Shinya Ootomo

Normally Off n-Channel GaN MOSFETs on Si Substrates Using an SAG Technique and Ion Implantation

We have demonstrated n-channel gallium nitride (GaN) MOSFETs using a selective area growth (SAG) technique and ion implantation on a silicon substrate. Both MOSFETs realized good normally off operations. The MOSFET using the SAG technique showed a large drain current of 112 mA/mm, a lower leakage current, and a high field mobility of 113 cm2/V . s, which is, to our knowledge, the best for a GaN MOSFET on a silicon substrate.

Normally off operation GaN-based MOSFETs for power electronics applications

Gallium nitride (GaN) is a promising electronic semiconductor material for high-power, high-temperature devices due to its remarkable material properties like wide bandgap, large critical electric field and high saturation velocity compared with Si. The metal-oxide–semiconductor (MOS) field-effect transistor (MOSFET) structure can be operated at a positive threshold voltage, namely the normally off mode, which is preferable for power transistors in terms of fail-safe operation. However in order to minimize the power losses in MOSFET operation, good interface quality at SiO2/GaN and low resistance in the n+-contact layer are strongly required. The MOS capacitors were used to characterize the interface states at SiO2/GaN, and the interface state density at Ec − 0.4 eV was less than 1 × 1011 cm−2 eV−1 after annealing at 900 °C for 30 min by the furnace. In addition, the activation annealing of Si-implanted GaN was performed at 1260 °C for 30 s in rapid thermal annealing (RTA) and its sheet resistance was 23 kΩ sq−1. Finally, we have fabricated GaN MOSFETs and have achieved more than 1 A operation in the normally off mode at more than 250 °C. The breakdown voltage was more than 1500 V. We also confirmed more than 100 h of consecutive operation at 250 °C at the moment.

Solid-Phase Diffusion of Carbon into GaN Using SiNx/CNx/GaN Structure

We performed a feasibility study on the solid-phase diffusion of carbon into GaN using a SiNx/CNx/GaN structure prepared by electron-cyclotron-resonance-assisted chemical vapor deposition. An X-ray photoelectron spectroscopy study on the CNx layer deposited on GaN showed that its energy positions and spectrum features are very close to those of a C–N bond, and the N composition was estimated to be 24%, indicating a highly C-rich layer. No degradation in the chemical properties of the GaN surface was found after the diffusion process at 1000 °C. A secondary ion mass spectrometry result clearly showed a diffusion of carbon into GaN. We also observed an increase in resistivity for the C-diffused GaN layer.