm−2 and -9.71 × 1017 ± 5.18 × 1016 cm−3, respectively. The time dependent dielectric breakdown (TDDB) characteristics exhibited longer lifetime estimation as the SiO2 thickness increased. The excellent reliability of the PECVD SiO2 film was validated for use as the gate oxide of recessed AlGaN/GaN MOS-HFET.

We have developed a silicon oxynitride (SiON) deposition process using a plasma-enhanced chemical vapor deposition system for the gate dielectric of GaN-on-Si metal–insulator–semiconductor field-effect transistors (MISFETs). The optimized SiON film had a relative dielectric constant of 5.3 and a breakdown field of 12MV/cm. A normally-off GaN-on-Si MISFET fabricated with a 33-nm SiON gate dielectric exhibited a threshold voltage of ~2 V, an ON-resistance of $7.85~\mathsf {m}\Omega \cdot \mathsf {cm}^{{\mathsf {2}}}$ , and a breakdown voltage of ~640 V at the OFF-state current density of $1~\mu \text{A}$ /mm. The extracted interface trap density was $\mathsf {1} \times \mathsf {10}^{{\mathsf {12}}}$ cm $^{{\mathsf {-2}}}\cdot \mathsf {eV}^{\mathsf {-1}}$ at ${E}_{c}- {E}_{t}= \mathsf {0.442}$ eV, which resulted in negligible hysteresis and excellent dynamic characteristics.

We have investigated the channel mobility of AlGaN/GaN-on-Si recessed-metal-oxide-semiconductor-heterojunction field-effect transistors (recessed-MOS-HFET) with SiO 2 gate oxide. Both field-effect mobility and effective mobility for the recessed-MOS channel region were extracted as a function of the effective transverse electric field. The maximum field effect mobility was 380 cm 2 /V·s near the threshold voltage. The effective channel mobility at the on-state bias condition was 115 cm 2 /V·s at which the effective transverse electric field was 340 kV/cm. The influence of the recessed-MOS region on the overall channel mobility of AlGaN/GaN recessed-MOS-HFETs was also investigated.

This paper reports a new method to enable the normally-off operation of AlGaN/GaN heterojunction field-effect transistors (HFETs). A capacitor was connected to the gate input node of a normally-on AlGaN/GaN HFET with a Schottky gate where the Schottky gate acted as a clamping diode. The combination of the capacitor and Schottky gate functioned as a clamp circuit to downshift the input signal to enable the normally-off operation. The normally-off operation with a virtual threshold voltage of 5.3 V was successfully demonstrated with excellent dynamic switching characteristics.

We have investigated the effects of a silicon nitride (SiNx) passivation process using plasma-enhanced chemical vapor deposition (PECVD) on a ultra-thin-barrier AlGaN/GaN heterostructure field-effect transistors (HFETs). The bulk charge characteristics of the PECVD SiNx films were dependent on the film deposition conditions, which strongly influenced the sheet resistance (Rsh) and flat-band voltage characteristics of AlGaN/GaN HFETs. The reduction in Rsh is a strong function of the amount of positive bulk charges in the SiNx passivation film. An optimized PECVD SiNx process was used to drastically decrease the Rsh from 45,450 Ω/sq to 732 Ω/sq. A Mo/Au Schottky-gate device fabricated with PECVD SiNx passivation exhibited a maximum drain current density of 172 mA/mm, quasi-normally-off operation, and breakdown voltage of > 1100 V

This study investigated the effects of a thin aluminum oxynitride (AlOxNy) gate insulator on the electrical characteristics of AlGaN/GaN-on-SiC metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). The fabricated AlGaN/GaN-on-SiC MIS-HEMTs exhibited a significant reduction in gate leakage and off-state drain currents in comparison with the conventional Schottky-gate HEMTs, thus enhancing the breakdown voltage. The effects of gate recess were also investigated while using recessed MIS-HEMT configuration. The Johnson’s figures of merit (= fT × BVgd) for the fabricated MIS-HEMTs were found to be in the range of 5.57 to 10.76 THz·V, which is the state-of-the-art values for GaN-based HEMTs without a field plate. Various characterization methods were used to investigate the quality of the MIS and the recessed MIS interface.

In this study, we have investigated the effects of SF6 plasma treatment on the isolation leakage current characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs). The device isolation was performed by Cl2/BCl3-based plasma etching process followed by SF6 plasma treatment for 1 min. It was found that the SF6 plasma post-etching treatment was an effective method to improve the electrical isolation characteristics. X-ray photoelectron spectroscopy analysis was carried out to investigate the chemical bonding states of the etched GaN surfaces.

We investigated the growth behavior of GaN grown on AlN along with Ⅴ/Ⅲ ratio and pressure variation, and found out lateral growth regime for fully coalesced channel layer of the AlN-based double-hetero structure high electron mobility transistor (HEMT). When the Ⅴ/Ⅲ ratio increases and pressure decreases, compressive stress in the GaN channel increases, and pit formation occurs to release the stress. The AlN-based HEMT structure was grown and the device was fabricated with an optimized channel layer. The two-dimensional electron gas mobility, sheet density, and sheet resistance were 1480 cm2/Vs, 1.32 × 1013 cm-2, and 319 Ω/, respectively, at room temperature. The device was characterized; direct current output result showed that the maximum current was ~620 mA/mm, on-resistance was 6.4 Ωmm, transconductance was ~140 mS/mm, and current on/off ratio was ~104, respectively.

We developed a high-quality plasma enhanced atomic layer deposited (PEALD) aluminum oxynitride (AlO x N y ) process for the metal-oxide-semiconductor (MOS) gate insulator of fully-recessed-gate AlGaN/GaN-on-Si MOS-heterojunction field-effect transistors (MOS-HFETs). It was found that the cyclic nitrogen incorporation into aluminum oxide (Al2O3) during PEALD process improved the conduction band offset at GaN interface resulting in higher forward breakdown field strength, which also contributed to suppressed trapping effects under forward gate bias stress. Improved interface characteristics, which resulted from suppressed surface oxidation, led to significant improvement of threshold voltage stability. A low threshold voltage hysteresis of 180 mV at maximum gate sweep voltage of 10 V was obtained with AlO x N y gate insulator. The MOS channel mobility was also improved to 235 cm2 V−1 s−1. The fabricated fully-recessed-gate AlGaN/GaN-on-Si MOS-HFET with PEALD AlO x N y gate insulator exhibited excellent overall performances such as a threshold voltage of 3.2 V, a maximum drain current density of 481 mA mm−1, an on/off current ratio of ~1010, an ON-resistance of 12 mΩ mm, and a breakdown voltage of 1050 V.

A normally-off AlGaN/GaN-on-Si metal–oxide–semiconductor-heterojunction field-effect transistor (MOS-HFET) with an integrated single-stage inverter is developed. The integrated single-stage GaN inverter consisted of an AlGaN/GaN driver MOS-HFET and a resistive load. With the monolithically integrated gate driver, the gate charging current was boosted from 10 to 170 mA, which reduced the charging and discharging times from 626 to 107 ns and 553 to 196 ns, respectively.

Radio frequency (RF)-sputtered ZnO gate dielectrics for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) were investigated with varying O2/Ar ratios. The ZnO deposited with a low oxygen content of 4.5% showed a high dielectric constant and low interface trap density due to the compensation of oxygen vacancies during the sputtering process. The good capacitance–voltage characteristics of ZnO-on-AlGaN/GaN capacitors resulted from the high crystallinity of oxide at the interface, as investigated by x-ray diffraction and high-resolution transmission electron microscopy. The MOS-HEMTs demonstrated comparable output electrical characteristics with conventional Ni/Au HEMTs but a lower gate leakage current. At a gate voltage of −20 V, the typical gate leakage current for a MOS-HEMT with a gate length of 6 μm and width of 100 μm was found to be as low as 8.2 × 10−7 mA mm−1, which was three orders lower than that of the Ni/Au Schottky gate HEMT. The reduction of the gate leakage current improved the on/off current ratio by three orders of magnitude. These results indicate that RF-sputtered ZnO with a low O2/Ar ratio is a good gate dielectric for high-performance AlGaN/GaN MOS-HEMTs.

We have investigated the effects of post-metallization-annealing (PMA) in oxygen atmosphere on recessed-gate GaN metal-oxide-semiconductor heterostructure field effect transistors (MOSHFETs). The flat band voltage of MOS is a function of bulk and interface charges in the oxide, which strongly depends on a post-annealing process as well as deposition conditions. A positive threshold voltage shift enabling normally-off operation has been achieved by an O2 PMA process where the GaN MOSHFET employed an ICPCVD SiO2 gate oxide with a Ni/Au metal gate. According to the analysis using energy dispersive x-ray spectroscopy in transmission electron microscopy and x-ray photoelectron spectroscopy, it is suggested that the improved SiO2/GaN interface quality with an enhanced metallic-like Ga level was responsible for the positive shift in threshold voltage.

We have developed a low-temperature ohmic contact process with a recessed overhang configuration for Au-free (complementary metal-oxide semiconductor) CMOS-compatible AlGaN/GaN heterostructure field effect transistors (HFETs). The recessed overhang configuration has a Ti/Al bilayer directly in contact with the AlGaN/GaN heterojunction interface at the recessed sidewall overlaid with the AlGaN barrier layer, which allows good and reproducible ohmic formation with low-temperature annealing. The optimum Ti/Al thickness was 40/200 nm, which resulted in an Rc of 0.76 Ω mm with excellent surface morphology when annealed at 550 °C for 1 min. The device fabricated with a Ni/Mo gate exhibited a maximum drain current density of ~500 mA mm−1, a specific on-resistance of 1.35 mΩ cm2 and a breakdown voltage of >1 kV.