In this letter, we describe the characteristics of Gallium Nitride (GaN) p–n junction diodes fabricated on free-standing GaN substrates with low specific on-resistance R on and high breakdown voltage V B. The breakdown voltage of the diodes with the field-plate (FP) structure was over 3 kV, and the leakage current was low, i.e., in the range of 10-4 A/cm2. The specific on-resistance of the diodes of 60 µm diameter with the FP structure was 0.9 mΩ·cm2. Baliga's figure of merit (V B 2/R on) of 10 GW/cm2 is obtained. Although a certain number of dislocations were included in the device, these excellent results indicated a definite availability of this material system for power-device applications.

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The forward current (I F) of GaN p–n diodes forward biased at 5 V was found to be proportional to anode perimeter and independent of temperature (273–373 K). Although this I F characteristic contributes to reduction in specific on-resistance (R on A) of small GaN p–n diodes, the mechanism involved is yet to be understood. Accordingly, in this study, an optical-thermo-transition model for the reduced R on A in small GaN p–n diodes is proposed. The energy of photons that are created through radiative recombination around the edge of anode electrode is considered to be used for optical-thermo-transition; that is, electrons are emitted from ionized acceptors to conduction bands, and the resultant neutralized acceptors are ionized through electron capture from valence bands. Since known self-heating and intrinsic-photon-recycling effects are considered to be negligible, optical-thermo transition and enhanced radiative recombination (due to large perimeter-to-area ratio) are concluded to play a dominant role in reducing R on A.

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