Fumimasa Horikiri

Vertical GaN p-n Junction Diodes With High Breakdown Voltages Over 4 kV

Vertical structured GaN power devices have recently been attracting a great interest because of their potential on extremely high-power conversion efficiency. This letter describes increased breakdown voltages in the vertical GaN p-n diodes fabricated on the free-standing GaN substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, the record breakdown voltages (VB) of 4.7 kV combined with low specific differential ON-resistance (RON) of 1.7 mΩcm2 were achieved. With reducing the Si-doping concentration of the top n-GaN drift layer adjacent to the p-n junction using well-controlled metal-organic vapor phase epitaxy systems, the peak electric field at the p-n junction could be suppressed under high negatively biased conditions. The second drift layer with a moderate doping concentration contributed to the low RON. A Baligas figure of merit (VB2/RON) was 13 GW/cm2. These are the best values ever reported among those achieved by GaN p-n junction diodes on the free-standing GaN substrates.

Bulk micromachined energy harvesters employing (K, Na)NbO3 thin film

In this paper, a micromachined energy harvester employing a lead-free (K, Na)NbO3 (KNN) thin film was reported. KNN is one of the lead-free piezoelectric materials. It is a promising alternative to Pb(Zr, Ti)O3 (PZT). In a micromachined energy harvester, a 2 µm thick KNN film was deposited on a silicon cantilever integrated with a proof mass. The energy harvester achieved an output power of 731 nW and a normalized power density (NPD) of 2.29 μW (g−2 mm−3) at the resonant frequency of 1509 Hz with the acceleration of 10 m s−2. The harvested energy was enhanced considerably by applying bulk micromachining, and was comparable to the PZT-based energy harvesters.

Wafer-level nondestructive inspection of substrate off-angle and net donor concentration of the n−-drift layer in vertical GaN-on-GaN Schottky diodes

In the mass production of GaN-on-GaN vertical power devices, a nondestructive simple inspection of the net donor concentration (N D − N A) of the n−-drift layer in the range of 1015 cm−3 is required. In this study, we demonstrate the wafer-level nondestructive inspection of GaN Schottky barrier diode epi-structures grown by metal organic vapor phase epitaxy (MOVPE) on free-standing GaN substrates. We found that the normalized yellow (YL) photoluminescence peak intensity of the near band edge (NBE), I YL/I NBE, is strongly related to the acceptor concentration N A of the n−-drift layer. This means that the N D − N A of the n−-drift layer can be inspected by photoluminescence measurement at a high speed, when Si concentration is not varying across the GaN wafers. Noncontact capacitance–voltage and secondary ion mass spectrometry measurements were used to investigate the cause of N D − N A variation across the GaN wafers. The discrepancy between C and N A indicates that compensation could be due to another electron trap.

Evaluation of Crystal Orientation for (K,Na)NbO3 Films Using X-ray Diffraction Reciprocal Space Map and Relationship between Crystal Orientation and Piezoelectric Coefficient

We have found an effective method for the evaluation of the crystal orientation of (K,Na)NbO3 (KNN) films in the (K,Na)NbO3/Pt/Ti/SiO2/Si structure using X-ray diffraction (XRD) reciprocal space maps. Previously, the crystal structure and orientation of such (K,Na)NbO3 films were evaluated using 2θ/θ XRD, and were considered to be the pseudocubic perovskite structure with preferential (001) orientation and no (111) orientation. Here, we applied the new method using XRD reciprocal space maps, and discovered that the (K,Na)NbO3 films had some degree of KNN(111) orientation. We calculated the KNN(001)- and KNN(111)-orientation volume fractions for the (K,Na)NbO3 films from the (101) diffraction peaks originating from the KNN(001)- and KNN(111)-orientation elements in the XRD reciprocal space maps, considering the calibration factors obtained from pole-figure simulations, and examined the relationship between the crystal orientation and d31 piezoelectric coefficient in the (K,Na)NbO3 films. The results indicated that the d31 piezoelectric coefficient increases with increasing (001)-orientation volume fraction.

5.0 kV breakdown-voltage vertical GaN p–n junction diodes

A high breakdown voltage of 5.0 kV has been achieved for the first time in vertical GaN p–n junction diodes by using our newly developed guard-ring structures. A resistance device was inserted between the main diode portion and the guard-ring portion in a ring-shaped p–n diode to generate a voltage drop over the resistance device by leakage current flowing through the guard-ring portion under negatively biased conditions before breakdown. The voltage at the outer mesa edge of the guard-ring portion, where the electric field intensity is highest and the destructive breakdown usually occurs, is decreased by the voltage drop, so the electric field concentration in the portion is reduced. By adopting this structure, the breakdown voltage (V B) is raised by about 200 V. Combined with a low measured on-resistance (R on) of 1.25 mΩ cm2, Baligas figure of merit was as high as 20 GW/cm2.