Jinyan Wang

High-Performance Normally-Off

This letter reports a normally-OFF Al₂O₃/GaN gate-recessed MOSFET using a low-damage digital recess technique featuring multiple cycles of plasma oxidation and wet oxide removal process. The wet etching process eliminates the damage induced by plasma bombardment induced in conventional inductively coupled plasma dry etching process so that good surface morphology and high interface quality could be achieved. The fully recessed Al₂O₃/GaN MOSFET delivers true enhancement-mode operation with a threshold voltage of +1.7 V. The maximum output current density is 528 mA/mm at a positive gate bias of 8 V. A peak field-effect mobility of 251 cm²/V·s is obtained, indicating high-quality Al₂O₃/GaN interface.

{\rm Al}_{2}{\rm O}_{3}/{\rm GaN}

In this letter, enhancement-mode (E-mode) AlGaN/ GaN high electron mobility transistors (HEMTs) were demon- strated based on lateral scaling of the 2-D electron gas channel using nanochannel array (NCA) structure. The NCA structure consists of multiple parallel channels with nanoscale width defined by electron-beam lithography and dry etching. Because of the improved gate control from the channel sidewalls and partially relaxed piezoelectric polarization, the fabricated 2 μm-gate-length NCA-HEMT with a nanochannel width of 64 nm showed a thresh- old voltage of +0.6 V and a higher extrinsic transconductance of 123 mS/mm, compared to -1.6 V and 106 mS/mm for the conventional HEMT with μm-scale channel width. The scaling of threshold voltages, peak transconductance, and gate leakage as a function of the nanochannel width were investigated. Small-signal RF performance of NCA-HEMTs were characterized for the first time and compared with those of conventional HEMTs.

MOSFET Using a Wet Etching-Based Gate Recess Technique

A mechanism of the formation of the bulges on the surface of Ti/Al/Ni/Au Ohmic contact in AlGaN/GaN high electron mobility transistors is proposed. According to the analysis of TEM images and corresponding electron dispersive x-ray spectra, the bulges were found to consist of Ni–Al alloy in the body and Au–Al alloy surrounding. We deduce that the bulges were formed due to Ni–Al alloy aggregation in some local areas during the rapid thermal annealing process, which accounts for the rough surface morphology.

Enhancement-Mode Operation of Nanochannel Array (NCA) AlGaN/GaN HEMTs

A self-terminating gate recess etching technique is first proposed to fabricate normally off AlGaN/GaN MOSFET. The gate recess process includes a thermal oxidation of the AlGaN barrier layer for 40 min at 615°C followed by 45-min etching in potassium hydroxide solution at 70°C, which is found to be self-terminated at the AlGaN/GaN interface with negligible effect on the underlying GaN layer, manifesting itself easy to control, highly repeatable, and promising for industrialization. The fabricated device based on this technique with atomic layer deposition Al2O3 as gate insulator exhibits a threshold voltage as high as 3.2 V with a maximum drain current over 200 mA/mm and a 60% increased breakdown voltage than that of the conventional high electron mobility transistors.

Analysis of surface roughness in Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN high electron mobility transistors

In this paper, we report the device performance of a high-voltage normally off Al₂O₃/GaN MOSFET on the Si substrate. Normally off operation is obtained by multiple cycles of O₂ plasma oxidation and wet oxide-removal gate recess process. The recessed normally off GaN MOSFET with 3 μm gate-drain distance exhibits a maximum drain current of 585 mA/mm at 9 V gate bias. The threshold voltage of the MOSFET is 2.8 V with a standard derivation of 0.2 V on the sample with an area of 2 × 2 cm². The gate leakage current is below 10^-6 mA/mm during the whole gate swing up to 9 V and the ION/IOFF ratio is larger than 109, indicating the good quality of Al₂O₃ gate insulator. The MOSFET with 10 μm gate-drain distance shows a three terminal OFF-state breakdown voltage (BV) of 967 V at zero gate-source bias with a drain leakage current criterion of 1 μA/mm. The specific ON-resistance (R_ON,SP) of the device is 1.6 mQ · cm² and the power figure of merit (BV²/R_ON,SP) is 584 MW/cm².

Fabrication of Normally Off AlGaN/GaN MOSFET Using a Self-Terminating Gate Recess Etching Technique

In this letter, we investigated the behaviors of surface- and buffer-induced current collapse in AlGaN/GaN high-electron mobility transistors (HEMTs) using a soft-switched pulsed I-V measurement with different quiescent bias points. It is found that the surface- and buffer-related current collapse have different relationship with the gate and drain biases (VGS0,VDS0) during quiescent bias stress. The surface-induced current collapse in devices without passivation monotonically increases with the negative VGS0, suggesting that an electron injection to the surface from gate leakage is the dominant mechanism and the Si3N4 passivation could effectively eliminate such current collapse. The buffer-induced current collapse in devices with intentionally carbon-doped buffer layer exhibits a different relationship with VGS0 after surface passivation. The buffer-related current collapse shows a bell-shaped behavior with VGS0, suggesting that a hot electron trapping in the buffer is the dominant mechanism. The soft-switched pulsed I-V measurement provides an effective method to distinguish between the surface- and buffer-related current collapse in group III-nitride HEMTs.

900 V/1.6

In this letter, a plasma-free etch stop structure is developed for GaN HEMT toward enhancement-mode operation. The self-terminated precision gate recess is realized by inserting a thin AlN/GaN bilayer in the AlGaN barrier layer. The gate recess is stopped automatically at the GaN insertion layer after high-temperature oxidation and wet etch, leaving a thin AlGaN barrier to maintain a quantum well channel that is normally pinched off. With addition of an Al2O3 gate dielectric, quasi normally OFF GaN MOSHEMTs have been fabricated with high threshold uniformity and low ON-resistance comparable with the normally ON devices on the same wafer. A high channel mobility of 1400 cm2/V·s was obtained due to the preservation of the high electron mobility in the quantum-well channel under the gate.

{\rm m}\Omega\cdot{\rm cm}^{2}

In this article, a detailed analysis of the wet-etching technique for AlGaN/GaN heterostructure using dry thermal oxidation followed by a wet alkali etching was performed. The experimental results show that the oxidation plays a key role in the wet-etching method and the etching depth is mainly determined by the oxidation temperature and time. The correlation of etching roughness with oxidation time and temperature was investigated. It is found that there exists a critical oxidation temperature in the oxidation process. Finally, a physical explanation of the oxidation procedure for AlGaN layer was given.

Normally Off

Based on our proposed self-terminating gate recess etching technique, normally-off recess-gated AlGaN/GaN MOSFET has been demonstrated with a novel method using GaN cap layer (CL) as recess mask, which, as a result, simplifies the device fabrication process and lowers the fabrication cost. The GaN CL is capable of acting as an effective recess mask for the gate recess process, which includes a thermal oxidation for 45 min at 650 °C followed by 4-min etching in potassium hydroxide (KOH) at 70°C. After gate recess process, no obvious change is observed in terms of the surface morphology of the GaN CL, the contact resistance of the Ohmic contact formed directly on the GaN CL as well as the sheet resistance of the two-dimensional electron gas (2-DEG) channel layer under the GaN CL. The fabricated device exhibits a threshold voltage (Vth) as high as 5 V, a maximum drain current (Idmax) of ~200 mA/mm, a high ON/OFF current ratio of ~1010 together with a low forward gate leakage current of ~10-5 mA/mm. Meanwhile, the OFF-state breakdown voltage (Vbr) of the device with gate-drain distance of 6 μm is 450 V.

{\rm Al}_{2}{\rm O}_{3}/{\rm GaN}

A test structure, which is called the ridge-furrow structure, is used to evaluate the leakage current of the Schottky contact on the mesa edge of an AlGaN/GaN heterostructure. The mesa edge leakage currents were measured at different temperatures from 300 to 500 K and analyzed. The conduction-band-edge energy distribution at the mesa edge is simulated by the Integrated Systems Engineering Technology Computer-Aided Design. Based on the simulation results, the electric field strength can be obtained as a function of reverse bias voltage. The mesa edge leakage current is found to agree with the predicted characteristics, which is based on the Frenkel-Poole emission model. Therefore, we believe that the Frenkel-Poole emission dominates the mesa edge leakage at temperatures between 300 and 500 K.