Hiroshi Ohta

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.

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.

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.

High breakdown voltage vertical GaN p-n junction diodes using guard ring structures

New guard ring structures were adopted to increase the breakdown voltages of the vertical GaN p-n diodes. With this structure, a voltage drop occurs in the guard ring portions by inserting resistance devices between the guard ring portions and the main p-n diode portions, and it is possible to attain higher breakdown voltages by the voltage drop at the most sensitive outer region of the diodes. By adopting this structure, the breakdown voltages of about 200 V has been improved compared with the normal circular p-n diodes.

Junction-barrier Schottky diodes fabricated with very thin highly Mg-doped p + -GaN(20 nm)/n-GaN layers grown on GaN substrates

Low turn-on voltage junction barrier Schottky (JBS) diodes were fabricated using very thin (20 nm) and extremely highly Mg doped (2×10²⁰ cm⁻³) p⁺-GaN layer placed on top of n⁻-GaN epitaxial layers grown on n-GaN substrates. By omitting p-GaN layers in conventional p⁺-GaN/p-GaN/n⁻-GaN vertical p-n junction diodes, device processing has been eased and low specific on-resistances (R<inf>on</inf><0.3 mΩcm²) have been obtained.

High quality free-standing GaN substrates and their application to high breakdown voltage GaN p-n diodes

Recent development in GaN free-standing substrates have enabled fabrications of high performance vertical structure GaN devices. This letter reviews fabrication method of the high quality GaN substrates and reports increased breakdown voltages in vertical GaN p-n diodes fabricated on the substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, record breakdown voltages (VB) of 4.7 kV combined with low specific differential on-resistance (Ron) of 1.7 mΩcm2 were achieved.