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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The effect of extrinsic photon recycling (EPR) in p-type gallium nitride (p-GaN), namely, increased ionization ratio (R) of magnesium acceptors owing to radiative recombination, was quantitatively investigated. The lateral extension (L) of EPR was determined by using transmission-line-model (TLM) patterns, formed with GaN p–n junction epitaxial layers on free-standing substrates, as well as by using device simulation. With increasing vertical current (I V) of the p–n junction, lateral current (I L) in the p-GaN layer (magnesium concentration: N Mg = 5×1017 cm-3) was found to increase within L of 10 µm from the edge of the TLM electrodes; the measured I L corresponded to a large R, namely, 30%. This lateral extension will contribute to reducing base resistance and enhancing conductivity modulation of GaN bipolar power-switching devices for power-electronics applications.

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Analytical solutions based on the thin surface barrier (TSB) model were utilized to reproduce the measured current–voltage characteristics of Pd/unintentionally doped Al0.08Ga0.92N/compositionally graded AlGaN/n-GaN Schottky barrier diodes (SBDs) formed on GaN free-standing substrates. The TSB thickness (D) and the Schottky barrier height were used as fitting parameters, while the measured surface concentration of shallow donors was used as a constant. Since the resultant D was close to the value determined from the oxygen concentration profiles, the surface oxygen was concluded to be deeply-involved in the TSB formation.