GaN Lateral Superjunction Schottky Diode - Power Electronics News

The superjunction (SJ) charge balancing concept has been successfully implemented in silicon (Si) MOSFET power devices. Greater focus is now being placed on improving the performance metrics of wide bandgap (WBG) power devices with this technique. In this article, we summarize initial results that show promise in the application of the SJ structure in a lateral gallium nitride (GaN) Schottky diode.

SJ device engineering 

For device voltage ratings exceeding a couple of hundred volts, the drift region contributes the majority share of the net device on-state resistance (R_DS(ON)). A lighter-doped, thicker drift region is needed to support higher off-state depletion voltages in the device. The R_DS(ON) in a Si MOSFET has the following approximate relationship to its breakdown voltage (BV):

  The SJ concept replaces the n-drift region with alternating layers of n- and p-doped regions that are in a state of charge balance. A near linear relationship between R_DS(ON) and BV can be achieved as depletion is achieved at much lower voltages, i.e. 

The n-drift layer doping can now be higher, providing a significant reduction in R_DS(ON). Conversely, a higher BV is achieved at a given R_DS(ON) using the same drift doping levels. One method for the fabrication of SJ MOSFETs involves trench etching into a large part of the drift region, followed by selective epitaxial growth of p-doped columns within these trenches. An alternative method can involve multi-epitaxial growth, where a masked p-implant can be done between successive n-doped epitaxial growths. 

SJ Si MOSFETs in the 600 V rating can theoretically provide approximately 30 times improvement in specific R_DS(ON) compared to conventional MOSFETs, though in reality this factor can be closer to 5 -10 (this depends on the SJ column pitch). 

Challenges in GaN SJ fabrication

Vertical GaN-on-GaN device fabrication has many challenges, starting with the difficulty of creating low-defect, large-area GaN substrates. Selective area doping, especially to create Mg-doped p-regions, remains problematic with GaN devices. Doping control, activation, and implant damage reduction are some of the main issues. Epitaxial regrowth on a non-planar substrate can be even more formidable. SJ devices require accurate charge balancing. A high-temperature regrowth is required to achieve good activation of the p-dopant, which can result in dopant intermixing and non-uniform incorporation. Defect formation in the regrown layers is often a trade-off for improved activation. Incorporation of impurities such as Si and O at the regrowth interface creates dopant compensation. 

A natural option would be to create a lateral SJ device that is based on standard lateral GaN HEMT fabrication technology. The use of a design that does not require selective regrowth or doping allows for simpler fabrication. 

Several approaches have been tried. One example is the use of a post-growth p-type conducting oxide such as NiO. The use of GaN/AlGaN alternating layers creates a superjunction from the alternating charge polarizations in these layers. In this article, we discuss a simpler all-GaN p-n-p lateral superjunction-based Schottky diode demonstrated by Zachary Biegler and his team at UCSB, USA [1]. 

Figure 1(a) depicts the basic device cross-sectional view of a standard lateral GaN Schottky diode. The n-GaN drift region between the Schottky anode and the ohmic cathode determines the device BV. In contrast, the p-n-p structure shown in Figure 1(b) represents the SJ diode. A lateral anode contact removes the need for regrown p-type layers. 

The use of an all-GaN structure instead of an AlGaN/GaN based design ensures minimal strain and a uniform E-field. Thicker layers and/or higher doping can be used to improve conductivity. The p-n-p structure can theoretically be repeated vertically, though the prototype demonstrated used a single sandwich.

Simulations were conducted on a stack consisting of p-GaN layers, each having a thickness of 50 nm and a doping density of 1×10¹⁸/cm³ that sandwich a 1 µm thick n-GaN layer doped at 1×10¹⁷/cm³ (i.e., net doping x thickness matches for the n- and p-type layers). A flat E-field profile of 2.3 MV/cm is seen across the drift region, with peaks in the field seen at the top p-GaN/dielectric interface. A 10 µm drift could achieve a 2.3 kV reverse BV in these simulations.

To create greater confidence in the charge balancing, SIMS charge doping analyses were conducted on molecular beam epitaxy (MBE) grown layers. The n-type Si doping could be controlled through the dopant cell temperature, while the p-type Mg doping could be controlled through the percentage valve opening. While metal-organic chemical vapor deposition (MOCVD) is the preferred growth choice for GaN in mass manufacturing, MBE has some advantages that include atomic-level precision that can generate a more abrupt interface and a lower growth temperature. 

]While MOCVD has a faster growth rate, precursors that contain carbon can compensate for dopants. Since MBE has a lower level of such compensating impurities, a wider range of doping concentrations can be achieved. UCSB has been pioneering the development of ammonia (NH₃) based MBE. The nitrogen-rich growth regime eliminates gallium droplets seen in the alternative plasma-enhanced MBE growth technique of GaN. 

In this work, the MBE was carried out at a growth temperature of 825 °C for the n-doped layers and 750 °C for the p-doped layers. Indium surfactant was used during the growth, which has the advantages of creating a smoother surface morphology and also reducing the background doping concentration. 

The SJ device was fabricated on Fe-doped GaN on sapphire substrates and included a bottom 100 nm p-layer (placed below the unintentionally doped buffer layer) to compensate the n-type donors at the substrate interface. In the actual device, the n SJ layer was 1.3 µm thick, while the p SJ layers were 65 nm thick. The standard Schottky fabricated for comparisons used only the 1.3 µm, 1×10¹⁷/cm³ n-doped layer.

Fabrication involved reactive ion etching to create the device mesas, followed by Al₂O₃ passivation through atomic layer deposition. The ohmic contact was formed using Ti/Au, while the Schottky contact consisted of a Pd/Au metal deposited through evaporation under rotation to ensure conformal sidewall coating.  Both the SJ and standard Schottky devices shown in Figure 1 followed the same fabrication steps, with the drift length varied from 10 µm to 50 µm.    

Results

The forward and reverse IV curves of the diodes are shown in Figure 2.

The forward characteristics between SJ and standard diodes are similar, showing no degradation with the use of the SJ layers. Both devices were more resistive than expected due to the high growth rate of 1 µm/hr used during the MBE for the n-layers, which creates lower electron mobility. A slower growth rate is expected to improve this. 

An interesting feature seen in the IV curves is the hump in the SJ Schottky reverse characteristics at around -40 V. This is attributed to a not perfectly ohmic contact on the p-GaN sidewalls on the anode contact (ideally, this would be a reverse-biased ohmic p-junction). A Schottky behavior at this junction increases leakage until the channel is pinched off with vertical depletion at higher reverse voltages.

The big improvement with the use of the SJ layers is in the BV of the diodes. The standard Schottky diode failures start at voltages as low as 10 V, while the SJ diodes survive up to 200 V. The highest BV of 860 V was achieved on the SJ Schottky diode with a drift length of 40 µm. 

While the leakage and BV on these SJ diodes need to be improved, this proof-of-concept study is promising and can provide the foundation for future improvements. 

References

[1] Z. J. Biegler, W. Y. Ho, E. Farzana, S. Krishnamoorthy, and J. S. Speck, “Lateral GaN Schottky superjunction diodes with buried p-GaN by NH3-MBE,” APL Electronic Devices 2, 026114 (2026).