Naoki Kaneda

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.

Over 3.0

This letter describes a new two-step electrode process on p-GaN and characteristics of GaN p-n junction diodes on free-standing GaN substrates with low specific ON-resistance R_οn and high breakdown voltage V_B. We develop a two-step process for anode electrodes in order to avoid plasma damage to the p⁺-GaN contact layer during the sputtering process. The specific ON-resistance is further improved due to a new low-damage process. The breakdown voltage of the diodes with the field-plate (FP) structure is over 1100 V, and the leakage current was low, i.e., in the range of 10^-9 A. The specific ON-resistance of the diodes of 50 μm in diameter with the FP structure was 0.4 mΩ · cm². Baligas figure of merit (V_B²/R_on) of 3.0 GW/cm² is obtained. These are the best values ever reported among those achieved by GaN p-n junction diodes on free-standing GaN substrates.

\hbox{GW/cm}^{2}

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 Ron and high breakdown voltage VB. 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 mΩcm2. Baligas figure of merit (VB2/Ron) 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.

Figure-of-Merit GaN p-n Junction Diodes on Free-Standing GaN Substrates

We report GaN p-n diodes on free-standing GaN substrates: a record high Baligas figure-of-merit (V<;sub>B<;/sub><;sup>2<;/sup>/ Ron) of 12.8 GW/cm<;sup>2<;/sup> is achieved with a 32 μm drift layer and a diode diameter of 107 μm exhibiting a BV > 3.4 kV and a R<;sub>on<;/sub> <; 1 mΩ-cm<;sup>2<;/sup>. The leakage current density is low: 10<;sup>-3<;/sup> - 10<;sup>-4<;/sup> A/cm<;sup>2<;/sup> at 3 kV. A record low ideality factor of 1.1-1.3 is signature of high GaN quality. These are among the best-reported GaN p-n diodes.

High-Breakdown-Voltage and Low-Specific-on-Resistance GaN p–n Junction Diodes on Free-Standing GaN Substrates Fabricated Through Low-Damage Field-Plate Process

We studied the roles of lightly doped carbon in a series of n-GaN Schottky diode epitaxial structures on freestanding GaN substrates, and evaluated the effects of the doping on diode performances. A large variation of compensation ratio was observed for carbon doping at (1–2) × 1016 cm−3. A model was proposed to explain this phenomenon, in which a vulnerable balance between donor-type CGa and deep acceptor CN strongly affected the free-carrier generation. Application of Norde plots and reverse biased leakage current in current–voltage measurements suggested provisional optimization for a free-carrier concentration of 8 × 1015 cm−3 to achieve a tradeoff between breakdown voltage and on-resistance of the n-GaN diodes.

GaN-on-GaN p-n power diodes with 3.48 kV and 0.95 mΩ-cm2: A record high figure-of-merit of 12.8 GW/cm2

Forward-current-density <i>J</i>_F/forward-voltage <i>V</i>_F characteristics of vertical gallium-nitride (GaN) Schottky-barrier diodes (SBDs) and p-n diodes on free-standing GaN substrates were computationally, as well as experimentally, investigated. Based on the thermionic emission model, electron-drift mobility μ<i>n</i> was used as a parameter to fit the <i>J</i>_F-<i>V</i>_F characteristics of both reported and fabricated GaN SBDs. At 300 K, μ<i>n</i> was fitted to be 600 cm²/V ·s when electron concentration <i>n</i> was 1 × 10¹⁶ cm^-3 and 750 cm²/V ·s when <i>n</i> was 5 ×10¹⁵ cm^-3. Accordingly, the theoretical μ<i>n</i>-<i>n</i> curve for a carrier compensation ratio of 0.90 was applied in the case of n-GaN layers on GaN substrates. With respect to GaN p-n diodes, the reported <i>J</i>_F -<i>V</i>_F characteristics were successfully fitted with dislocation-mediated carrier lifetimes in the high-injection region and with Shockley-Read-Hall lifetimes in the generation-recombination current region.

Roles of lightly doped carbon in the drift layers of vertical n-GaN Schottky diode structures on freestanding GaN substrates

A Si-doped GaN layer was grown by MOVPE (metal-organic vapor phase epitaxy) using tetraethylsilane (TeESi) as the dopant. The Hall effect was studied between 11 K and 300 K. The electron concentration was observed to increase with the increase in the TeESi flow rate. The activation energy E D for ionization of shallow donors was determined to be 27 meV.

Numerical Analysis of Forward-Current/Voltage Characteristics of Vertical GaN Schottky-Barrier Diodes and p-n Diodes on Free-Standing GaN Substrates

Photon recycling was used to increase ionization of magnesium in GaN p-n diodes by reducing anode radius to 20 µm. On-resistance of GaN p-n diodes (with breakdown voltages of 0.7–0.8 kV) was extremely low (i.e., 0.5 mΩcm2 at 5 V) in the temperature range of 273–373 K. This temperature-independent extremely low on-resistance is a promising characteristic for power-electronics applications such as fast, high-voltage freewheeling diodes.

Si-Doping in GaN Grown by Metal-Organic Vapor Phase Epitaxy Using Tetraethylsilane

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 (IV) of the p–n junction, lateral current (IL) in the p-GaN layer (magnesium concentration: NMg = 5×1017 cm-3) was found to increase within L of 10 µm from the edge of the TLM electrodes; the measured IL 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.

Photon-recycling GaN p-n diodes demonstrating temperature-independent, extremely low on-resistance

The influence of surface recombination on forward current-voltage (I_F-V_F) characteristics of gallium nitride (GaN) p⁺n diodes formed on GaN free-standing substrates was both experimentally and computationally investigated. In the temperature range of 373-273 K, the surface-recombination velocity 5 of 0.5-μm n GaN overetched circular-mesa diodes was derived, respectively, as 2-10 × 10⁷ and 1 × 10⁷ cm/s for diodes with and without a mesa-field-plate termination structure consisting of dielectric and metal layers. It was shown that IF (V_F <; 2.9 V) is dominated by carrier recombination at the side surface of the etched n GaN. The I_F-V_F characteristics of the fabricated diodes were compared with the reported GaN p⁺n diodes with almost-zero overetched depth of n^- GaN. The large I_F of the latter diodes is attributed to enhanced photon recycling through photon reflection at the metal mirror. Moreover, reducing S to less than 10⁵ cm/s is considered indispensible for achieving similar enhanced photon recycling in overetched terminated diodes.