Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001476.pdf

Why nitrogen cannot lead to p-type conductivity in ZnO

In this paper, we report on 1.2-kV-class vertical GaN-based trench metal–oxide–semiconductor field-effect transistors (MOSFETs) on a free-standing GaN substrate with a low specific on-resistance. A redesigned epitaxial layer structure following our previous work with a regular hexagonal trench gate layout enables us to reduce the specific on-resistance to as low as 1.8 mΩcm2 while obtaining a sufficient blocking voltage for 1.2-kV-class operation. Normally-off operation with a threshold voltage of 3.5 V is also demonstrated. To the best of our knowledge, this is the first report on vertical GaN-based MOSFETs with a specific on-resistance of less than 2 mΩcm2.

http://iopscience.iop.org/article/10.7567/APEX.8.054101/pdf

1.8 mΩ·cm2 vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate for 1.2-kV-class operation

Emerging trends in wide band gap semiconductors (SiC and GaN) technology for power devices

The impact of gate metals on the threshold voltage (VTH) and the gate current of p-GaN gate high-electron-mobility transistors (HEMTs) is investigated by fabricating p-GaN gate HEMTs with different work function gate metals-Ni and W. p-GaN gate HEMTs incorporate a p-GaN layer under the gate electrode as the gate stack on top of the AlGaN/GaN layer. In comparison to the Ni-gate p-GaN HEMTs, the W-gate p-GaN HEMTs showed a higher VTH of 3.0 V and a lower gate current of 0.02 mA/mm at a gate bias of 10 V. Based on TCAD device simulations, we revealed that these high VTH and low gate current are attributed to the low gate metal work function and the high Schottky barrier between the p-GaN and the W gate metal.

p-GaN Gate HEMTs With Tungsten Gate Metal for High Threshold Voltage and Low Gate Current

In this letter, 600-V normally-OFF SiNx/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) is reported. Normally-OFF operation and low OFF-state gate leakage are obtained by using fluorine plasma ion implantation in conjunction with the adoption of a 17-nm SiNx thin film grown by plasma-enhanced chemical vapor deposition as the gate insulator. The normally-OFF MIS-HEMT exhibits a threshold voltage of +3.6 V, a drive current of 430 mA/mm at a gate bias of 14 V, a specific ON-resistance of 2.1 mΩ·cm2 and an OFF-state breakdown voltage of 604 V at a drain leakage current of 1 μA/mm with VGS=0 V, and the substrate grounded. Effective current collapse suppression is obtained by AlN/SiNx passivation as proved by high-speed pulsed I-V and low-speed high-voltage switching measurement results.

600-V Normally Off ${\rm SiN}_{x}$ /AlGaN/GaN MIS-HEMT With Large Gate Swing and Low Current Collapse

This letter studies the current collapse phenomenon during switching in p-GaN gate AlGaN/GaN high-electron-mobility transistors. It is found that channel hot electrons play a major role in increasing the current collapse and that adding a field plate significantly reduces the effect. By stressing the device with OFF-state pulses of 100 μs× 10 μs with a VGS rise/fall time of 10 ns at Vdc 400 V, compared to the ON-resistance before stress, the ON-resistance was 78 times larger after stress without field plates. With a field plate, it was only 1.8 times larger.

Impact of Channel Hot Electrons on Current Collapse in AlGaN/GaN HEMTs

A p-GaN/AlGaN/GaN based normally-off HEMT device has been demonstrated on a Si substrate. Our p-GaN based device shows not only a high threshold voltage of 3 V but also low gate leakage current. Buffer and device breakdown voltages exceed 1600 V with 5.2 um GaN buffer thickness and specific on-state resistance is 2.9mΩ cm2. The calculated figure of merit is 921 MV2/Ωcm2, which is the highest value reported for the GaN E-mode devices.

1.6kV, 2.9 mΩ cm 2 normally-off p-GaN HEMT device

An Enhancement-mode (E-mode) high electron mobility transistor (HEMT) includes a channel layer with a 2-Dimensional Electron Gas (2DEG), a barrier layer inducing the 2DEG in the channel layer, source and drain electrodes on the barrier layer, a depletion layer on the barrier layer between the source and drain electrodes, and a gate electrode on the depletion layer. The barrier layer is recessed below the gate electrode and the depletion layer covers a surface of the recess and extends onto the barrier layer around the recess.

E-Mode High Electron Mobility Transistors And Methods Of Manufacturing The Same

A pathway to increase the threshold voltage $(V_{\rm TH})$ of p-GaN gate high-electron-mobility transistors (HEMTs) is presented. The hole depletion width in the p-GaN layer at the gate interface is one of the key controlling factors of $V_{\rm TH}$ in p-GaN gate HEMTs. In order to increase the depletion width, we devise a new device structure of p-GaN gate HEMT having a source-connected p-GaN bridge. We demonstrate that a bridged p-GaN gate HEMT structure increases the $V_{\rm TH}$ from 0.93 to 2.44 V.

In-jun Hwang

Papers

p-GaN Gate HEMTs With Tungsten Gate Metal for High Threshold Voltage and Low Gate Current

Impact of Channel Hot Electrons on Current Collapse in AlGaN/GaN HEMTs

1.6kV, 2.9 mΩ cm 2 normally-off p-GaN HEMT device

E-Mode High Electron Mobility Transistors And Methods Of Manufacturing The Same

Source-Connected p-GaN Gate HEMTs for Increased Threshold Voltage