High Breakdown Voltage with Low On-state Resistance of InGaN/GaN Vertical Conducting Diodes
Atsushi Nishikawa, Kazuhide Kumakura, and Toshiki Makimoto
Materials Science Laboratory
Due to wide band gap and high critical electric field of GaN, GaN-based electronic devices are promising for high-power and high-temperature operation. Although a lateral conducting structure using a semi-insulating substrate such as sapphire is usually fabricated for GaN-based devices, a vertical conducting structure using a conductive substrate is advantageous for lower-loss and higher current density. Therefore, GaN-based vertical conducting devices are expected to be preferable for high-power electronic device applications. So far, the study on GaN-based vertical pn junction diode was insufficient because of a high-resistance p-GaN layer and a buffer layer to grow a GaN layer on a hetero-substrate. We have overcome these difficulties by using an InGaN layer for the p-type layer and GaN substrate on which a GaN layer can be directly grown. The diode fabricated in this study exhibits a high breakdown voltage with a low on-state resistance .
The samples were grown on n-GaN substrates, using metalorganic vapor phase epitaxy (MOVPE). The sample structure consists of a 1.8-mm or 3.6-mm-thick lightly doped n-GaN layer and a 140-nm-thick p-InGaN layer. For comparison, the same diode structure with 1.8-mm-thick n-GaN was grown on a conductive SiC substrate. Figure 1 shows reverse current-voltage (I-V) characteristics. The leakage current of the diode on a GaN substrate is one order of magnitude lower than that on a SiC substrate due to the better crystal quality of GaN grown on a GaN substrate. Since a thick crack-free GaN layer can be grown on a GaN substrate, the breakdown voltage increases with increasing n-GaN layer thickness. For the 3.6-mm-thick GaN layer, the breakdown voltage (VB) reaches as high as 571 V. At the same time, we obtained the low on-state resistance of 1.23 mWcm2 because of absence of the buffer layer, as shown in Fig. 2. The figure-of-merit, V2/Ron, is calculated to be 265 MW/cm2, which is the highest value among those ever reported for GaN-based vertical conducting Schottky and pn junction diodes.
 A. Nishikawa, K. Kumakura, and T. Makimoto, Appl. Phys. Lett. 89 (2006) 153509.
Fig. 1. Reverse I-V characteristics.
Fig. 2. Forward I-V characteristics.
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