Gallium oxide (β-Ga2O3) single crystal is a direct band gap ultra-wide band gap oxide semiconductor with a band gap of 4.8 to 4.9 eV. It has unique ultraviolet transmission characteristics (absorption cut-off edge ~ 260 nm); The electric field strength is as high as 8MV / cm, which is nearly 27 times that of Si, more than 2 times that of SiC and GaN, and Barrier Plus values are 10 times and 4 times that of SiC and GaN. Single crystal, so β-Ga2O3 has become one of the preferred materials for ultra-high voltage power devices and deep ultraviolet optoelectronic devices.
The State University of New York at Buffalo announced the development of a β-Ga2O3 MOSFET with a breakdown voltage in excess of 1800V [Ke Zeng et al, IEEE ElectronDevice Letters, published online 23 July 2018], and the withstand voltage level before that Located below 750V. The increase in breakdown voltage is attributed to improvements in the field plate design, using a composite dielectric to support the field plate metal. Atomic layer deposition (ALD) is used to deposit dielectric in the expected high-field region to improve the material quality.
The device is manufactured using a semi-insulating iron-doped Ga2O3 substrate, and a 200 nm thick unintentionally doped (UID) and tin-doped (Sn) Ga2O3 epitaxial layer are deposited on the substrate. Then, in the source / drain regions, spin doping technology is used to selectively perform Sn doping. The source / drain contact metal is annealed titanium / gold. The gate dielectric is a 20 nm thick silicon dioxide film deposited by the ALD method. Plasma-enhanced chemical vapor deposition (PECVD) and silicon dioxide deposited by ALD methods support the field plate structure. The stack also includes a thin ALD-deposited aluminum oxide layer that is designed to serve as a stop layer for gate trench reactive ion etching. The aluminum oxide etch stop layer at the bottom of the trench is removed by wet etching. The gate and field plate metals consist of titanium / aluminum / nickel / gold.
In the electronic fluorinert environment, the breakdown voltage of the device reaches 1850V, and the gate-drain distance at this time is 20 μm. Other factors remain the same. The breakdown voltage of the device is only 440V in the air environment. By changing the gate-drain distance, the researchers found that the breakdown voltage of the device in the Fluorinert environment was about 4 times that in the air environment.
The researchers compared the performance of their device with the performance of lateral Ga2O3 transistors reported by other research teams. At present, the overall performance is hovering near the theoretical limit of Si-based devices, which is lower than the expected limits of GaN and Ga2O3. The team hopes to improve the withstand voltage level through more elaborate device processes. In addition, Fluorinert fluids will also be replaced by other passivation materials.