Ronghui Hao

Normally-off p-GaN/AlGaN/GaN high electron mobility transistors using hydrogen plasma treatment

In this letter, we report a method by introducing hydrogen plasma treatment to realize normally-off p-GaN/AlGaN/GaN HEMT devices. Instead of using etching technology, hydrogen plasma was adopted to compensate holes in the p-GaN above the two dimensional electron gas (2DEG) channel to release electrons in the 2DEG channel and form high-resistivity area to reduce leakage current and increase gate control capability. The fabricated p-GaN/AlGaN/GaN HEMT exhibits normally-off operation with a threshold voltage of 1.75 V, a subthreshold swing of 90 mV/dec, a maximum transconductance of 73.1 mS/mm, an ON/OFF ratio of 1 × 107, a breakdown voltage of 393 V, and a maximum drain current density of 188 mA/mm at a gate bias of 6 V. The comparison of the two processes of hydrogen plasma treatment and p-GaN etching has also been made in this work.

Fabrication of normally-off AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors by photo-electrochemical gate recess etching in ionic liquid

We characterized an ionic liquid (1-butyl-3-methylimidazolium nitrate, C8H15N3O3) as a photo-electrochemical etchant for fabricating normally-off AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs). Using the ionic liquid, we achieved an etching rate of ~2.9 nm/min, which is sufficiently low to facilitate good etching control. The normally-off AlGaN/GaN MIS-HEMT was fabricated with an etching time of 6 min, with the 20 nm low-pressure chemical vapor deposition (LPCVD) silicon nitride (Si3N4) gate dielectric exhibiting a threshold voltage shift from −10 to 1.2 V, a maximum drain current of more than 426 mA/mm, and a breakdown voltage of 582 V.

Degradation of AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors under off-state electrical stress

In this paper, the authors have fabricated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors employing the low-pressure chemical vapor deposition (LPCVD) SiNx as the gate insulator with field plate structure and the long-term degradation was investigated under off-state stress with degradation process monitoring. The gate leakage and drain leakage under off-state electrical stress showed different change rules while the former was expected to be effectively suppressed by LPCVD-SiNx dielectric. The output and transfer characteristics between the stress were obtained periodically to investigate the degradation process. Through the analysis of the degradation of the parameters and the shifts of the Raman spectra, the inverse piezoelectric effect is believed to be the dominant degradation mechanism.

Gate leakage mechanisms in normally off p-GaN/AlGaN/GaN high electron mobility transistors

In this letter, gate leakage mechanisms in different gate contact normally off p-GaN/AlGaN/GaN high electron mobility transistors (HEMTs) have been studied by the temperature-dependent current-voltage (IG-VG) measurement. It is found that two-dimensional variable range hopping is responsible for gate leakage current at the reverse gate bias and low forward gate bias in both high-leakage and low-leakage Schottky gate contact devices. At high forward gate bias, in the case of high-leakage Schottky contact, the dominant current conduction mechanism is found to be thermionic field emission while it is Poole–Frenkle emission (PFE) for the case of low-leakage Schottky contact and the activation energy of trap states for PFE current is derived as 0.6 eV. Besides, related models are also proposed to describe the gate leakage current in p-GaN gate HEMTs and they match well with the experimental gate leakage current within a wide range of temperatures and gate biases.In this letter, gate leakage mechanisms in different gate contact normally off p-GaN/AlGaN/GaN high electron mobility transistors (HEMTs) have been studied by the temperature-dependent current-voltage (IG-VG) measurement. It is found that two-dimensional variable range hopping is responsible for gate leakage current at the reverse gate bias and low forward gate bias in both high-leakage and low-leakage Schottky gate contact devices. At high forward gate bias, in the case of high-leakage Schottky contact, the dominant current conduction mechanism is found to be thermionic field emission while it is Poole–Frenkle emission (PFE) for the case of low-leakage Schottky contact and the activation energy of trap states for PFE current is derived as 0.6 eV. Besides, related models are also proposed to describe the gate leakage current in p-GaN gate HEMTs and they match well with the experimental gate leakage current within a wide range of temperatures and gate biases.

Influence factors and temperature reliability of ohmic contact on AlGaN/GaN HEMTs

In this paper, we have studied the performance of Ti/Al/Ni/Au ohmic contact with different Al and Au thicknesses and pretreatments. The temperature dependence of contact resistances (Rc) was investigated and it shows that there are different optimal annealing temperatures with different metal thicknesses and pretreatments. The optimal annealing temperature is affected by Al and Au thickness and AlGaN thickness. The etched AlGaN barrier is useful to achieve good ohmic contact (0.24 Ω·mm) with a low annealing temperature. Only the contact resistances of the samples with 130 nm Al layer kept stable and the contact resistances of the samples with 100nm and 160 nm Al layers increased with the measurement temperatures. The contact resistances showed a similar increase and then keep stable trend for all the samples in the long-term 400 °C aging process. The ohmic metal of 20/130/50/50 nm Ti/Al/Ni/Au with ICP etching is the superior candidate considering the contact resistance and reliability.In this paper, we have studied the performance of Ti/Al/Ni/Au ohmic contact with different Al and Au thicknesses and pretreatments. The temperature dependence of contact resistances (Rc) was investigated and it shows that there are different optimal annealing temperatures with different metal thicknesses and pretreatments. The optimal annealing temperature is affected by Al and Au thickness and AlGaN thickness. The etched AlGaN barrier is useful to achieve good ohmic contact (0.24 Ω·mm) with a low annealing temperature. Only the contact resistances of the samples with 130 nm Al layer kept stable and the contact resistances of the samples with 100nm and 160 nm Al layers increased with the measurement temperatures. The contact resistances showed a similar increase and then keep stable trend for all the samples in the long-term 400 °C aging process. The ohmic metal of 20/130/50/50 nm Ti/Al/Ni/Au with ICP etching is the superior candidate considering the contact resistance and reliability.

Design and simulation of a novel E-mode GaN MIS-HEMT based on a cascode connection for suppression of electric field under gate and improvement of reliability*

We proposed a novel AlGaN/GaN enhancement-mode (E-mode) high electron mobility transistor (HEMT) with a dual-gate structure and carried out the detailed numerical simulation of device operation using Silvaco Atlas. The dual-gate device is based on a cascode connection of an E-mode and a D-mode gate. The simulation results show that electric field under the gate is decreased by more than 70% compared to that of the conventional E-mode MIS-HEMTs (from 2.83 MV/cm decreased to 0.83 MV/cm). Thus, with the discussion of ionized trap density, the proposed dual-gate structure can highly improve electric field-related reliability, such as, threshold voltage stability. In addition, compared with HEMT with field plate structure, the proposed structure exhibits a simplified fabrication process and a more effective suppression of high electric field.

Interface Si donor control to improve dynamic performance of AlGaN/GaN MIS-HEMTs

In this letter, we have studied the performance of AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) with different interface Si donor incorporation which is tuned during the deposition process of LPCVD-SiNx which is adopted as gate dielectric and passivation layer. Current collapse of the MIS-HEMTs without field plate is suppressed more effectively by increasing the SiH2Cl2/NH3 flow ratio and the normalized dynamic on-resistance (RON) is reduced two orders magnitude after off-state VDS stress of 600 V for 10 ms. Through interface characterization, we have found that the interface deep-level traps distribution with high Si donor incorporation by increasing the SiH2Cl2/NH3 flow ratio is lowered. It’s indicated that the Si donors are most likely to fill and screen the deep-level traps at the interface resulting in the suppression of slow trapping process and the virtual gate effect. Although the Si donor incorporation brings about the increase of gate leakage current (IG...

High-resistivity unintentionally carbon-doped GaN layers with nitrogen as nucleation layer carrier gas grown by metal-organic chemical vapor deposition

In this letter, high-resistivity unintentionally carbon-doped GaN layers with sheet resistivity greater than 106 Ω/□ have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). We have observed that the growth of GaN nucleation layers (NLs) under N2 ambient leads to a large full width at half maximum (FWHM) of (102) X-ray diffraction (XRD) line in the rocking curve about 1576 arc sec. Unintentional carbon incorporation can be observed in the secondary ion mass spectroscopy (SIMS) measurements. The results demonstrate the self-compensation mechanism is attributed to the increased density of edge-type threading dislocations and carbon impurities. The AlGaN/GaN HEMT grown on the high-resistivity GaN template has also been fabricated, exhibiting a maximum drain current of 478 mA/mm, a peak transconductance of 60.0 mS/mm, an ON/OFF ratio of 0.96×108 and a breakdown voltage of 621 V.