Study On AlGaN-based Novel Power Electronic Devices

As the invention of high brightness blue gallium nitride(GaN) LED won 2014 Nobel Prize in Physics, the wide band gap material has been known to the public. The power electronic devices based on GaN have attracted great attentions from both the academia and industry, for their high breakdown voltage, high current density, high switching speed, low on-resistance, and excellent thermostability. According to some statistics, about 10% of the total electricity is wasted during power conversion, equaling to the sum of the clear resources including wind, solar, and hydroelectricity. With the growing of global energy crisis, people are urgently in need of more efficient power conversion system, of which the power electronic devices are the core technique. Equipment using GaN power devices can not only save a lot of electricity energy, but also simplify the cumbersome heat sink, which is very meaningful to the miniaturization of the appliances and promoting the performance of some weapons.In this context, this paper has tried to realize some new AlGaN-based novel power electronic devices with both high breakdown voltage and low on-resistance by replacing the traditional GaN with AlGaN(with a bettter breakdown characteristics) as the buffer layer, and by optimizing the structure of the buffer layer, channel layer, and the HEMTs(high electron mobility transistors). The results obtained are shown as follows:1. The high performance Al0.30Ga0.70N/GaN/Al0.07Ga0.93 N DH HEMTs(double heterostructure high electron mobility transistors) is fabricated by adopting an AlxGa1-xN graded buffer layer,. The optimized DH features a RMS(root mean square roughness) of 0.16 nm, a FWHM(full width at half maximum) of 540 arcsec of(10-12) plane rocking curve, a carrier mobility of 1744 cm2/V·s, a 2DEG(2-dimensional electron gas) sheet carrier density of 1.09×1013 cm-2. The device characterizations demonstrate that the presented DH HEMTs have fully surpassed the traditional single heterostructure HEMTs in performance, including saturation drain current promoted from 990 mA/mm to 1014 mA/mm, peak transcondutance improved from 182 mS/mm to 194 mS/mm, subthreshold swing(SS) reduced from 113 to 78 mV/dec, Ion/Ioff ratio promoted from 105.3 to 106.2, and drain induced barrier lowering(DIBL) reduced from 24 and 14 mV/V, reverse Schottky leakage current reduced by one order in magnitude, and breakdown voltage promoted from 59 to 109 V.2. We have investigated the effect of the AlGaN barrier thickness and the GaN channel thickness on the performance of GaN DH HEMTs by comparision. We found that the crystal quality of the sample with a 1400 nm AlxGa1-xN buffer layer and a 70 nm GaN channel layer is superior to that of the sample with a 800 nm AlxGa1-xN buffer layer and a 12 nm GaN channel layer, including RMS reduced from 0.22 nm to 0.17 nm, edge dislocation density reduced from 2.4×109 to 1.3×109 cm-2,electron mobility promoted from 1535 to 1602 cm2/V·s, sheet carrier density(2DEG) improved from 0.87×1013 to 1.15×1013 cm-2, saturation drain current promoted from 757 to record 1050 mA/mm, on-resistance reduced from 5.3 ?·mm to 3.6 ?·mm, mesa leakage reduced by two orders in magnitude, and breakdown voltage promoted from 72 to 108 V. Noticeably, 1050 mA/mm is a record value among all the reported Al0.30Ga0.70N/GaN/Al0.07Ga0.93 N DH HEMTs.3. We have successfully obtained high performance Al0.40Ga0.60N/Al0.18Ga0.820 N HEMTs by adopting an AlGaN/GaN composite buffer layer. The crystal quality, RMS, 2DEG sheet density and mobility, and sheet resistance of Al0.40Ga0.60N/Al0.18Ga0.820 N can be greatly improved by the AlGaN/GaN composite buffer layer. The fabricated AlGaN channel HEMTs have also presented excellent performance, including the saturation drain current promoted by 148%, peak transconductance promoted by 72%, and on-resistance reduced from 31.2 ?·mm to 8.1 ?·mm.4. The AlGaN channel MIS-HEMTs(metal-insulator-semiconductor high electron mobility transistors) have been presented in the world for the first time. Adopting SiNx dielectrics in the device is beneficial to reduce the gate leakage, leading to a breakdown voltage of 359 V at gate-drain distance of 2 ?m and an average breakdown electric field of 1.8 MV/cm. The breakdown voltage increases non-linearly with the gate-drain distance and reaches 1661 V with a gate-drain distanceof 20 ?m, meanwhile the average breakdown electric field drops to 0.83 MV/cm because the reverse gate bias cannot fully deplete the channel layer and the buffer layer near the drain edge as the LGD gradually increases. The distinguished breakdown characteristics of the AlGaN channel thickness results from the introduction of the gate dielectrics, and the wider band-gap barrier and channel materials.5. We have also found that AlGaN channel MIS-HEMTs feature an excellent thermostability. As temperature increases from 25 to 275?, the saturation drain current decreases slightly by 20% from 211 to 169 mA/mm and theon-resistance increases only by 24%.The high performance mainly benefits from the stable electron mobility at elevated temperatures. The AlGaN channel MIS-HEMTs are very promising for applications under a tough environment, such as spacecraft, aviation dynamotor, and mining industry.