AlGaN/GaN Schottky Barrier Diodes for High Power and Fast Switching Applications

Abstract

In recent years, aluminum gallium nitride (AlGaN)/ gallium nitride (GaN) heterostructure devices have received great attention as high efficiency power devices due to their superior properties, such as high current density, high breakdown field, and fast recovery time. This dissertation presents the investigations of AlGaN/GaN Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs) as a power switching devices. Schottky HEMTs were fabricated on the AlGaN/GaN heterostructure with a silicon carbon nitride (SiCN) capping layer. With a 5-nm-thick SiCN capping layer, two-dimensional electron gas (2DEG) carrier concentration was increased about 51 % compared with a conventional AlGaN/GaN heterostructure, due to the neutralization of negative polarization charges at the AlGaN surface. Thus, maximum drain current of the Schottky HEMT with SiCN capping layer was increased about 10 % compared with a conventional Schottky HEMT. Schottky HEMT with an integrated Schottky-drain protection diode was fabricated. Low turn-on voltage of 0.7 V and blocking capability over 100 V was measured with a SiCN-SBD. Recessed Schottky contact was studied as an AlGaN/GaN SBD scheme. Two-step etching composed of dry etching and wet digital etching with 5 nm/cycle etch rate was proposed as an epi-layer etching method. Due to the surface treatment of wet digital etching, the ideality and Schottky barrier height of SBDs were improved from 1.35 and 0.60 eV to 1.19 and 0.73 eV, respectively. Influence of anode recess depth was analyzed and optimum recess depth was determined. As a result, the AlGaN/GaN SBD with turn-on voltage of 0.52 V, ideality factor of 1.17, Schottky barrier height of 0.76 eV, forward current of 127 mA/mm at 1.5 V, and reverse current of 2 μA/mm at -1100 V was fabricated. Failure of the fabricated diode was not observed at the 200 °C, 200 V reverse stress condition during 200 hours. Double field plate structure to increase the breakdown voltage of AlGaN/GaN SBD was also studied. The electric field distribution effect of the field plates was clearly verified by device simulation. Experimental result shows the increase of breakdown voltage from 925 V to 1065 V with the double field plate structure. AlGaN/GaN SBD with anode edge terminated SiNx layer was proposed to reduce the reverse current. Reverse leakage current of the proposed diode was decreased about 103 order due to the suppression of the surface current. The fabricated diodes shows the good forward and reverse current uniformity. The multi-finger lateral-type AlGaN/GaN SBDs were successfully implemented with a 5 mm2 active area. The fabricated diode exhibits the forward current of 7.1 A at 1.5 V, specific on-resistance of 6.1 mΩ·cm2, reverse current of 12.5 µA at -600 V, and breakdown voltage of 1080 V. The figure of merit (VBR2/Ron) is 192 MW/cm2, which is one of the highest value of the lateral-type large area GaN SBDs ever published. Reverse recovery time of the fabricated diode was measured as 10 ns. These results indicate that the proposed AlGaN/GaN SBDs are adequate for fast switching applications with low losses.